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Is sugar bad for athletes?

Sugar is often labelled as being “bad” for health. Some headlines claim sugar causes obesity, as well as cancer, cardiovascular disease or premature death. In contrast, sugars are labelled as “good” for athletes during exercise. Various sports nutrition products are marketed based on their high sugar content to fuel performance. In the first blog in this series, we discussed what sugar is. In the second blog we discussed the importance of sugar in metabolism. Now we will turn to the main question: “Is sugar bad for athletes?”

Infographic on claims around sugar intake

The recommendations for sugar

The World Health Organisation (WHO) have advised that a diet high in free sugars can be harmful to health as it is associated with dental decay and may lead to excess consumption of energy (calories), which over time can cause overweight and obesity.

  • Adults and children should reduce their intake of sugar to less than 10% of their total daily energy intake. On average, this equals about 12 teaspoons (50 grams) of sugar per day for an adult. This refers to all added sugars, including the naturally-occurring sugars in honey, fruit juices, syrups and fruit-juice concentrates.

  • Reducing your intake to less than 5% of total energy intake (6 teaspoons or 25 grams) would provide even more health benefits. Read the nutrition panel on the food label. If the total sugar exceeds 15g of sugar per 100g of the food, check the list of ingredients to see if any added sugars are high on the list.

The Dietary Guidelines for Americans 2020-2025 also advise that all Americans 2 years and older limit added sugars in the diet to less than 10% of total calories. For a 2,000 calorie/day diet, that translates into 200 calories from of sugar daily (again, about 50 grams or 12 teaspoons of sugar). Toddlers and infants younger than 2 years should not be given solids or beverages with any added sugars. They will still get sugar from milk for example.

In the UK, current recommendations are that free sugar intake should not exceed 5% of daily intake. Average intakes of free sugars, however, is at least twice this dietary recommendation. Most of that comes from soft drinks and fruit juices, sugars that are added to food and drink, including jams and chocolate spread, biscuits, pastries, and cakes.

The exact recommendations vary from country to country, but the general consensus is that excessive sugar is unhealthy

What is the difference between added sugar and natural sugar?

Sugar in our diet is usually referred to as added sugar or natural sugar. Natural sugar refers to that which is naturally present in our diet, such as sugar in milk and fruit. In a separate blog we will explain the differences between different types of sugar in the diet but the main conclusion relevant for this discussion is that chemically there are no differences and to our bodies the sugar (for example glucose) from one source is handled in the same way as sugar from another source. However, the food in which it is packed and the other nutrients that food contains may be different.

Honey and sugar

The negative health effects of sugar

There is overwhelming evidence that excessive sugar intake is bad for your health. For example, the National Health and Nutrition Examination Survey (NHANES) provided evidence from a prospective cohort study (a group of people who were followed over time) in the US regarding the effect of added sugar on cardiovascular disease risk (1). In this study the sugar intake of over 11,000 adults was estimated and CVD mortality was determined. Compared with individuals who consumed 8% of energy intake from sugars, those that consumed around 20%, had a 38% higher risk of CVD mortality.

There are also epidemiological studies that shown relationships between sugar intake and all-cause mortality. An example is a recent prospective cohort study by Andersen and colleagues (2) who studied a group of 198,285 men and women aged 40–69 years of whom 3,166 (1.6%) died over a 7 year follow-up. All-cause mortality was associated with total sugar intake and intake of sugar-sweetened beverages, but not artificially sweetened beverage intake. In addition to all-cause mortality, studies have shown links between sugar intake and non-alcoholic fatty liver disease as well as high blood pressure. Claims that it causes cancer, have not been substantiated.

It is also important to point out that sugar intake is not directly related to diabetes. However, diabetes can develop as a result of overweight and obesity which can be caused by excessive sugar intake.

It is beyond the scope of this blog to discuss the negative health consequences in detail. The point is that there is overwhelming evidence that excessive sugar intake has a wide range of health consequences. The next big question is: What exactly does “excess” mean? For example: does excessive mean the same for the average population, as it does for a Tour de France cyclist who rides his bike for 5 hours a day at high intensities.

