Lesson 4: Protein

Table of Contents


Introduction

At this point we’ve talked about:

  • metabolism
  • estimating our total daily energy expenditure (TDEE)
  • setting a goal weight
  • determining our caloric intake
  • tracking our progress over time
  • adjusting our calories accordingly

Now we are in a better position to talk about actual nutrition. We will start with the three primary macronutrients: protein, carbohydrate, and fat. In this lesson we will go over protein.

Note: Alcohol is another macronutrient distinct from protein, carbohydrate, and fat. Fiber is sometimes considered a macronutrient but most consider it a subset of carbohydrates. I will discuss both of these in Lessons 8 & 7, respectively.


There are three primary benefits to consuming additional protein when attempting to lose weight:

  • Protein intake stimulates muscle protein synthesis (“MPS”)(Jäger, 2017; Trommelen, 2019), which is crucial for skeletal muscle hypertrophy.
      •  This helps build new muscle or at least maintain current muscle while losing body fat (“BF”).
  • Protein is more filling than carbohydrate and fat.(Dhillon, 2016) This is great for losing weight as most people prefer to minimize hunger during the process. On the other hand, for people who struggle to gain weight I recommend not increasing protein intake too high as this makes it difficult to consume additional calories.
      • There is some evidence that above a certain threshold (around the levels described below) additional protein intake may not have further satiety benefits, at which point higher protein intake may be beneficial only to alleviate cravings.(Roberts, 2018)
  • Protein has an increased thermic effect of feeding relative to carbohydrate and fat. This leads to less net calorie absorption.

Example: The thermic effect of feeding reflects what percentage of calories consumed are required to digest and absorb the food we eat. As indicated in lesson 1 this is estimated to be up to 30% for protein in some situations. Suppose a person increases their protein intake by 100 grams daily. This equals 400 kcal assuming 4 kcal per gram of protein. 30% of this equals 120 kcal. Thus, their net caloric absorption increase may be closer to 280 kcal than 400 kcal.


How much protein should we consume?

This is a somewhat controversial topic as different lines of evidence lead to different recommendations.


Minimum for general health

Standard dietary guidelines suggest:

  • 2-3 years old: 5-20% of one’s calories come from protein
  • 4-18 years old: 10-30% of one’s calories come from protein
  • ≥19 years old: 10-35% of one’s calories come from protein

The recommended dietary allowance (“RDA”) of protein intake in adults is 0.8 grams per kilogram (g/kg) body weight (“BW”) daily (0.36 grams per pound (g/lb)). Please note that this does not match the recommendation from the standard dietary guideline above (see further discussion below).

There are several lines of evidence that this may be too low for optimal health, particularly when exercising regularly. In youth the current recommendations do not account for physical activity, the method used to determine the RDA (which varies by age in youth) likely underestimates actual requirements, and it’s unclear if minimum recommendations should be based on “normal” growth or a potentially different “optimal” growth.(Moore, 2019; Hudson, 2021) Evidence in seniors suggests 1.2 g/kg daily(Phillips, 2016; Putra, 2021) may be a better target, and an argument has even been made for up to 1.6 g/kg daily.(Nunes, 2021) In healthy populations, higher protein intakes have been found to have a small benefit on several cardiometabolic markers without detrimental effects.(Vogtschmidt, 2021)

One caveat In the elderly is that any advice to increase protein intake must be balanced against the increased satiation effect. If aiming for a certain protein intake level does not allow for consumption of sufficient calories to prevent undesired weight loss then it may be necessary to lower the protein target target.

Note: It is technically challenging to determine how much protein is actually needed, as most methods are known to underestimate or overestimate the true amount. If curious, for a good overview of the various technologies consider reading through section 4 of this review. It is written with children in mind but the information applies to adults as well.

Example: If a person weighs 80 kilograms (176 pounds), the RDA is 80 kilograms * 0.8 g/kg = 64 grams of protein daily. For the elderly, 80 * 1.2 = 96 grams of protein daily; this may yield better health outcomes.


