Table of Contents
In the last lesson we introduced the topic of food groups and discussed grains, fruits & vegetables, nuts, and oils. All of these can have significant health benefits, and there is relatively little controversy regarding them. In this lesson we will talk about dairy, eggs, and meat. All of these food groups have controversial components, and I will attempt to provide a fair overview of the evidence below.
There is some controversy with dairy products regarding health benefits as well as risks of whole-fat vs reduced-fat options. Thankfully, this does not seem to be due to an overly significant influence from the dairy industry(Chartres, 2020), but rather due to varying lines of research. I will attempt to discuss the research in an unbiased fashion here.
Health effects in children
Several reviews regarding dairy consumption in children have been published:
- A 2016 systematic review and meta-analysis (SR/MA) of prospective cohort studies with children found increased dairy intake was associated with lower odds of having overweight or obesity.(Lu L, 2016)
- A 2017 SR of randomized controlled trials (RCTs) in children and adolescents, however, found that dairy consumption does not have a significant effect on body composition but does aid bone development.(Kouvelioti, 2017)
- A 2020 SR of studies in children found that neither whole-fat nor reduced-fat dairy had strong associations with body composition or cardiometabolic health, with authors concluding there is no strong evidence to support limiting whole-fat dairy intake in children, although they acknowledge more trials are needed.(O’Sullivan, 2020)
- A 2021 review on the impact of various aspects of nutrition on bone health highlighted research showing that low milk intake during childhood or adolescence is associated with an increased risk of fractures, dairy product consumption is positively associated with bone mineral density, and milk consumption is associated with attained height.(Rizzoli, 2021)
Thus, given the known benefit of dairy & calcium consumption for bone development in youth, it seems advantageous for children and adolescents to aim for multiple servings of dairy daily (the DGA suggests 2-2.5 servings daily if <8 years old and 3 servings daily if ≥9 years old), without strong preference for a specific fat content.
However, a 2020 review examining the overall role of milk as it relates to health described several lines of evidence showing that meeting the dietary guidelines of 3 servings daily, principally derived for bone development, is not needed for bone development in individuals consuming an overall nutritious diet with other sources of calcium and vitamin D.(Willett, 2020) Of note, there were several critical commentaries of this review (I include the citation to these commentaries with the citation to the review down below). It seems while milk can be a great nutritional source for individuals who do not have access to healthy nutrition otherwise, for children who can consume an otherwise healthy diet with adequate calcium and vitamin D from other sources aiming for 3 servings daily is likely not needed.
Of note, dairy intake has been associated with increased acne, which can certainly be a concern as acne typically worsens in adolescence. One MA evaluating this mainly found an association with reduced-fat milk(Aghasi, 2019), but a separate SR/MA that includes additional studies and corrects some of the methodological flaws of this prior MA found all dairy types are associated with increased acne risk (with odds ratios of 1.22-1.36).(Juhl, 2018 – of note, this was actually written after the Aghasi, 2019 MA)
Health effects in adults
Several reviews regarding dairy consumption in adults have been published:
- In 2018 a SR/MA of observational studies concluded that dairy, milk, and yogurt are all associated with decreased risk of metabolic syndrome, with this being largely driven by whole-fat dairy as low-fat dairy did not have an impact.(Lee, 2018)
- A 2018 SR/MA of prospective cohort studies found that studies consistently show total & reduced-fat dairy have neutral or inverse associations with type 2 diabetes mellitus (T2DM) as well as other cardiometabolic disease processes.(Soedamah-Muthu, 2018)
- A 2019 SR/MA of RCTs concluded that higher dairy intake was associated with very minimal decreases in insulin resistance, waist circumference, and body weight; the results they report are likely not clinically significant.(Sochol, 2019)
A 2020 review discusses various studies regarding the impact of different types of dairy and finds more consistent favorable evidence regarding cheese and yogurt for cardiometabolic risk reduction.(Timon, 2020) Both of these can be fermentable sources of dairy, and the live active cultures may impact our gut microbiome to provide beneficial effects. Cheese in particular contains a high amount of calcium and phosphorus as well as a particular fat structure that collectively aids cholesterol levels upon consumption.
- In 2020 a MA of prospective cohort studies found 200 grams daily of yogurt intake was associated with a 5% decreased risk of ACM and an 8% decreased risk of cardiovascular (CV) mortality.(Gao, 2020)
- A 2020 SR found that dairy intake is beneficial for bone health in older adults with a lack of evidence precluding valid conclusions in most other age groups.(Wallace, 2020b)
- A 2020 SR/MA of the impact of fermented dairy intake in prospective cohort studies found yogurt led to a 27% decrease in T2DM risk and 20% decrease in metabolic syndrome risk, cheese led to a 24% increase in T2DM risk (95% confidence interval of 1.03-1.49), and fermented milk led to a 4% decrease in risk of stroke, ischemic heart disease, and cardiovascular disease (CVD) mortality.(Companys, 2020)
- In 2020 an umbrella review of observational studies found dairy had beneficial effects for several disease processes, though some findings were restricted to reduced-fat dairy products.(Godos, 2020) However, they also found that dairy can increase the risk of prostate cancer and Parkinson’s disease.