What does "excessive" sugar intake mean?

This question is more complex than it seems at first sight, and it may depend on the context. As an example: 100 grams of sugar may be a very large amount for someone who is inactive and has a daily energy expenditure of 2,000 kcal per day. But for someone who has a daily energy expenditure of 5,000 kcal and exercises at high intensity for 3 hours and uses 500 grams of carbohydrate as a fuel for that exercise session, 100 grams of sugar may not be as much. Even 100 grams per hour of exercise (300 grams in total) may not be that much, even though this could be 30% of the daily energy intake.

Another consideration is that “excessive” may not just refer to an amount of carbohydrate ingested, it may also refer to “positive carbohydrate balance”. Carbohydrate balance means the calories from carbohydrate consumed sugar (and other carbohydrate sources) minus the calories from carbohydrates utilised on a 24 h basis. In positive carbohydrate balance, excess carbohydrate is converted to fat and this will also result in an elevation of blood lipids (triglycerides), a risk factor for cardiovascular disease.

Infographic of excess sugar intake

The timing of intake may also be something to consider. Carbohydrate intake during prolonged exercise is likely going to be used as a fuel during that activity. Carbohydrate immediately after exercise will be used to replenish glycogen stores in liver and muscle.

We will discuss these points in more detail later. For now, just hold on to the idea that what “excessive” means depends on the context... and it may not be as simple as a certain number of teaspoons of sugar that is going to bad for everyone in all situations.

Does sugar cause glucose spikes?

One argument that is often used is that sugar causes glucose spikes in the blood and high blood glucose is linked to various negative health effects long term. First of all, not all sugars cause large elevations in blood glucose. Fructose for example does not, whereas glucose does. Another factor to consider is that there are many modifiers of the blood glucose response and exercise is a very strong one. For example, when carbohydrate is ingested during exercise there is little or no insulin or glucose spike. During intense exercise, insulin may even decrease despite glucose ingestion.

Thirdly, and importantly, these changes in insulin and glucose are normal physiology: transient changes that reflect the balance between uptake and disposal of glucose. These transient changes are distinctly different from chronically high glucose concentrations. Chronically elevated glucose concentrations usually happen as a result of long term excess intake of food with weight gain. Excess intake of food usually means that we have elevated lipids in addition to elevated blood glucose, which over time may reduce insulin sensitivity. Following carbohydrate or sugar ingestion, insulin is released to help the body remove glucose from the blood. The insulin “spikes” that we see in healthy individuals are there to store glucose in the appropriate tissues. Chronically elevated blood glucose develops because insulin becomes less effective (poor insulin sensitivity). With poor insulin sensitivity, it becomes more and more difficult to maintain normal glucose concentrations. In other words, insulin still increases in response to sugar (carbohydrate) ingestion, but the effect of insulin is reduced; it becomes more challenging to take up glucose into our cells after a meal, including the muscle cell. Glucose concentrations stay higher for longer, especially when you are not active.

Transient changes in blood glucose concentration in response to feeding should not be confused with chronically elevated blood glucose. Spikes in blood glucose is normal, chronically elevated blood glucose is not.

People with non-insulin dependent diabetes (Type II) have a problem with the effectiveness of insulin, we call that insulin resistance. You then need more insulin for the same effect. However, athletes, also those that take sugar regularly and in larger amounts, are more insulin sensitive not less, despite their regular spikes in glucose and insulin. In conclusion, short term changes in blood glucose are often confused with chronically elevated glucose concentrations but there is no evidence that sugar per se causes insulin resistance. One good review concluded “Excess sugar can promote weight gain, thus type II diabetes mellitus through extra calories, but has no unique diabetogenic effect at physiological levels” (3).