Minimum to maximally stimulate muscle protein synthesis (“MPS”)

Recently, a systematic review and meta-analysis (SR/MA) concluded consuming up to 1.62 g/kg of BW (0.73 g/lb) daily when following a resistance training program increases lean body mass (“LBM”) acquisition.(Morton, 2018) There is individual variability; some may benefit from consuming more and many will do well with less (see figure below). An even more recent SR/MA looking at studies with and without resistance training finds significant benefit to increasing protein intake for LBM gain up to 1.3 g/kg of BW (0.59 g/lb) daily, with diminishing results beyond this, while also finding resistance training augments the results.(Tagawa, 2020) Going slightly above this while actively losing weight may additionally benefit some individuals,(Helms, 2017) but practically speaking the studies do not indicate this will be a very significant difference. Therefore, I do not recommend purposely aiming for protein intake much above 1.3-1.6 g/kg (0.59-0.73 g/lb) per day for the hope of additional benefit.

This figure shows how changes in lean body mass change with different levels of protein intake
This figure reproduced from (Morton, 2018) depicts segmental linear regression of the change in fat free mass (y axis) against the average protein g/kg per day ("g/kg/d") intake (x axis) in resistance training study populations (each circle is a separate study). They find a breakpoint at 1.62 g/kg/d, with a wide confidence interval that doesn't quite reach statistical significance (p = 0.079), implying many people can do well with less than 1.6 g/kg/d intake.

There is evidence(Schoenfeld, 2018) that 20g of high quality (quality is discussed below) protein per meal may come close to maximizing MPS, at least in the acute setting (this threshold increases to 40g after whole body resistance exercise). A case can be made for slightly higher protein consumption in the last meal of the day prior to fasting overnight. Thus, eating regular meals with at least 20 grams of high quality protein, with more in the last meal of the day, should come close to maximizing MPS.

Scaled for body weight, 0.25 g/kg of BW can come close to maximizing MPS after a meal in young men, and adding two standard deviations to this (to capture outliers) yields 0.4 g/kg.(Schoenfeld, 2018) Eating 4 meals of 0.4 g/kg body weight yields 1.6 g/kg per day, which as mentioned above is thought to maximize MPS when not in a caloric deficit.

Note: For individuals with significant obesity the recommended values above on a per BW basis will lead to excessive protein consumption. Physiologically, much of the excess weight with obesity is comprised of adipose tissue, with only a small proportion being increased skeletal muscle mass and other organ tissue. As adipose tissue uses less dietary protein relative to skeletal muscle and other organs, using the 1.3-1.6 g/kg BW target is unnecessary. In this instance sticking to the per meal recommendation of 20-40 grams of high quality protein is likely more logical.

Example:

  • If a person weighs 100 kg (220 lbs) then 100 kg * 1.3 g/kg = 130 grams of protein daily. Split up into 4 meals this equals 32 grams of protein per meal. This will likely be sufficient to maximize (or come very close to maximizing) MPS.
  • If a person weighs 180 kg (397 lbs), then aiming closer to the high end of the 20-40 gram per meal recommendation will be more appropriate; thus aiming for 30-40 grams of protein in 4 meals during the day should be sufficient.


Maximum to avoid health risks

Potential health risks of high protein diets are frequently cited. Some of the most common concerns include kidney health, bone health, and development of diabetes.