- A 2021 umbrella review of SR/MAs regarding the health impact of milk intake found generally beneficial associations with CVD, stroke, hypertension, colorectal cancer, metabolic syndrome, obesity, osteoporosis, T2DM, and Alzheimer’s disease.(Zhang X, 2021) The authors found harmful associations with increased risk of prostate cancer, Parkinson’s disease, acne, and iron-deficiency anemia in infancy.
The above analyses collectively show lots of health benefits, but there were risks mentioned above. The risk of iron-deficiency anemia in children is valid if excessive milk is consumed; consult with your child’s healthcare provider if this is a concern. I provided data regarding the risk of acne above. Regarding Parkinson’s disease and cancer concerns:
- Specifically regarding Parkinson’s disease, a 2014 MA specifically examining the increased risk of Parkinson’s disease found a small absolute increase in risk due to dairy intake, predominantly in men, with 2-4 new cases per 200 grams daily intake of milk and 1-3 new cases per 10 grams daily intake of cheese per 100,000 person-years.(Jiang, 2014)
- Regarding cancer as a whole, a 2019 overview of reviews showed that for most cancers there was no significant association of dairy intake and change in risk.(Jeyaraman, 2019) Possible exceptions included colorectal cancer where 20 reviews showed no association but 9 found a decreased risk, endometrial cancer where only 1 review was found and this showed an increased risk, non-Hodgkin’s lymphoma where 5 reviews found no association but 4 found an increased risk, and prostate cancer where 13 reviews found no association, 2 found a decreased risk, and 13 found an increased risk.
- A 2016 MA examining cancer mortality risk found an increased risk only with prostate cancer, not other types.(Lu W, 2016)
Thus, the increased risk of Parkinson’s disease is small and it’s unclear if this is due to actual dairy or due to other confounding factors. The risk of prostate cancer seems more significant; I discuss this body of literature more thoroughly in the next section.
Dairy and prostate cancer
There is much concern in the literature regarding increased prostate cancer risk from dairy intake. Various published literature includes:
- A 2015 SR/MA specifically examined the link between dairy products, calcium, and prostate cancer risk.(Aune, 2015) This included 32 prospective studies and found small relative risks of prostate cancer development with increased dairy consumption in the 1.03-1.14 range per serving of all milk, no significant association with whole milk, and when comparing high versus low consumption, relative risks of 1.07 for cheese & 1.12 for yogurt.
- As stated above, a 2016 meta-analysis examining cancer mortality risk found an increased risk only with prostate cancer.(Lu W, 2016) Here they only included two studies, and for high compared to low whole milk intake the relative risk of prostate cancer mortality was 1.50 with a 95% confidence interval of 1.03-2.17. For reduced-fat milk the risk was 1.00 with a 95% confidence interval of 0.75-1.33.
- Since then a 2017 prospective study has found whole milk intake to be significantly associated with increased risk of death once diagnosed with localized prostate cancer, while reduced-fat milk intake was not associated with an increased risk of death, though there are small sample numbers and wide confidence intervals in this study.(Downer, 2017)
- A prospective study published in 2019, however, found no association of dairy intake and prostate cancer incidence overall and only a small increased risk of late-stage prostate cancer with regular-fat dairy and a small increased risk of advanced prostate cancer with 2% milk.(Preble, 2019)
- In 2019 an overview of SR/MAs was published regarding milk/dairy product consumption and prostate cancer risk & mortality, citing several of the references discussed above through 2016.(López-Plaza, 2019) They included 6 total studies and found that for total prostate cancer the relative risk of high vs low consumption ranged from 1.09-1.68 for total dairy, 1.11-1.50 for milk, 0.74-1.18 for cheese, and 0.95-1.03 for other dairy. For advanced prostate cancer and prostate cancer mortality they note there was no statistically significant association seen. They note that the relative risks have decreased with sequential meta-analyses as new literature has emerged.
Note: It is not clear to me why the authors here state no association with prostate cancer mortality was seen as they do include (Lu W, 2016) which shows a statistically significant association between whole milk consumption and prostate cancer mortality.
- A separate review in 2019 attempting to examine the link between dairy intake and prostate cancer and the various proposed mechanisms found that much of the literature is inconsistent but nonetheless most of it does point to an association.(Vasconcelos, 2019)
- Most recently, a 2021 SR evaluating 20 studies through May, 2020 (3 ecological studies, 7 prospective cohort studies, 10 case-control studies) found almost all of them do observe an association between increased milk intake and prostate cancer.(Sargsyan, 2021) It is still not clear to what degree milk fat content impacts this; the literature more conclusively shows whole-fat milk leads to an increased risk. The authors note it is still possible confounding factors explain the association.
