Lesson 6: Carbohydrates - Healthier with Science

Introduction

In the last two lessons I discussed protein and fat, the benefits they may or may not confer for dieting and weight loss, the recommended amounts and types for health purposes, and other relevant considerations. In this lesson I will do the same for carbohydrates while also considering the topics of added sugar, artificial sweeteners, and the glycemic index and load. I will discuss sources of carbohydrates as well as gluten in Lesson 10 when I talk about grains as a food group.

Specific considerations for exercise and weight management

In general no meaningful differences are seen in weight management outcomes when comparing low carbohydrate vs. low fat diets if calories are matched.^{(Blaak, 2021; Yang, 2021)}This seems to also extend to weight maintenance after weight loss.^{(van Baak, 2021)}

Carbohydrate intake can aid glycogen replenishment to provide energy for workouts.^{(Murray, 2018)} For people eating at maintenance calories who are not doing extended aerobic training sessions this is generally not a significant concern unless you train twice within an 8-hour period. However, when eating at a caloric deficit it can be harder to replenish glycogen levels effectively to fuel a workout without sufficient carbohydrate intake.

Some people subjectively find consuming carbohydrates prior to a workout allows them to perform their workout with higher intensity. In the protein lesson I discussed that the timing of protein intake in relation to a workout is not very significant. For carbohydrate intake, though, it can be significant. As different people respond differently, it can take trial and error to determine the best source, quantity, and timing of pre-workout carbohydrates to best fuel your workout sessions.

Tip: A “fundamental principal” of a healthy lifestyle including exercise sessions is to make the exercise sessions the best they can be. Being able to train with more focus and intensity will yield better results. For this reason, I recommend trialing a variety of different food/drink items prior to a workout (and potentially during a workout) to determine what allows you to train to the best of your ability. I discuss eating in relation to exercise in more detail in Lesson 15 of the general exercise course. Some considerations:

Consuming too much food/drink too close to the workout can lead to gastrointestinal discomfort
Eating carbohydrates alone will lead to faster digestion than adding a source of protein or fat, which may or may not be helpful
Some people do best eating a meal several hours prior to a workout and a small snack shortly before a workout
Some people find they train best when they are fasting first thing in the morning
If training for >60 minutes it can be helpful to consume a source of carbohydrates during the workout (ie, a sports drink)

This largely comes down to personal preference and experimentation. Try different things and determine what works best for you.

Note: There are people who train while on a ketogenic diet, meaning very little carbohydrate intake, and this has been studied heavily. It is generally shown this can lead to effective cardiovascular adaptations without a negative impact on performance.^{(McSwiney, 2019)} A caveat though is that this is only seen after being “keto-adapted”, meaning one must be on a ketogenic diet for at least several weeks prior to it no longer being detrimental.^{(Sherrier, 2019)} For unclear reasons it seems more difficult to build new lean body mass in response to resistance training while on a ketogenic diet^{(Vargas, 2018)} and it seems difficult to maintain lean body mass while losing weight.^{(Ashtary-Larky, 2021)} I will discuss ketogenic diets further in Lesson 13.

Minimum carbohydrate intake for health purposes

The recommended dietary allowance (“RDA”) for carbohydrate intake at all ages >1 year old is 130 grams of carbohydrate daily. The acceptable macronutrient distribution range (“AMDR”) is 45-65% of your total daily calories.

Note: The RDA was set with the thought that 100 grams of carbohydrate intake daily will provide enough glucose for brain function for ~50% of people; adding two standard deviations to this yields 130 grams. However, we know that many people function well on fewer carbohydrates than this. Those on a ketogenic diet (at times prescribed for medical purposes) generally eat considerably fewer than 50 grams of carbohydrates daily.

The AMDR was set with an upper limit based on data that low fat, high carbohydrates worsen blood cholesterol levels and thus increase the risk of coronary heart disease. The lower limit comes from the thought that low carb, high fat diets will lead to people overeating and increase their risk of developing obesity.