High protein diets can harm those with severe chronic kidney disease.(Rhee, 2018) If you have chronic kidney disease I suggest discussing appropriate levels of protein intake with your healthcare provider. There is currently no strong evidence that high protein diets will harm healthy kidneys.(Devries, 2018a)

However, we currently lack sufficiently long term studies with high protein intakes to prove conclusively no kidney damage is done over a many year time span. What has been seen is that higher protein intake can lead to glomerular hyperfiltration (a measure of kidney function) in people with healthy kidneys at baseline, and the concern is that over the longer term (ie, several years) this will “wear out” the kidneys and they will ultimately decline faster.(Ko, 2020) It is possible that long term damage may be a risk only in a subset of the population that experiences acute glomerular hyperfiltration.(Jhee, 2020) Much more research is needed to confirm if this is a valid concern; if so then perhaps someday we will be able to test people to see if they are at higher risk of this occurring.

Negative impacts on bone health may be overstated and are likely mitigated with adequate vitamin D & calcium consumption as well as resistance training.(Pedersen, 2013; Antonio, 2018) While there is an association of higher protein intake and risk of developing type 2 diabetes or other cardiometabolic disease(Mittendorfer, 2020), particularly if excess calories are being consumed(Fappi, 2020), it is unclear how meaningful this is when consuming an overall healthy diet, exercising regularly, and maintaining a healthy body fat percentage.

In the elderly, one recent very small study indicated that high protein intake (in the study they reached ~1.76 g/kg daily) may lead to overall net loss of protein compared to ~0.8 g/kg daily.(Højfeldt, 2020) This needs to be replicated and studied further but does give pause to the belief that “more is better”. This does not refute the recommendation of aiming for 1.2 g/kg for the elderly listed above (as this level of intake was not studied here), it just indicates going well above 1.2 g/kg may do more harm than good.

Note: Another consideration is confounding variables; several studies have found higher protein intakes from red & processed meat are associated with unhealthy lifestyle factors. This leads to associations with poor health.(Song, 2016; Shab-Bidar, 2018) Overall, more research is needed on the impact of high protein diets and general health. An effective compromise, and my general recommendation for those with a primary goal of optimizing health, is to consume a higher protein diet (closer to the higher ranges of 1.6 g/kg daily or ≥30 grams per meal as mentioned above) while actively losing weight, and then decrease protein intake once the weight loss journey is complete and weight maintenance (or gain) is the goal (closer to 1.3 g/kg daily or ≥20 grams of high quality protein per meal). Serious athletes & lifters may benefit from staying at the higher ranges.

Having said that, if you go to lower protein levels and find this is making it harder for you to maintain a healthy BW or quality of life (due to increased hunger or for some other reason), then increasing protein intake is reasonable as there is no conclusive evidence that higher levels will cause harm and in this situation the benefits likely outweigh the risks.

Example: In lieu of strong recommendations of a maximum intake, let’s reconsider the guidelines stated above. For adults the RDA of protein is 0.8 g/kg per day. The acceptable macronutrient distribution range (“AMDR”) is 10-35% of daily calories from protein. If an adult weighs 80 kilograms and consumes 2,500 kcals daily, then:

  • 80 * 0.8 = 64 grams of protein daily to meet the RDA
  • 2,500 * [10%-35% range] = 250-875 kcal. Assuming 4 kcal per gram of protein, this comes to 62-219 grams of protein daily
  • If consuming 1.3-1.6 grams per kilogram daily, this would equal (80 * 1.3) to (80 * 1.6) = 104-128 grams of protein daily
  • If simply aiming for 30-40 grams of protein in 3-4 meals, this equals 90-160 grams of protein daily

It should be clear that all of the protein recommendations discussed here are well within what is considered safe from the standard dietary guidelines. It is also interesting to note that there are several considerations regarding why the RDA may be too low and how this relates to the AMDR.(Wolfe, 2017)

What if we have someone who is dieting and only consuming 1,500 kcal/day? Well 35% of this would yield an upper limit of 131 grams of protein daily. This would still meet the 1.6 gram per kilogram goal for an 80 kilogram person, but would not quite meet a 40 gram 4x daily goal for somebody considerably heavier. In this situation it would be ok to go temporarily higher on the protein intake while eating a low calorie diet under medical supervision. If not dieting under medical supervision this points to the fact that people with severe obesity generally do not need to be on very low calorie diets to steadily lose weight; a 2,000 kcal/day diet (which would allow 40 grams protein 4x daily and be within the AMDR) would likely be more appropriate.