Thus, overall, it seems there probably is a small increased risk of prostate cancer with dairy consumption, at least at levels of typical consumption, though it’s unclear if it’s the dairy itself that is contributing or pesticides/hormones/other factors that are present in dairy products. If that is the case then organic dairy products may be safer, but this still remains to be seen. Given no other consistent significantly harmful risks seen with dairy consumption and the association with several health benefits, I do not believe that the concern of prostate cancer or Parkinson disease warrants elimination of dairy from one’s diet, but for people who are concerned by this limiting dairy to no more than 2-3 servings daily may be wise, though more research needs to be done.
Whole-fat vs reduced-fat?
Several analyses have been done considering differential health effects of reduced-fat vs. whole-fat dairy.(Mozaffarian, 2019; Astrup, 2019; Schmidt, 2021) Collectively these tend to show few if any significant detriments comparing whole-fat relative to reduced-fat, and at times positive outcomes are associated with whole-fat dairy that are not seen with low-fat dairy, particularly with yogurt and cheese, though occasionally the opposite is seen.(Hirahatake, 2020) A general lack of detriment with whole-fat dairy despite the relatively high saturated fatty acid content would likely be due in part to different saturated fatty acid profiles in dairy vs non-dairy products, as this seems to impart different health effects.(Unger, 2019)
However, a 2021 SR/MA of prospective cohort studies found no indication of benefit or harm when comparing high vs low fat dairy as a whole, but when specifically looking at milk intake the high fat intake was associated with ACM, CVD mortality, and cancer mortality, with a 6-13% increased risk per additional daily serving.(Naghshi, 2021) A separate 2021 SR/MA of cohort studies found high-fat milk was associated with an 8% increase in coronary heart disease (CHD) per 200 grams intake; cheese was associated with a 4-6% decreased risk of CHD per 20 grams intake, while total milk, low-fat milk, yogurt, and butter had no association.(Jakobsen, 2021)
Of note, many of these associations are seen in observational studies where total consumption of whole-fat dairy products may not be very high. A recent study did show detrimental effects of high levels of dairy fat (41 grams daily, for comparison a typical serving of whole-fat milk has 8 grams of fat) which were significantly attenuated by substituting some of the saturated fatty acid content with monounsaturated fatty acids (by feeding the cows 1 kilogram of sunflower oil daily for 4 weeks prior to dairy collection).(Vasilopoulou, 2020) Thus, at typically consumed dairy levels choosing whole-fat options may not be overly detrimental, but if consumed at high levels the large amount of saturated fat may impose harm.
A late 2020 narrative review describes much of the literature regarding dairy intake and cardiometabolic health, as well as various dietary guidelines, and provides an overview of different lines of evidence showing why dairy fat intake may be beneficial or harmful depending on the source and context of consumption, ultimately concluding we will need longer term randomized controlled trials to better tease apart the controversy.(Poppitt, 2020)
Many people find as they get older that they become intolerant of dairy products. This is due to a reduction of the lactase enzyme that lines the small intestine and helps to digest the lactose sugar found in dairy. For people where this is a problem, they may find supplementing with Lactaid to be helpful. Of note, many people will have only partial lactose-intolerance and still be able to consume some dairy without symptoms, typically no more than 12 grams of lactose daily. Additionally, many people who have symptoms with certain dairy products will not have symptoms with yogurt, particularly strains of yogurt with more live active cultures, as these microorganisms can help break down the lactose so it does not cause symptoms.(Kok, 2018) If none of these are options then soy milk (with calcium fortification) would be a reasonable alternative as a source of calcium if your diet doesn’t have much calcium elsewhere.
Throughout youth consuming 2-3 servings of dairy daily seems beneficial for bone development, without significant preference for fat content unless saturated fat intake is otherwise high in one’s diet (in which case choosing a reduced-fat dairy option makes more sense). Choosing reduced-fat options for weight concerns may also be beneficial if either tracking calories or in some way restricting ad-libitum food intake.
In adulthood dairy intake is associated with several health benefits, with some evidence of additional benefit with fermented dairy products. Those with lactose intolerance may benefit from taking Lactaid or preferentially consuming yogurt with live active cultures. There seems to be some risk of increased acne with dairy intake, there may be a very small increased risk of Parkinson’s disease, and there is likely an association between dairy products and prostate cancer that needs to be better characterized. I don’t advise decreasing dairy intake for these concerns specifically, but if you have acne and want to try eliminating dairy from your diet (as opposed to using medication to treat the acne) to see if this makes a difference then give it a 2-3 month trial. Regarding Parkinson’s disease and prostate cancer, I have outlined the evidence above and you can decide if the risk seems significant enough to warrant decreasing consumption accordingly.