However, the data used to generate these recommendations are several decades old, and it is now more clear that the source of nutrients rather than just the quantities play a large role in dictating health outcomes. Therefore, while these recommendations exist and are frequently cited, in my opinion they deserve to be updated.

In light of the above, I do not believe there is a true minimum amount of carbohydrates needed daily; there are technically no essential carbohydrates that the body requires and energy can be generated from fat and protein. However, for health purpose, there are several sources of carbohydrates with health benefits, such as fruits^{(Fardet, 2019)}, vegetables^{(Wallace, 2019)}, and whole grains^{(Jones, 2020)}. A healthy diet including these food sources will require some level of carbohydrate intake. These food groups, and thus sources of carbohydrates, are discussed more fully in Lesson 10.

Maximum carbohydrate intake for health purposes

There are some common concerns of high-carbohydrate diets, including that they lead to weight gain, diabetes, or high cholesterol. However, there are several studies indicating these concerns are not valid, particularly when the source of carbohydrates are fruits, vegetables, and fiber-rich grains.^{(Sartorius, 2018; Sylvetsky, 2017)} In fact, there is a specific diet, referred to as the Ma-Pi 2 diet, that has shown good outcomes in short term trials on cardiovascular risk factors and glycemic control in adults with type 2 diabetes; in these trials on average 70% of calories came from carbohydrates.^{(Porrata-Maury, 2014)}

Note: I do not actually recommend the Ma-Pi 2 diet as it is difficult to follow, restrictive, and expensive, but I wanted to include it as an example of a high-carbohydrate diet that can yield good outcomes, at least in the short term in adults with type 2 diabetes. In general, low-carbohydrate diets have been found to be helpful in individuals with type 2 diabetes, at least for up to 6 months duration, but it’s unclear if this is due to the lower carbohydrate intake or due to the greater calorie restriction that this type of diet typically induces.^{(Jayedi, 2022)}

Therefore, the total quantity of carbohydrates one consumes seems less relevant, at least when the food sources are fruits, vegetables, and fiber-rich grains, though there may be some concern of cardiovascular disease with >60% of energy intake coming from carbohydrates in Asian populations.^{(Jo, 2023)} Overall, I do not believe there is a true maximum amount of carbohydrates above which one will be at risk of harm, assuming this does not lead to excessive caloric consumption and the carbohydrate intake is coming from more nutritious sources.

Consideration of added sugar

The general recommendation for added sugar intake is to limit this to <10% of your daily calories. However, this is largely to help ensure there is room for other healthy nutrients while still keeping total calories at a reasonable level as explained in the United States Department of Agriculture and Health and Human Services (USDA-HHS) response to the Dietary Guidelines Advisory Committee’s Report that was used to develop the 2020-2025 Dietary Guidelines for Americans:

"However, after careful consideration of the totality of evidence presented by the Committee, the Dietary Guidelines for Americans, 2020-2025 retains the recommendation to limit intakes of added sugars to less than 10 percent of calories per day in the 2015-2020 Dietary Guidelines. The introduction of this quantitative recommendation was based on significant scientific agreement from data analysis, systematic reviews, and food pattern modeling, and largely, the science has not changed. According to food pattern modeling, the amount of calories available from added sugars varies depending on caloric needs...For those people who need higher calorie intakes per day, an upper limit of 10 percent of calories from added sugars may be consumed while still meeting food group recommendations in nutrient-dense forms.
See full discussion here.