Protein quality

There are several considerations when discussing the quality of dietary protein. In general the quality of protein intake becomes more important when engaging in resistance training if the overall quantity of protein intake is on the lower side.(Morgan, 2021)

Note: I am discussing quality conceptually, but there are actual ranking systems of different proteins based upon to what degree they provide various amino acids. These include the PDCAAS (protein digestibility-corrected amino acid score) and the more recent and theoretically improved DIAAS (digestible indispensable amino acid score).

  • The PDCAAS considers the amino acid content of a consumed food item and the amino acid content in fecal material, assuming the remainder is used by the body. However, it’s known this is not accurate:
    • For example, some amino acids will be metabolized by the gut microbiome and thus appear to be digested with the PDCAAS method even though they are not used by the body.
  • The DIAAS compares the most limiting indispensable amino acid in a protein source and the amino acid content in end-ileal material (material at the end of the ileum is unlikely to be further absorbed). This is considered more accurate. However, this is difficult to sample in humans.
    • This is frequently tested in pigs and we are still building up a catalogue of values for different food items.

With the exception of people with very restricted diets I do not believe the DIAAS is worth considering at this point when choosing individual sources of protein beyond the considerations described below. Over time it may become easier to use the DIAAS more practically and if so I will update this. Feel free to read through this link for further material on these topics.


Amino acids

There are 9 essential amino acids (building blocks of protein) that are required in our diet; assuming you eat a variety of foods it is generally not difficult to obtain sufficient quantities. Of these, the amino acid leucine is most strongly associated with MPS. It is unclear how much leucine is required to stimulate MPS fully in the context of consuming sufficient protein in your diet overall, and estimates range anywhere from 1-3 grams per meal (usually closer to 2.5 grams).(Jäger, 2017) There may be merit in specifically choosing sources of protein that are leucine-rich (typically animal-based sources) or supplementing leucine directly if following a lower protein diet for any reason. This has been studied in older adults (who typically consume relatively lower amounts of protein and calories in general) with mixed results.(Devries, 2018b; Volpi, 2018; Putra, 2021)

The 2020-2025 Dietary Guidelines for Americans lists different protein sources in ounce-equivalents; this helps standardize portion sizes between different types. However, this ignores the varying essential amino acid content between different sources and thus the ounce-equivalents do not seem metabolically equivalent.(Park, 2021) This will not be a practically relevant distinction for most but it’s worth keeping in mind for individuals who are on a lower protein diet or with restricted food choices.

Tip: I mentioned cronometer.com in a prior lesson; feel free to plug a typical day of eating into that website to see if you are deficient in any of the amino acids or leucine in particular. Of note, it is actually quite difficult to be deficient in essential amino acids unless following a very restrictive diet.


Speed of digestion

Another consideration is fast vs. slow digesting protein. Whey protein is faster digesting than casein protein, and likely due to that reason absorption of whey protein can increase MPS moreso than casein protein in the acute setting when consumed in isolation, while consuming casein protein prior to bed may be beneficial due to a slower absorption throughout the night.(Jäger, 2017) However, when we eat mixed meals (meaning protein with fat as well as fiber) all digestion slows significantly relative to consuming whey protein in isolation. Thus, this distinction in rates of digestion is unlikely to significantly matter for the general population who is consuming food in meals as opposed to isolated nutrients.

Tip: Many individuals will not need to supplement protein powder (typically purchased as whey or casein) directly as it is generally not too difficult to obtain sufficient quantities of protein through diet alone. However, for those actively losing weight and thus eating relatively few calories, and for those who do not eat meat, supplementation may be helpful. There are many brands of protein out there, and some are lower quality than others (due to addition of unnecessary ingredients or amino acid spiking where cheap amino acids such as glycine are over-represented in the mixture). One brand with very reasonable prices and generally thought to be of high quality is MyProtein, which can be purchased from https://us.myprotein.com/ or https://www.myprotein.com/, depending on your location.