Many people think science goes back and forth about whether eggs are healthy or not, and this is for good reason as there has been a lot of evidence published both ways. I will discuss recent literature regarding this topic below, including recommendations from the American Heart Association.
Impact on cholesterol
Regarding the impact of egg intake on cholesterol, one 2020 SR/MA of RCTs in healthy individuals found that increased egg consumption primarily increases LDL (the “bad” type) cholesterol (by 8.14 mg/dL on average) with no significant impact on HDL (the “good” type), leading to an increase of the LDL:HDL ratio on average by ~0.14.(Li M, 2020) However, a separate 2020 SR/MA evaluated 66 RCTs to determine the impact of egg intake on many different types of cholesterol.(Khalighi Sikaroudi, 2020) All studies included 1 or more eggs daily in the intervention group and cholesterol levels were checked after a 12 hour fast. Findings comparing high vs low egg consumption included:
- Total cholesterol (TC) increased 9.1 mg/dl.
- Triglycerides did not change.
- LDL increased 7.4 mg/dl.
- HDL increased 1.4 mg/dl.
- VLDL did not change.
- LDL:HDL ratio did not change.
- TC:HDL ratio increased 0.11.
- apoA1 increased 0.025 g/L.
- This is the primary protein in HDL particles.
- apoB100 increased 0.058 g/L.
- This is the primary protein in LDL particles.
- A linear correlation between egg intake and effect was seen with TC, HDL, apoA1 and apoB100.
- A nonlinear correlation was seen for LDL and TC:HDL ratio.
Thus, there does seem to be an impact on cholesterol levels, larger on the LDL than HDL subtypes, with at most a small change in the LDL:HDL and TC:HDL ratios (which are arguably better risk markers than LDL, HDL, or TC in isolation). This impact on cholesterol may occur primarily in hyper-responders as some research indicates 2/3 to 3/4 of people do not experience much change in cholesterol levels when increasing cholesterol intake.(Clayton, 2017; Godos, 2021)
- A 2019 analysis of 6 prospective cohort studies in the US examining CVD outcomes found that for an additional 0.5 eggs intake daily the hazard ratios for incident CVD and ACM were 1.06 & 1.08, respectively; this was no longer significant when adjusting for cholesterol consumption.(Zhong, 2019)
- A 2020 analysis of the PURE, ONTARGET, and TRANSCEND cohorts found an increased risk of heart failure with the latter two cohorts at higher egg consumption levels.(Dehghan, 2020)
- A 2020 overview of SRs summarized the various reviews through December, 2019 in the figure reproduced below, depicting the evidence base for egg consumption and various aspects of health (beneficial, harmful, no association, or controversial).(Zhang X, 2020)
- Evaluating various health outcomes, a 2020 umbrella review regarding egg consumption and human health found that there is no consistent evidence of a harmful effect on most health outcomes examined.(Marventano, 2020)
- A 2020 SR/MA of prospective cohort studies examining the risk of egg consumption on T2DM incidence found that overall there was a relative risk of 1.07 with 1 egg consumed daily, with a risk of 1.18 in US cohorts and no increased risk in European & Asian cohorts.(Drouin-Chartier, 2020b) Of interest, randomized trials, such as the DIABEGG study, have shown eggs can be a regular part of a weight maintenance or weight loss program in individuals with prediabetes or T2DM and have no harmful effects on any of the classic cardiovascular risk factors.(Fuller, 2018)
- An analysis of a US cohort (NHANES) that may be more representative of the general population than the 2019 pooled cohort analysis above did not find an association of increased egg intake with ACM or CV mortality with intake of ≥1 eggs daily (though there are small sample sizes at >1 egg intake daily).(Xia, 2020)
- Additionally, a 2020 SR/MA of prospective cohort studies found no indication that consuming 1 egg daily led to any increased risk of CVD, with the exception of increased risk in people with T2DM.(Drouin-Chartier, 2020a) There also was no risk of harm when consuming 2 eggs daily but very few people consumed eggs this frequently. An inverse association of risk was actually seen in Asian populations, but they provide a strong rationale that this is due to confounding lifestyle factors and not an underlying biological mechanism (the highest average egg intake level in the largest Asian cohort was only 0.76 eggs daily (less than the other Asian cohorts) and the inverse associated risk was seen in urban but not rural settings).
- A 2020 SR/MA evaluating the influence of egg intake on risk of stroke found overall no impact when comparing higher vs lower intake, with an inverse association in studies from Asian populations, and a possible decreased risk with intake of 1-4 eggs weekly and an increased risk at intake ≥1 egg daily.(Tang, 2020)
- A 2020 cohort study of a Chinese population that followed individuals for a median of 15 years with repeat measurements of egg intake found increased egg consumption was associated with decreased total mortality (relative risk of 0.78 for 1-2 eggs intake daily and 0.64 for averaging ≥2 eggs intake daily).(Zhuang, 2020) The authors note that egg consumption has been associated with a higher risk of health problems in Western populations relative to Asian populations, possibly due to the association of egg consumption with poor overall diet quality among Western populations.