There are several lines of evidence that added sugar intake >10% of your total daily calories can be safe in individuals who are keeping total caloric intake at a level to obtain/maintain a healthy body weight and composition while exercising regularly.^{(Rippe, 2015)} This extends to considerations of blood pressure, lipids, and glycemic control in some scenarios, though less likely with sugar-sweetened beverages.^{(Prinz, 2019a; Perrar, 2019; Prinz 2019b)} A 2022 review highlights that added sugar in sugar-sweetened beverages likely does contribute to health risks, but this may not extend to added sugar in solid foods.^{(Yan, 2022)} On the other hand, there is a possible increased risk of some cancer types with added sugar intake, although this evidence is epidemiological in nature.^{(Makarem, 2018)} Fructose in particular may be the most harmful when consumed in excess, and as it is frequently found in sugar-sweetened beverages (which are associated with worse health outcomes) it makes sense to minimize sugar-sweetened beverage consumption to keep added sugar at <10% of your daily calories.^{(Blaak, 2021; Wang, 2022; Chiavaroli, 2023)}

Overall, keeping added sugar at <10% of your daily calories is a reasonable goal, though occasionally going over this is unlikely to pose significant health risks.

Note: To be clear, as recently reviewed, the potential safety of going above 10% daily calories with added sugar depends on this not leading to elevated caloric intake and undesired weight gain.^{(Veit, 2022)} If tracking calories this can be well-controlled but if not tracking calories it can be rather difficult to eat substantial amounts of food with added sugar and not start to gain weight. This is in part due to processed foods with added sugar being less satiating than unprocessed whole foods.^{(Hall, 2019)}

Artificial sweeteners as a substitute for added sugar?

While not technically carbohydrates, artificial sweeteners are frequently used as a substitute for added sugar and thus they are worth discussing here. This is a controversial topic; at this point the majority of the literature suggests that artificial sweeteners can be used safely in moderation. For an overview of the research on this topic, click below.

Click here for an overview of some of the literature regarding the health implications of artificial sweeteners

Artificial sweeteners, also referred to as low-calorie sweeteners (“LCS”) and non-nutritive sweeteners (“NNS”), pose yet another controversial topic in the literature, primarily regarding their impact on body weight as well as glucose and insulin control. I will discuss some of the more recent literature here.

Some select 2018 references:

In a 12-week trial, 100 healthy adults with body mass index (“BMI”) 18-25 and low habitual consumption of LCS were split into 3 groups (control, 350 mg of aspartame daily (equivalent to ~1 can of low calorie soda), and 1,050 mg of aspartame daily).^{(Higgins, 2018)} An oral glucose tolerance test (OGTT) conducted before and after the 12-week period showed no differences in glucose or insulin response, and there were additionally no differences in weight or appetite.
In a 14-day trial, 30 healthy adults with normal BMI and low habitual consumption of LCS were given ~45% of the acceptable daily intake for sucralose divided into 3 doses daily.^{(Romo-Romo, 2018)} With an OGTT blood glucose response did not change but insulin sensitivity decreased 17.7% after the trial (a control group had a decrease of 2.8%).
A review highlighted some of the harmful effects of LCS consumption on weight control and metabolic diseases seen in epidemiological studies and contrasted this with the neutral or even beneficial effects seen in controlled trials, indicating longer, well-controlled intervention studies are needed.^{(Sylvetsky, 2018)}

Some select 2019 references:

A policy statement by the American Academy of Pediatrics regarding LCS consumption in children and adolescents also acknowledged the literature is conflicting, there may be reverse causality explaining some of the negative associations seen in the observational literature, and trials indicate artificial sweeteners can have positive benefits for weight management.^{(Baker-Smith, 2019)}
In a 12-week trial, 123 healthy adults with BMI 25-40 who were not habitual consumers of LCS at baseline were randomly assigned into 5 groups.^{(Higgins, 2019)} One group was given a beverage to drink daily consisting of sucrose (sugar); the volume was based on the individual’s body weight and the beverages had either 400, 480, or 560 kcals. The other four groups were given beverages without sucrose that had sweetness matched with one of four tested LCS (saccharin, aspartame, rebA, sucralose). There was more body weight gain with sucrose (1.85 kg) than with any of the artificial sweeteners (1.18 kg for saccharin, no significant gain in the other groups). There were no differences between fasting glucose, fasting insulin, or glucose & insulin responses to an OGTT before and after the intervention.