Regarding plant-based protein powders, soy is the most studied although others such as pea protein also show promise if consumed in sufficient quantities; more research on these other options needs to be done.(Putra, 2021)

Note: it is important to recognize much of the literature regarding leucine, fast vs slow digesting protein, and supplements involve nutrients consumed in isolation. There are several studies in the literature indicating whole food sources have different effects than would be expected simply from the individual protein components.(Burd, 2019) While the above can be considered for general guidance, especially if one is consuming protein supplements, simply eating a generally healthy diet of whole food sources with sufficient protein will likely net the vast majority of the benefit that will be obtained from dietary protein; I believe additional benefit of attempting to manipulate leucine and speed of protein digestion will be relatively minor.


Bioavailability

Amino acids from different protein sources have variable bioavailability (meaning what percentage of the consumed nutrients are actually absorbed). However, this is difficult to measure accurately.(Watford, 2018) Practically speaking, if one is consuming a variety of food sources I would not worry about this detail.


Source (plant vs animal)

Lastly, another consideration is plant vs animal sources of protein. In general the literature is favorable towards plant-based sources for overall health, with particularly negative outcomes seen with processed meat and to a lesser degree unprocessed red meat.(Song, 2016; Mariotti, 2019) Part of this is likely residual confounding alluded to above, but at the same time it is likely safest to minimize processed meat consumption and to limit red meat consumption if possible. This is discussed further in Lesson 10.

When exercising regularly and working to build muscle, plant protein sources seem less optimal relative to animal protein sources due to decreased essential amino acid content (specifically leucine).(Kerksick, 2021) Having said that, there is also evidence suggesting when consuming at least 1.0 g/kg of protein daily there may not be that much of a meaning influence long term between predominantly plant and animal sources.(Lim, 2021) Thus, aiming for the higher ranges of protein intake suggested above is likely beneficial if the majority of one’s protein intake will come from plant sources, but for individuals who are not competitive athletes or attempting to maximize their lean body mass gain this is likely not something to be overly concerned about.

                           


Protein distribution throughout the day

Studies generally show splitting protein into multiple meals throughout the day will better continuously stimulate MPS than only consuming 1-2 meals daily.(Jäger, 2017; Schoenfeld, 2018) A recent systematic review of studies that did not include resistance training found a small benefit to a more even distribution of protein intake throughout the day regarding lean body mass.(Jespersen, 2021) The more even distribution allowed more meals to meet the per meal protein recommendations listed above.

If consuming meals with a decent amount of protein (at the levels recommended above) several times throughout the day, the timing of protein intake with respect to the timing of a workout is less relevant, and protein supplementation periworkout has not shown benefit relative to other times of day.(Wirth, 2020) Therefore, if consuming close to 1.3-1.6 grams of protein per kilogram of body weight daily in at least 3 meals, I do not believe there is any additional meaningful benefit to ensuring you consume protein directly before or after a workout. One exception here wold be individuals who perform resistance training late in the evening; consuming protein after the workout in this instance may be helpful.(Antonio, 2020)

For individuals employing a time-restricted feeding or intermittent fasting strategy (discussed in Lesson 11), I recommend consuming a decent amount of protein at the edges of the eating window.



Conclusion

Protein intake is helpful for several reasons and attempting to aim for the minimum amounts recommended above, particularly when attempting to lose weight, can make the dieting process easier. As mentioned in prior lessons, maintaining LBM while losing BF is very helpful to keep the weight off long term, and consuming sufficient protein makes this easier.