- A 2020 SR/MA of 23 prospective cohort studies found egg intake was not associated with CVD events or stroke, consuming on average >1 egg daily was associated with a 11% decrease risk of coronary artery disease, and subgroup analyses based on geography did not alter these findings.(Krittanawong, 2020)
With the somewhat conflicting data regarding the impact of egg intake and heart health, in 2020 the American Heart Association published a science advisory discussing both egg and cholesterol consumption.(Carson, 2020) They highlighted that observational studies generally do not show a negative impact of cholesterol consumption and that there is lots of confounding in studies with eggs. They performed a meta-regression of trials that had similar ratios of polyunsaturated-to-saturated fatty acid intake and varied between 155-1000 mg of cholesterol intake daily and found a small positive association between dietary cholesterol and total cholesterol levels with no significant association between dietary cholesterol and LDL or HDL cholesterol individually. They concluded that healthy individuals can include up to 1 whole egg daily in their diet without issues, people with abnormal lipid levels should be cautious of increased dietary cholesterol intake, and due to the highly nutritious content of eggs older individuals with normal cholesterol can have 2 eggs daily in the context of a heart-healthy dietary pattern.
More recent evidence and opinions
- Not everybody agrees with this advice; more recently in the Journal of the American Heart Association an opinion was published citing concern that the negative impact of dietary cholesterol occurs after meal consumption, not when fasting, and thus fasting tests do not see this.(Spence, 2021) The authors note that research indicates dietary cholesterol with saturated fat together can lead to significantly worse cholesterol outcomes (ie, eating eggs with bacon). They note that in populations that generally have healthier dietary habits than the USA (where a classic Western diet predominates) there is greater indication of harm with low levels of egg intake (though as indicated above this is not seen in some Asian cohorts). Lastly, they state that much of the negative health effects arise from toxic metabolites, and individuals with poor kidney function should be especially careful to avoid egg consumption.
- A 2021 analysis of 3 prospective cohort studies found an increased risk of mortality with higher cholesterol intake in black individuals, a mostly neutral risk in white individuals, and an inverse risk in Asian individuals, at least with intake up to 500mg cholesterol daily or 1 egg daily.(Pan, 2021)
- A 2021 analysis of the Women’s Health Initiative (>90,000 post-menopausal women) found egg consumption of at least 1 daily compared to <1 weekly was associated with a ~14% increased risk of ACM and CVD.(Chen, 2021) Of note, there is significant concern of residual confounding given all of the known unhealthy lifestyle habits that associated with higher egg intake in this cohort.
- In 2021, a dose-response MA looking at CV outcomes evaluated 39 prospective cohort studies and determined(Godos, 2021):
- Intake of up to 6 eggs per week has a relative risk of 0.96 for CVD relative to no consumption.
- A decreased risk for CVD mortality was seen with egg consumption of 1 per day.
- CHD incidence/mortality decreased with up to 2 eggs per week (relative risk 0.96) and plateaued beyond this.
- There was no association with stroke.
- With heart failure, there was an increased relative risk of 1.15, 1.19, and 1.23 for 7, 8, and 9 weekly eggs, respectively, relative to no consumption.
- In people with T2DM the risk of CVD incidence/mortality peaked at 1 egg daily (relative risk 1.22).
- Looking specifically at women there was a decreased risk of CVD, CHD, and heart failure when consuming 4 eggs weekly (relative risks 0.89, 0.89, and 0.84) but when consuming 1 egg daily there was only a decrease in CVD risk (relative risk 0.90).
Eating 1 whole egg daily seems safe. Given the small amount of literature regarding egg intake beyond 1 daily, as well as the frequent association of egg consumption with unhealthy lifestyle habits (at least in US studies), it is hard to derive an evidence-based viewpoint on greater egg intake. Short-term trials such as the DIABEGG study listed above have shown higher egg intake can be included in healthy weight-maintenance or weight-loss efforts. As indicated above, perhaps 25-33% of people are hyper-responders to cholesterol consumption. For individuals who do wish to consume multiple eggs daily, it may be best to obtain a blood lipid profile prior to increasing egg consumption. Repeating the blood lipid profile at least 4 weeks after increasing egg consumption would then show if an individual’s cholesterol numbers worsened significantly; if so, it would be worth considering cutting back on the egg intake.
For an extended period of time, as evidenced in the DGA, the majority of the literature indicated that eating patterns that include seafood are associated with positive health effects, as are eating patterns low in red meat and processed meat. The DGA does make a distinction for lean red meat, however, stating that when consumed in recommended quantities this can also be part of a healthy eating pattern.
Note: “Lean meat” is generally defined as containing <10 grams of fat, ≤4.5 grams of saturated fat, and <95mg of cholesterol per 100g serving. When buying beef at a store this would be at least 90% lean.