Some select 2020 references:

Some select 2021 references:

A SR/MA of intervention studies of at least 1-week duration examined LCS vs sugar, LCS vs water or nothing, and LCS vs placebo.^{(Rogers, 2021)} Overall there was a benefit for body weight control and energy intake in the LCS vs sugar comparison group with no difference in adverse events. No consistent effects were seen in the other two comparison groups.
A SR/MA evaluated 34 articles of prospective cohorts regarding the associations between both sugar-sweetened beverages (SSBs) and artificially-sweetened beverages (ASBs) with type 2 diabetes, cardiovascular disease, and all-cause mortality.^{(Meng, 2021)} There was more consistent evidence for an increased risk with the SSBs (risk of each outcome increased 27%, 9%, and 10%, respectively, with 1 additional drink daily, though the risk of all-cause mortality was not statistically significant). With 1 additional drink of ASBs daily the risk increased 13%, 8%, and 7%, respectively, though the association with type 2 diabetes and cardiovascular disease was less evident after adjusting for confounding variables and the risk with all-cause mortality was not statistically significant. Of note, there was a statistically significant increased risk of all-cause mortality when comparing high vs low intakes for both SSBs and ASBs intake (14% and 15% increased risk, respectively).
- Thus, from the observational literature it appears that ASBs are less harmful than SSBs, and while they may impose some level of risk this is minimized when adjusting for confounding variables. It’s possible there is unmeasured confounding that explains the remainder of the risk.

Some select 2022 references:

Some select 2023 references:

A 2023 SR/MA included 25 reports of 36 acute feeding trials examining the impact of a single exposure to artificial sweeteners on various measures of glucose control and hunger/satiety hormones when consumed with or without calories, finding in general no impact of the artificial sweeteners in people regardless if they were generally healthy, had obesity, or had type 2 diabetes.^{(Zhang, 2023)}

Recommendations:

Most of the better-done research (from a methodology perspective) thus far does not observe a negative impact of LCS on body weight control or glucose and insulin levels in the body, though the most recent trial listed above did find an impact on glycemic response. However, it is difficult to draw concrete solutions due to the lack of long-term studies, potential differences between the different LCS types, and individual variability that may be at play (ie, people with different gut microbiomes may metabolize the LCS differently).

Overall, at this point in time the majority of the evidence indicates LCS consumption in moderation is safe. If using products with LCS in moderation in lieu of actual sugar aids you in sticking to an overall healthy eating plan and caloric goal then I believe this is a reasonable strategy.

Consideration of the glycemic index and glycemic load

While the body can perhaps handle increased amounts of added sugar when keeping total calories in a healthy range, another consideration is if the added sugar is consumed all at once in a rapidly digesting form (ie, a sugar-sweetened beverage) or spread out in meals that slow down its digestion. This gets into the notion of glycemic index and glycemic load:

Glycemic index: this is defined as the area under the curve for blood glucose response over a 2-hour period after consuming a 50 gram carbohydrate portion of both a test food and a standard food (50 grams of glucose or a 50 gram carbohydrate portion of white bread) and expressing the data as a percentage of the test food relative to the standard food.
Glycemic load: this takes portion size into account; the glycemic index for a food is multiplied by the quantity of carbohydrates and divided by 100.