The jury is still out regarding very high protein intake over many years, but there are several lines of evidence indicating within the time frame of months to 1-2 years high protein intake is safe. While the RDA is 0.8 g/kg per day, this does not include considerations of resistance training which generate higher requirements, as well as other considerations.(Wolfe, 2017) I thus recommend individuals without health conditions such as chronic kidney disease to aim for at least 1.2 grams per kilogram per day on a regular basis while undergoing resistance training and increasing this further while dieting to lose weight.

Spreading this out throughout the day will likely be additionally helpful. Attempting to make the majority of this come from plant sources, fish, and unprocessed meat will likely aid overall health. It is also ok to eat more than this for satiety-inducing purposes or for personal preference, though it’s unclear if there may be negative consequences to this if done over many years. For individuals who are not tracking calories regularly, you can track grams of protein every now and then to see if you are meeting these thresholds and if not then adjust dietary intake accordingly.

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References

  1. Antonio J, Ellerbroek A, Evans C, Silver T, Peacock CA. High protein consumption in trained women: bad to the bone?. J Int Soc Sports Nutr. 2018;15:6. Published 2018 Jan 31. doi:10.1186/s12970-018-0210-6
  2. Antonio J, Candow DG, Forbes SC, Ormsbee MJ, Saracino PG, Roberts J. Effects of Dietary Protein on Body Composition in Exercising Individuals. Nutrients. 2020 Jun 25;12(6):1890. doi: 10.3390/nu12061890. PMID: 32630466; PMCID: PMC7353221.
  3. Burd NA, Beals JW, Martinez IG, Salvador AF, Skinner SK. Food-First Approach to Enhance the Regulation of Post-exercise Skeletal Muscle Protein Synthesis and Remodeling. Sports Med. 2019;49(Suppl 1):59-68. doi:10.1007/s40279-018-1009-y
  4. Devries MC, Sithamparapillai A, Brimble KS, Banfield L, Morton RW, Phillips SM. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis. J Nutr. 2018a;148(11):1760‐1775. doi:10.1093/jn/nxy197
  5. Devries MC, McGlory C, Bolster DR, et al. Protein leucine content is a determinant of shorter- and longer-term muscle protein synthetic responses at rest and following resistance exercise in healthy older women: a randomized, controlled trial. Am J Clin Nutr. 2018b;107(2):217‐226. doi:10.1093/ajcn/nqx028
  6. Dhillon J, Craig BA, Leidy HJ, et al. The Effects of Increased Protein Intake on Fullness: A Meta-Analysis and Its Limitations. J Acad Nutr Diet. 2016;116(6):968‐983. doi:10.1016/j.jand.2016.01.003
  7. Fappi A, Mittendorfer B. Dietary protein intake and obesity-associated cardiometabolic function. Curr Opin Clin Nutr Metab Care. 2020 Nov;23(6):380-386. doi: 10.1097/MCO.0000000000000689. PMID: 32868684; PMCID: PMC7781151.
  8. Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: a case for higher intakes. Int J Sport Nutr Exerc Metab. 2014;24(2):127‐138. doi:10.1123/ijsnem.2013-0054
  9. Højfeldt G, Bülow J, Agergaard J, Asmar A, Schjerling P, Simonsen L, Bülow J, van Hall G, Holm L. Impact of habituated dietary protein intake
    on fasting and postprandial whole-body protein turnover and splanchnic amino acid metabolism in elderly men: a randomized, controlled,