The NutriRECS publications
However, at the end of 2019 a new series of SRs culminating in a guideline suggested that current levels of processed meat and unprocessed red meat intake do not need to be decreased. This garnered a lot of press and many responses. I’m going to briefly discuss this here.
Cohort studies and cardiometabolic outcomes
One of these SR/MAs evaluated cohort studies examining the impact of red & processed meat consumption on risk for ACM & cardiometabolic outcomes.(Zeraatkar, 2019a) The authors found studies through July, 2018 from multiple databases and performed a final MEDLINE search for further references in April, 2019. They considered 120 grams to be a serving of unprocessed red meat, 50 grams to be a serving of processed meat, and 100 grams to be a serving of meat from mixed sources. Results showed that decreasing 3 servings per week of red/processed meat yielded relative risk for ACM of 0.93/0.92, CV mortality 0.90/0.90, total stroke 0.94/0.94, fatal stroke 0.94/0.95, total myocardial infarction 0.93/0.94, and T2DM 0.90/0.78, all with evidence rated low or very low due to the observational design of the studies. The GRADE system was used to determine the level of evidence.
Note: GRADE by default will rank SR/MAs of observational evidence as low quality initially. Thus, even if all of the cohort studies were perfectly done the level of evidence would have still initially been considered low. This is in part as GRADE was not developed with long term nutrition studies in mind but rather was developed to help address clinical questions. A separate system, NutriGrade, has been proposed to better evaluate nutrition literature specifically.(Schwingshackl, 2016)
They note in the text that per GRADE the level of evidence can be upgraded if a dose-response relationship is seen, and they do see a dose-response relationship, but they actually choose not to upgrade the level of evidence due to concerns that other dietary confounding is what actually explains the dose-response relationship. They imply that if red & processed meat were the primary drivers of the negative health outcomes then larger associations would be seen.
Note: Keep in mind the results are considering a reduction of 3 servings weekly; reducing servings by 1 serving daily would thus yield stronger associations as they do see evidence of a dose-response relationship.
Additionally, it does not logically make sense to me that they would not upgrade the evidence due to concern for confounding as a primary reason that GRADE downgrades observational research in the first place is due to the concern for unmeasured confounding. Thus, to not upgrade it essentially punishes the literature base twice for unmeasured confounding.
Randomized trials and cardiometabolic outcomes
A second of these SRs evaluated randomized trials examining the impact of red meat intake on cardiometabolic & cancer outcomes.(Zeraatkar, 2019b) The authors searched for trials in a similar manner as the above-mentioned SR/MA and found 12 unique randomized trials. They did not find any trials involving processed meat so this was not included. Of the 12 trials they found, only the Lyon Diet Heart Study & the Women’s Health Initiative (WHI) actually addressed ACM & other major cardiovascular disease outcomes. The former only had 605 people and per the authors was stopped early presumably due to an unrealistic large benefit.
Note: I do not think it is fair to just assume the results of the Lyon Diet Heart Study were unrealistic. There are certainly fair criticisms that can be made, however.(Kris-Etherton, 2001)
Therefore, they only present the results of the WHI for the ACM and CVD outcomes. Thus, this systematic review actually only includes 1 trial for the main outcome components. The WHI enrolled postmenopausal women and was actually designed to study the impact of a low-fat diet. The low-fat group decreased their red meat consumption by ~20% (1.4 servings per week). Unsurprisingly with such a small decrease in red meat consumption, no impact was seen on ACM, CVD, T2DM, cancer, or quality of life.
The NutriRECS guideline
I will not discuss the other analyses in detail but these include a SR/MA regarding the impact of red & processed meat on cancer incidence & mortality in cohort studies, a SR/MA regarding the impact of red & processed meat on cardiometabolic & cancer outcomes in cohort studies, and a SR/MA on health-related values & preferences regarding meat consumption.(Han, 2019; Vernooij, 2019; Valli, 2019) This culminated in a guideline that I will now address.(Johnston, 2019)
In this guideline they acknowledge that they ignore any environmental & animal welfare concerns as they just want to focus on the health aspects. Additionally, their target audience is people who consume red meat or processed meat (thus, they are not trying to convince non-meat eaters to begin eating meat) and they take the perspective of individual decision making rather than a public health perspective.
Note: This last point may seem like semantics but it is actually quite relevant for their conclusions. From a population health perspective even a small decrease in risk can have a significant impact when applied to millions of people. For example, a 1% decrease in risk applied to 1 million people may benefit 10,000. From an individual perspective, however, a 1% decrease in risk may not be a very strong motivational factor to change habits that have already been ingrained.