There are several considerations here:

First, when calculating glycemic index values there is both interindividual variability (testing the same food item on different people) and intraindividual variability (replicating the test in the same individual), with one study showing coefficients of variation of 17.8% (interindividual) and 42.8% (intraindividual) when testing white bread in triplicate relative to glucose.^{(Vega-López, 2007)}
Second, while the glycemic index technically should be tested in the morning after an overnight fast, when eating throughout the day prior meals can impact and hence alter glycemic responses of the current meal being consumed.^{(Ando, 2018)}
Third, based on other real world conditions (as opposed to lab-testing conditions) and interindividual variability, specific meals may generate improved glycemic responses in some individuals while the same meals may generate worsened glycemic responses in others.^{(Zeevi, 2015)}

For these reasons, it is not very surprising that the totality of evidence does not indicate significant health implications of the glycemic index or glycemic load, with large systematic reviews and meta-analyses indicating at most a mild impact on body weight, diabetes, cardiovascular disease, cancer, or all-cause mortality.^{(Vega-López, 2018; Turati, 2019; Reynolds, 2019; Gaesser, 2021)}

A 2020 umbrella review of meta-analyses of prospective cohort studies found moderate quality evidence of an association between glycemic index and^{(Jayedi, 2020)}:

gallbladder disease (relative risk 1.26, 2 studies)
bladder cancer (relative risk 1.26, 2 studies)
type 2 diabetes (relative risk 1.18, 15 studies)
coronary heart disease (relative risk 1.14, 10 studies)
colorectal cancer (relative risk 1.12, 12 studies)
breast cancer (relative risk 1.06, 11 studies)

There was no increased risk of cardiovascular mortality or all-cause mortality. For glycemic load, there was moderate quality evidence of an association with:

gallbladder disease (relative risk 1.41, 2 studies)
stroke (relative risk 1.21, 7 studies)
type 2 diabetes (relative risk 1.11, 14 studies)

Again, there was no increased risk of cardiovascular mortality or all-cause mortality. Thus, a diet with a higher glycemic index is associated with a small risk of increased health consequences; mechanistically the biggest concern would be type 2 diabetes and it seems the risk of this when comparing high vs low glycemic index and load is only increased by <20%.

As glycemic index can reflect many other aspects of the diet (ie, fruits/vegetables/whole grains will generally have a lower glycemic index than sources of added sugar and refined grains), it is more logical to me that it is the underlying food groups (discussed in Lessons 10 and 11) and not the glycemic index per se that would have a significant health impact. I expect the glycemic index and load to be even less relevant for individuals consuming an appropriate amount of calories to attain or maintain a healthy body composition while exercising regularly.

Having said all of that, for people with significant insulin resistance or diabetes mellitus it will make more sense to consume less of the high glycemic index/load foods, particularly in isolation, as these individuals are more prone to developing much more significant elevations in their blood glucose levels. Additionally, a recent trial found that individuals who experience a larger decrease in their blood glucose level 2-3 hours after consuming a meal experience greater levels of hunger and subsequently consume more calories, and there was some evidence that meals that increase blood glucose to the highest degree subsequently led to a greater decrease in glucose.^{(Wyatt, 2021)} While this will likely not affect everyone to the same degree, certain individuals may find that consuming high glycemic index/load foods in isolation will lead to a relative increase in their hunger levels.

Note: For individuals who are interested in looking up the glycemic index of specific foods, an updated table of values was published in 2021 and can be accessed by clicking here.^{(Atkinson, 2021)}

Conclusion

Overall, much of dietary carbohydrate intake comes down to personal preference. Some people feel better and perform better with higher carbohydrate diets while some people prefer lower carbohydrate diets. Indeed, the literature as a whole does not generally suggest a preference for low-carb/high-fat vs high-carb/low-fat with regards to weight management.^{(Seid, 2019)} Trial and error helps to determine what works best for any given individual, particularly with regards to pre-workout nutrition.

Fruits, vegetables, and whole grains with fiber all have several beneficial health properties and these sources of carbohydrates should be prioritized if sticking to lower carbohydrate intakes. If consuming higher intakes, limiting added sugar to <10% of total daily calories is the safest option, though less worrisome if one maintains a healthy body weight and composition and exercises regularly. Sugar-sweetened beverages should in particular be avoided, especially when consumed in isolation.

As I have now discussed the three primary macronutrients, in the next two lessons I will discuss fiber and alcohol.

Click here to proceed to Lesson 7

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