    crossover trial. Am J Clin Nutr. 2020 Jul 25:nqaa201. doi:10.1093/ajcn/nqaa201. Epub ahead of print. PMID: 32710741.
  10. Hudson JL, Baum JI, Diaz EC, Børsheim E. Dietary Protein Requirements in Children: Methods for Consideration. Nutrients. 2021 May 5;13(5):1554. doi: 10.3390/nu13051554. PMID: 34063030; PMCID: PMC8147948.
  11. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20. Published 2017 Jun 20. doi:10.1186/s12970-017-0177-8
  12. Jespersen SE, Agergaard J. Evenness of dietary protein distribution is associated with higher muscle mass but not muscle strength or protein turnover in healthy adults: a systematic review. Eur J Nutr. 2021 Feb 6. doi: 10.1007/s00394-021-02487-2. Epub ahead of print. PMID: 33550490.
  13. Jhee JH, Kee YK, Park S, Kim H, Park JT, Han SH, Kang SW, Yoo TH. High-protein diet with renal hyperfiltration is associated with rapid decline rate of renal function: a community-based prospective cohort study. Nephrol Dial Transplant. 2020 Jan 1;35(1):98-106. doi: 10.1093/ndt/gfz115. PMID: 31172186.
  14. Kerksick CM, Jagim A, Hagele A, Jäger R. Plant Proteins and Exercise: What Role Can Plant Proteins Have in Promoting Adaptations to Exercise? Nutrients. 2021 Jun 7;13(6):1962. doi: 10.3390/nu13061962. PMID: 34200501; PMCID: PMC8230006.
  15. Ko GJ, Rhee CM, Kalantar-Zadeh K, Joshi S. The Effects of High-Protein Diets on Kidney Health and Longevity. J Am Soc Nephrol. 2020 Aug;31(8):1667-1679. doi: 10.1681/ASN.2020010028. Epub 2020 Jul 15. PMID: 32669325; PMCID: PMC7460905.
  16. Lim MT, Pan BJ, Toh DWK, Sutanto CN, Kim JE. Animal Protein versus Plant Protein in Supporting Lean Mass and Muscle Strength: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2021 Feb 18;13(2):661. doi: 10.3390/nu13020661. PMID: 33670701; PMCID: PMC7926405.
  17. Mariotti F. Animal and Plant Protein Sources and Cardiometabolic Health. Adv Nutr. 2019;10(Suppl_4):S351‐S366. doi:10.1093/advances/nmy110
  18. Mittendorfer B, Klein S, Fontana L. A word of caution against excessive protein intake. Nat Rev Endocrinol. 2020;16(1):59‐66. doi:10.1038/s41574-019-0274-7
  19. Moore DR. Protein Metabolism in Active Youth: Not Just Little Adults. Exerc Sport Sci Rev. 2019 Jan;47(1):29-36. doi: 10.1249/JES.0000000000000170. PMID: 30334848.
  20. Morgan PT, Harris DO, Marshall RN, Quinlan JI, Edwards SJ, Allen SL, Breen L. Protein Source and Quality for Skeletal Muscle Anabolism in Young and Older Adults: A Systematic Review and Meta-Analysis. J Nutr. 2021 Jul 1;151(7):1901-1920. doi: 10.1093/jn/nxab055. PMID: 33851213; PMCID: PMC8245874.
  21. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376‐384. doi:10.1136/bjsports-2017-097608
  22. Nunes EA, Currier BS, Lim C, Phillips SM. Nutrient-dense protein as a primary dietary strategy in healthy ageing: please sir, may we have more? Proc Nutr Soc. 2021 May;80(2):264-277. doi: 10.1017/S0029665120007892. Epub 2020 Oct 14. PMID: 33050965.
  23. Park S, Church DD, Schutzler SE, Azhar G, Kim IY, Ferrando AA, Wolfe RR. Metabolic Evaluation of the Dietary Guidelines’ Ounce Equivalents of Protein Food Sources in Young Adults: A Randomized Controlled Trial. J Nutr. 2021 May 11;151(5):1190-1196. doi: 10.1093/jn/nxaa401. PMID: 33693735; PMCID: PMC8112772.
  24. Pedersen AN, Kondrup J, Børsheim E. Health effects of protein intake in healthy adults: a systematic literature review. Food Nutr Res. 2013;57:10.3402/fnr.v57i0.21245. Published 2013 Jul 30. doi:10.3402/fnr.v57i0.21245
