Fructose - in Daily Life and Why it Matters for Sport and Endurance

Fructose, Digestion and Gut Health

Fructose is a naturally occurring sugar (a type of carbohydrate) found mainly in fruit, vegetables, and also honey. In whole foods, it comes packaged with fibre, vitamins, and minerals, making it a healthy, balanced source of energy.

So, What’s the Issue?

The problem isn’t the fructose in whole foods - it is the large amounts added to processed foods and drinks. The food industry often uses fructose or high-fructose corn syrup as a cheap sweetener. These added sugars are far more concentrated than what we would naturally consume and are stripped of fibre and nutrients, which normally help slow digestion and balance energy release.

How the Body Processes Fructose – and Why It Can Cause Gut Issues

Fructose is absorbed in the small intestine and transported to the liver, where it is metabolised. Unlike glucose, it doesn’t trigger insulin and isn’t directly used by most body cells - almost all of it is processed in the liver.

In moderate amounts (from fruit or vegetables), fructose is converted to glucose or glycogen and used for energy - this is normal and healthy.

However, when large amounts are consumed (especially from sweetened drinks or processed foods) the liver and small intestine can become overloaded. Unabsorbed fructose remains in the gut, which can lead to bloating, cramps or diarrhoea.

Over time, consistently high intake may also promote fat storage in the liver and body fat, increasing the risk of fatty liver disease, insulin resistance, and metabolic syndrome.

Why Fructose Can Be Tricky - In Daily Life and Sport

Digestive issues from fructose are not limited to athletes. Even people who tolerate fructose normally may experience gut discomfort from large intakes in daily life.

During intense exercise, digestion can become even more sensitive:

• Blood flow shifts from the digestive system to muscles, heart, and lungs, slowing absorption.

• Consuming large amounts of fructose at this time - from gels, fruit-based snacks or sports drinks - can increase the risk of bloating, cramps or diarrhoea.

Why Is Fructose Added to Sports Gels?

In recent years, there has been growing interest in combining glucose and fructose in sports nutrition, although evidence on its benefits remains mixed. The sports industry has adopted this approach to help provide more available energy during exercise:

• The body can absorb only a limited amount of glucose per hour, but fructose follows a different absorption pathway, giving athletes access to additional energy.

• Sports gels are formulated with specific glucose-to-fructose ratios to optimise absorption, and taking them together speeds the uptake of both sugars.

While glucose is efficiently absorbed on its own, fructose absorbs more effectively when it “piggybacks” on glucose. Without enough glucose, fructose is absorbed more slowly and may cause digestive discomfort in sensitive individuals. During long training sessions or races, digestion can become even more sensitive and slower to process nutrients.

Consuming additional sources of fructose - think: bananas, lollies, dried fruit, date balls, muesli or nut bars, electrolyte drinks, lemonade and similar foods - can upset the glucose-to-fructose ratio, increasing the risk of gut discomfort. Sodium and other electrolytes are also important, as low levels can reduce carbohydrate absorption.

To support energy needs while minimising digestive issues, athletes can monitor total fructose intake, time additional sources carefully, and choose gels and sports drinks thoughtfully.

Practical Tips for Athletes

There is no one-size-fits-all solution, but you can manage fructose intake and reduce gut issues:

• Check your overall fructose intake, including hidden sources in processed foods.

• Test gels and drinks during training, not on

race day.

• Pay attention to electrolytes and hydration.

“And that bargain multipack of glucose-fructose gels from the last expo?” Don’t toss it - use it strategically and see how your gut responds.

Fructose metabolism is still being studied, but both dose and timing matter. For most people, whole foods are perfectly fine. For athletes using sports nutrition products, watching fructose intake can make a big difference for performance and digestive comfort.

In general, it’s not about eliminating fructose — it’s about using it in the right amounts at the right times.

Need Help?

If you struggle with gut issues in daily life or during training or racing, give me a call or send a message. I have lots of experience with these issues, and together we can create a plan that works for you.

References:

Benardout, M., Gresley, A.L., ElShaer, A. and Wren, S.P. (2022). Fructose malabsorption: causes, diagnosis and treatment. British Journal of Nutrition, [online] 127(4), pp.481–489. doi:https://doi.org/10.1017/S0007114521001215.

Dean, E., Osborne, A., Subar, D., Hendrickse, P. and Gaffney, C. (2025). Comparative Effects of a Glucose-Fructose bar, Glucose-Fructose hydrogel, and a Maltodextrin gel on Carbohydrate Oxidation and Sprint Performance in Tier two Athletes. https://doi.org/10.51224/SRXIV.553. doi:https://doi.org/10.51224/srxiv.553.

Eckstein, M.L., Erlmann, M.P., Aberer, F., Haupt, S., Zimmermann, P., Wachsmuth, N.B., Schierbauer, J., Zimmer, R.T., Herz, D., Obermayer-Pietsch, B. and Moser, O. (2022). Glucose and Fructose Supplementation and Their Acute Effects on Anaerobic Endurance and Resistance Exercise Performance in Healthy Individuals: A Double-Blind Randomized Placebo-Controlled Crossover Trial. Nutrients, [online] 14(23), p.5128. doi:https://doi.org/10.3390/nu14235128.

Fischer, A., Corr, L., Birdsey, L.P., Hearris, M., Morgan, P., Hodson, N., Martinho, D., Fleming, J., Mohr, M., Krustrup, P., Russell, M., Sale, C., Harper, L.D. and Field, A. (2025). Negligible Effects of Fructose-Glucose Composite Carbohydrates on Performance Across a Prolonged Soccer Match Simulation Compared With a Glucose-Only Control in Semiprofessional Soccer Players. International journal of sport nutrition and exercise metabolism, [online] (https://doi.org/10.1123/ijsnem.2024-0253), pp.1–12. doi:https://doi.org/10.1123/ijsnem.2024-0253.

Hannou, S.A., Haslam, D.E., McKeown, N.M. and Herman, M.A. (2018). The Journal of Clinical Investigation. The Journal of Clinical Investigation, [online] 128(2). doi:https://doi.org/10.1172/JCI96702.

Herman, M.A. and Birnbaum, M.J. (2021). Molecular aspects of fructose metabolism and metabolic disease. Cell Metabolism, 33(12), pp.2329–2354. doi:https://doi.org/10.1016/j.cmet.2021.09.010.

Kishida, K., Iida, T., Yamada, T. and Toyoda, Y. (2023). Intestinal absorption of D-fructose isomers, D-allulose, D-sorbose and D-tagatose, via glucose transporter type 5 (GLUT5) but not sodium-dependent glucose cotransporter 1 (SGLT1) in rats. British Journal of Nutrition, 130(11), pp.1852–1858. doi:https://doi.org/10.1017/s0007114523001113.

Merino, B., Fernández-Díaz, C.M., Cózar-Castellano, I. and Perdomo, G. (2019). Intestinal Fructose and Glucose Metabolism in Health and Disease. Nutrients, 12(1), p.94. doi:https://doi.org/10.3390/nu12010094.