Infographic of factors affecting sugar for health

Sugar intake and fatty liver

Excess sugar intake can also result in the production of triglycerides, and chronically elevated triglycerides are a risk factor for cardiovascular disease. Excess sugar can also be converted into fat in the liver and can be stored in the liver. In fact, this storage of fat in the liver seems to be caused by excess calories in general, not just sugar. Fatty liver disease is thus caused primarily by the intake of excess calories which causes fat to be stored in the liver, an organ that is not designed to store fat.

Sugar intake when in energy balance

Now we get to the bigger question: Does sugar intake cause negative health effects when in energy balance? If energy intake is not in excess of energy expenditure, so people are either in energy balance or they are losing weight, there is no convincing evidence that sugar intake has negative health consequences. A thorough review by Moore and Fielding (4) concluded that the link between sugar intake and various negative health outcomes are clear from overfeeding studies and in free living conditions where unfortunately we deal with overconsumption. In energy balance, however, there is far less information and no convincing evidence that there are any negative health effects of sugar. The authors also note (as we discussed above) that glucose responses are complex and dependent on many factors. Sugar intake is just one factor and not necessarily the major component that determines glucose responses, especially long term. If there was such a strong correlation between sugar intake and chronically high blood glucose concentration, we would expect to see that endurance cyclists and triathletes, who have the highest sugar intakes, would also suffer the most from insulin resistance. The opposite is the case; endurance athletes are far more insulin sensitive than the general population, despite their higher intakes of sugar-containing sports drinks, gels, and energy bars.

It is also interesting to note that markers of health seem to improve slightly in weight loss studies despite a relatively high sugar intake. This at least suggests that sugar is not the primary cause, but excess calories are likely a much more important factor. This doesn’t mean that we should now recommend unlimited sugar intake for individuals in energy balance or in negative energy balance. For instance, health markers seem to improve more when whole grains are ingested instead of refined grains (5). So, although there may not be negative effects of sugar, it is still better to eat whole grains for markers of health.

Types of sugar in bowls

In summary, there is no convincing evidence that sugar intake has negative effects when in energy balance. This does not mean that in energy balance that you can eat as much sugar as you want. If you are consuming high sugar products it is likely that these are not very nutritious foods. These foods will contain calories but not much else and thus you will be missing out on health benefits from foods that would have more nutrients. But, again, the problem is not sugar per se. The problem is missing out on health promoting nutrients.

In summary, there is not convincing evidence that sugar intake has negative effects when in energy balance

Sugar intake and physical activity

I think one really important point is that in almost every single study, the effects of exercise are often ignored. This is an issue as we know that exercise is an important modifier of glucose responses. For example, exercise will improve insulin sensitivity. These effects are both acute (the effects of a single exercise bout may last 48h), and to some degree they are also chronic (a training effect). Many studies have not controlled or even measured physical activity in any detail when assessing the impact of sugar on health outcomes. This underappreciation of the broad effects of physical exercise are a general problem in the nutrition literature.

What about very high intakes of sugar in athletes?

In previous blogs we discussed the recommendations for carbohydrate intake. Especially during longer exercise activities intakes of 90 g/h are recommended. In some studies, and by some athletes, intakes of 120 g/h are used. A large amount of these carbohydrates would be consumed in the form of sugars.

Let’s take the example of an athlete who has a very high carbohydrate intake of 90 g/h for 4 hours and all of these carbohydrates are sugars. This would mean a total intake of 360 grams. Compared to the general population recommendations where 50 grams would mark the upper limit, this would be an extraordinary amount of sugars. However, during exercise, metabolism is increased 10-fold or more, and we increase the use of sugars. The sugars ingested during exercise would be used for fuel and the majority of studies show that oxidation of ingested carbohydrate is somewhere between 70-80% (6) (we refer to this as the oxidation efficiency). This means that of the 90 grams of carbohydrate ingested in an hour, 70-80% (or 63-72 grams) would immediately be used to fuel the exercise bout. Carbohydrate use during exercise in a moderately trained athlete during low to moderate exercise intensity would be at least 100 g/h, but more likely 120 g/h. In a highly trained person this could easily be as high as 250 g/h. So, even with high intakes of 90-120 g/h, we would not even match the sugar use during exercise, and we will always tap into our carbohydrate reserves. These reserves will be replenished as soon as we stop exercising. The main source of carbohydrate for this restoration of the carbohydrate stores in liver and muscle are the sugars ingested during exercise (and just after exercise). So, in essence, pretty much all of the sugars ingested during (and in the hours after exercise) will be used to fuel exercise or to replenish glycogen stores. The 360 grams that seemed excessive in the context of the general population-based recommendations, is not excessive at all in this context where the exercise alone uses up 400-480 grams of sugar.