  25. Phillips SM, Chevalier S, Leidy HJ. Protein “requirements” beyond the RDA: implications for optimizing health. Appl Physiol Nutr Metab. 2016;41(5):565‐572. doi:10.1139/apnm-2015-0550
  26. Putra C, Konow N, Gage M, York CG, Mangano KM. Protein Source and Muscle Health in Older Adults: A Literature Review. Nutrients. 2021 Feb 26;13(3):743. doi: 10.3390/nu13030743. PMID: 33652669; PMCID: PMC7996767.
  27. Rhee CM, Ahmadi SF, Kovesdy CP, Kalantar-Zadeh K. Low-protein diet for conservative management of chronic kidney disease: a systematic review and meta-analysis of controlled trials. J Cachexia Sarcopenia Muscle. 2018;9(2):235-245. doi:10.1002/jcsm.12264
  28. Roberts J, Zinchenko A, Mahbubani K, et al. Satiating Effect of High Protein Diets on Resistance-Trained Subjects in Energy Deficit [published correction appears in Nutrients. 2019 Jul 08;11(7):]. Nutrients. 2018;11(1):56. Published 2018 Dec 28. doi:10.3390/nu11010056
  29. Schoenfeld BJ, Aragon AA. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. J Int Soc Sports Nutr. 2018;15:10. Published 2018 Feb 27. doi:10.1186/s12970-018-0215-1
  30. Shab-Bidar S, Golzarand M, Hajimohammadi M, Mansouri S. A posteriori dietary patterns and metabolic syndrome in adults: a systematic review and meta-analysis of observational studies. Public Health Nutr. 2018;21(9):1681-1692. doi:10.1017/S1368980018000216
  31. Song M, Fung TT, Hu FB, et al. Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality. JAMA Intern Med. 2016;176(10):1453-1463. doi:10.1001/jamainternmed.2016.4182
  32. Tagawa R, Watanabe D, Ito K, et al. Dose-response relationship between protein intake and muscle mass increase: a systematic review and meta-analysis of randomized controlled trials. Nutrition Reviews. 2020 Nov 3:nuaa104. doi.org/10.1093/nutrit/nuaa104
  33. Trommelen J, Betz MW, van Loon LJC. The Muscle Protein Synthetic Response to Meal Ingestion Following Resistance-Type Exercise. Sports
    Med. 2019 Feb;49(2):185-197. doi: 10.1007/s40279-019-01053-5. PMID: 30659499.
  34. Vogtschmidt YD, Raben A, Faber I, de Wilde C, Lovegrove JA, Givens DI, Pfeiffer AFH, Soedamah-Muthu SS. Is protein the forgotten ingredient: Effects of higher compared to lower protein diets on cardiometabolic risk factors. A systematic review and meta-analysis of randomised controlled trials. Atherosclerosis. 2021 Jul;328:124-135. doi: 10.1016/j.atherosclerosis.2021.05.011. Epub 2021 May 26. PMID: 34120735.
  35. Volpi E. Is leucine content in dietary protein the key to muscle preservation in older women?. Am J Clin Nutr. 2018;107(2):143‐144. doi:10.1093/ajcn/nqy009
  36. Watford M, Wu G. Protein. Adv Nutr. 2018;9(5):651‐653. doi:10.1093/advances/nmy027
  37. Wirth J, Hillesheim E, Brennan L. The Role of Protein Intake and its Timing on Body Composition and Muscle Function in Healthy Adults: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. J Nutr. 2020;150(6):1443-1460. doi:10.1093/jn/nxaa049
  38. Wolfe RR, Cifelli AM, Kostas G, Kim IY. Optimizing Protein Intake in Adults: Interpretation and Application of the Recommended Dietary Allowance Compared with the Acceptable Macronutrient Distribution Range. Adv Nutr. 2017;8(2):266-275. Published 2017 Mar 15. doi:10.3945/an.116.013821
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