For both unprocessed red meat & processed meat they suggest continuing current intake rather than decreasing intake by 3 servings weekly, and they consider this a weak recommendation. Of note, only 11 of the 14 panel members agree with this; 3 believe they should advise decreased consumption. They base these recommendations on the low quality of evidence and the small absolute risk reduction seen from decreasing intake by 3 servings weekly, while also considering that their SR on values indicates omnivores will not want to decrease meat consumption.
There has been much commentary on this guideline.(Correspondence, 2020) Dissenters highlight the fact that some studies were excluded, using GRADE was not appropriate, and the portion size they use is smaller than most prior analyses, which decreases the significance found.(Fadnes, 2020; Qian, 2020; Neuhouser, 2020) People in favor of it point to the fact that the analyses were very comprehensive(Dyer, 2019), and that confounding likely explains the small associations that are seen.(Correspondence, 2020) There has also been concern of conflicts of interest on both sides.(Rubin, 2020)
Interpretation of the guideline
My takeaway is that I do not believe their guideline is consistent with the evidence they put forth. They find risk reduction with smaller amounts of red & processed meat intake even when considering a reduction of only 3 servings weekly, and as indicated above they evaluate the evidence in a biased fashion in part due to the inherent flaws of using the GRADE approach for this type of analysis. Additionally, while the risk of confounding is always present in observational studies and it is certainly possible unmeasured confounding may account for the observed decrease in risk, to simply state that is the explanation for their findings without providing a rationale is in my opinion inappropriate.
More recent research
- Generally, with nutrition literature evidence is considered stronger when randomized trials support the large observational findings. When this does not occur the concern for confounding is greater. However, a MA of RCTs showed that replacing both red meat and lean red meat with an isocaloric equivalent of plant-based foods yielded a significant benefit for total and LDL cholesterol levels, which is thought to impart risk reduction of CV events.(Guasch-Ferré, 2019)
- A 2020 SR/MA found processed meat increases the risk of colorectal cancer in a dose-response fashion (given a dose-response relationship it is less likely for confounding to account for the full association).(Händel, 2020)
- A 2020 MA of prospective cohort studies found total meat, red meat, and processed meat intake were associated with T2DM (with a 36% increased risk/100 grams daily intake, 22% increased risk/100 grams daily intake, and 46%increased risk/50 grams daily intake, respectively).(Yang, 2020)
- A 2020 analysis of 6 US cohorts found that when comparing 2 servings per week to 0 servings per week, processed meat yielded a 7%/3% increased risk of CVD/ACM, respectively, unprocessed red meat yielded a 3% increased risk for CVD and ACM, poultry yielded a 4% increased risk of CVD with no increased risk of ACM, and fish yielded no increased risk of CVD or ACM.(Zhong, 2020) The authors note it’s unclear how foods were prepared (ie, fried chicken vs baked chicken) and this may influence the outcomes.
- More recent US cohort analyses show concerns of red meat consumption, at least pertaining to incident T2DM.(Würtz, 2021) In this analysis, replacement of 1 serving of red meat daily by a variety of different food groups was generally associated with a 10-18% decreased risk of T2DM.
- In a 2021 analysis of the PURE study (a prospective cohort study including many countries), while unprocessed red meat consumption was not found to increase the risk of mortality or CVD, processed red meat did have this effect (with hazard ratios ~1.50 and an evident dose-response relationship).(Iqbal, 2021) Of note, the PURE study participants averaged only 37 grams of unprocessed red meat daily, considerably less than in other cohorts where an increased risk was found. As indicated above, differences in portion sizes may underlie the variable results that are seen.
- A 2021 analysis of the UK Biobank data found increased meat consumption associated with a variety of different medical conditions, though there was significant attenuation when adjusting for BMI (implying there may have been further unmeasured confounding).(Papier, 2021a)
- In 2021 a SR/MA of prospective cohort studies (mostly of white European ancestry) demonstrated that each 50 grams per day increased intake of unprocessed red meat and processed meat led to 9% and 18% increased risk of ischemic heart disease, respectively.(Papier, 2021b) There was no increased risk with poultry intake.
- A 2021 narrative review described various aspects of meat processing and the potential impacts this can have on health properties.(Geiker, 2021) The authors also reviewed evidence indicating that eating meat with certain other foods (ie, high fiber foods) may help to decrease some of the negative health outcomes that are seen. While this is largely speculative, it gives reason to think that we may someday be able to recommend specific food processing methods as well as food intake combinations to help optimize overall health.
Tip: For those who do consume red meat, benefits of lean red meat have been seen relative to non-lean red meat.(Guasch-Ferré, 2019) As lean red meat can be relatively expensive, one option is to ground beef and rinse/blot out the fat thoroughly. This can convert beef that is initially 70% lean to >90% lean, yielding a healthier product while saving money.(Love, 1992)
Other types of meat
As indicated above, white meat (poultry) is generally not associated with significant positive or negative health outcomes when consumed in typical ranges. A recent MA of prospective cohort studies evaluated white meat consumption and when comparing high vs. low consumption found a 6% decreased risk of ACM with no change in risk of CV events or CV mortality.(Lupoli, 2021) This may not seem overly significant and it’s possible that the benefit was seen from white meat consumption in lieu of red meat, but regardless it helps indicate white meat intake is not detrimental to health.