Infographic of sugar during exercise

Following exercise, further replenishment of glycogen stores from more nutrient-rich carbohydrate sources is recommended, especially if there is 24 hours or so to recover. To make it abundantly clear: the recommendation is not to consume gels and sports drinks the entire day, but there is a role during exercise, in trained athletes, when performance is important.

The recommendation is not to consume gels and sports drinks the entire day, but there is a role during exercise, in trained athletes, when performance is important

At the population level it may be useful for governments to have population targets (i.e., less than 10% of added sugar per day). However, at an individual level, such advice is much less valuable because there are many different modifiers of the effects of sugar on health, that are not considered in population-based recommendations. These modifiers will make the effects of sugar more or less pronounced, or even remove the negative effects completely.

Some important modifiers of the effect of sugar on health are:

  • Energy balance

  • Levels of physical activity

  • Training status

  • Overweight or obesity

  • Existing insulin resistance

For athletes specifically, the following factors will determine the effect of sugar on health:

  • Energy expenditure during exercise

  • Carbohydrate use during exercise (exercise intensity)

  • Exercise duration

  • Timing of intake in relation to exercise

So how should athletes handle sugar?

Athletes should follow sports nutrition guidelines when performance is important and the regular nutrition guidelines most of the time. When the quality of training is important, fuelling with carbohydrates and sugars are a good option. Afterwards when quick recovery is necessary (if there is another round or competition or hard training sessions several hours later, use sugars to replenish glycogen quickly). If there is more time (>24 h), athletes can also use other carbohydrate sources to replenish glycogen. Throughout the day eat wholesome foods with a lot of nutrients and minimise highly processed foods that often contain very few ingredients apart from sugar and fat. And for everyone: be physically active. Use your carbohydrate reserves regularly!


  1. Yang Q, Zhang Z, Gregg EW, Flanders WD, Merritt R, Hu FB. Added Sugar Intake and Cardiovascular Diseases Mortality Among US Adults. JAMA Intern Med. 2014;174(4):516–524.

  2. Anderson JJ, Gray SR, Welsh P, Mackay DF, Celis-Morales CA, Lyall DM, Forbes J, Sattar N, Gill JMR, Pell JP. The associations of sugar-sweetened, artificially sweetened and naturally sweet juices with all-cause mortality in 198,285 UK Biobank participants: a prospective cohort study. BMC Med. 2020 Apr 24;18(1):97.

  3. Lean, MEJand Te Morenga, L. Sugar and Type 2 diabetes. British Medical Bulletin, 2016, 120:43–53.

  4. Moore JB, Fielding BA. Sugar and metabolic health: is there still a debate? Curr Opin Clin Nutr Metab Care. 2016 19(4):303-9.

  5. Malin SK, Kullman EL, Scelsi AR, Haus JM, Filion J, Pagadala MR, Godin JP, Kochhar S, Ross AB, Kirwan JP. A whole-grain diet reduces peripheral insulin resistance and improves glucose kinetics in obese adults: A randomized-controlled trial. Metabolism. 2018 82:111-117.

  6. Jeukendrup, AE. Carbohydrate intake during exercise and performance, Nutrition, Volume 20, Issues 7–8, 669-677, 2004.

Recently, I recorded a podcast with Trevor Connor and Rob Pickles from FASTTALK labs about the same topic. We discussed the topics in this blog, but also touched on topics like inflammation and the quality of carbohydrates. Click here to listen:


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