Fish intake, on other hand, is associated with several positive health outcomes. A 2020 umbrella review found that this is most prevalent for most assessed conditions at 2-4 servings weekly, with better effects seen with intake of fish high in essential fatty acids.(Li N, 2020) These benefits extended to a range of health conditions including mortality, various types of heart disease, gastrointestinal cancer, Alzheimer’s disease, dementia, and metabolic syndrome, with other benefits found as well. Separately, one 2020 MA(Zhang B, 2020) of prospective cohort studies found a 4% decrease in CHD incidence and mortality for every 20 grams of fish consumption per day (with mortality risk plateauing at 60 grams intake daily) while a 2021 MA(Jiang, 2021) found a 4% decrease of CVD mortality risk with every 20 grams intake daily (plateauing at 40 grams daily intake).
However A 2021 SR of cohort studies did not find consistent benefit of fish intake on CV health and mortality but did find specifically that fried fish was associated with worse health outcomes; perhaps fatty fish intake benefits overall health while fried fish intake harms overall health and combining them in analyses makes it harder to see a meaningful impact.(Krittanawong, 2021)
Note: One must take care not to consume fish with too much mercury. This is especially true for women who are or may become pregnant as well as children. The below card is in accordance with the DGA regarding concerns of mercury. Of the “Best” choices, the following may be higher in the desirable omega-3 fatty acids: salmon, herring, Atlantic and Pacific mackerel (not King mackerel, which is high in mercury), sardines, trout, pacific oysters, shad, and anchovy. Whenever purchasing fish with a goal of obtaining omega-3 fatty acids be careful to avoid purchasing products that have had the fat removed.
Fish intake should be consumed at the levels indicated above, ideally 2-3 servings weekly of fish rich in the omega-3 fatty acids EPA & DHA while also being low in mercury. Meat otherwise is generally not associated with the healthiest eating patterns but there isn’t direct evidence to indicate a negative health impact of unprocessed poultry. Unprocessed red meat seems more problematic in excess, though occasional consumption of lean unprocessed red meat is unlikely to significantly increase one’s risk of harm. When consumed, lean red meat will generally be healthier than non-lean red meat. Processed meat, whether red or poultry, does seem harmful and intake should likely be limited for health purposes.
Tip: As alluded to within the NutriRECS guideline, some people love meat and do not want to give it up. For people who are going to eat a lot of meat (multiple servings daily) no matter what, considerations to make this healthier include:
- preferentially choosing fish (canned, fresh, or frozen is fine) that is low in mercury
- preferentially choosing unprocessed poultry
- if consuming red meat, preferentially choosing unprocessed lean options or removing as much fat as possible (perhaps add extra seasoning to make this more palatable)
- trying to avoid processed meat
- attempting to cook at relatively low temperatures to decrease the production of harmful byproducts, though meat should be cooked thoroughly enough to minimize the risk of food poisoning
There are many considerations when choosing what foods to eat, particularly for the more controversial food groups. To summarize the above:
- Dairy can be a great source of calcium and other nutrients and most research generally shows positive associations with health benefits. With significantly high intake there is a potential risk of harm given elevated saturated fatty acid consumption, increased risk of acne, a possible small increased risk of Parkinson’s disease, and, in men, a potential increased risk of prostate cancer. Thus, limiting dairy to no more than 3 servings daily for men and using reduced-fat versions if going beyond this intake level may be advantageous. The link with prostate cancer, and the small link with Parkinson’s disease, require further study.
- Most research suggests it is safe to consume 1 whole egg daily and potentially more if consuming a heart-healthy diet without significant cardiovascular risk factors. More research is needed to better determine why the controlled trials do not indicate the same level of risk seen in some, but not all, of the prospective cohort studies, as well as the impact of intake of >1 egg daily.
- Fish consumption is generally correlated with positive health outcomes at any typical level of intake; caution to avoid sources with significant mercury should be taken (see chart above). Dietary patterns with elevated meat consumption otherwise are generally associated with worse health outcomes, though there is likely confounding of other lifestyle factors and multiple different types of meat in these diets. Unprocessed poultry seems fairly neutral with respect to health, and lean unprocessed red meat seems relatively safe at low levels of consumption. Non-lean red meat and processed meat are associated with worse health outcomes and their intake should be reserved for occasional use.
We have now gone over the health implications of nutrition considering the nutrients of the foods and drinks we consume as well as the food groups themselves. After reading all of the lessons thus far it should be possible to construct a diet aimed to achieve a healthy body composition and to improve overall health. In the next lesson we will discuss the implications of the timing of when we eat.
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