Is Fructose The Reason Your Gut Hurts While Training?
Updated: Dec 12, 2020
For endurance athletes, research exists to show the benefit of multiple-transportable carbohydrates, or using different types of carbs to increase the amount a person can absorb per hour, resulting in more energy entering the bloodstream for the working muscles to utilize.
This is a good thing.
Yet this is a bad thing if you’re one of the roughly 30% of the population* with a fructose intolerance. For endurance athletes, they may experience negative, performance-limiting symptoms if the intolerance is left undiagnosed and the athlete is improperly dosing with during-training carbohydrates. This could result in a training session being shortened, intensity reduced, or even disqualifying oneself from a race.
In this article, I’ll explain what fructose is, how it’s absorbed, what types of symptoms occur, and how to reduce your fructose intake.
*This will depend on the definition of intolerance. Rao et al. (2007) provided prevalence by fructose dose delivered: "100% of healthy volunteers could absorb 15 [grams] of fructose, 90% could absorb 25 g of fructose but only 20-30% could absorb 50 g."
What is fructose?
Fructose is a carbohydrate found in both naturally-occurring foods and as an additive. Table sugar (sucrose) is half glucose, half fructose. Fructose, also called fruit sugar, is sweeter compared to sucrose and glucose, but has the least impact on increasing blood sugars.
There are many foods, beverages, and sport products that contain fructose:
Fruits: Apples, applesauce, bananas (ripe), cherries, figs, mangoes, pears, watermelon.
Vegetables: Artichokes, asparagus, broccolini, pickled onions.
Beans: Baked, broad, fava.
Sweeteners: Agave nectar, fructose, high-fructose corn syrup, honey.
Beverages: Apple juice, cranberry cocktail, orange juice.
Bars: KIND (e.g., Healthy Grains, Nut Bar, Pressed), LARA Bar.
Sports drinks: Cheribundi, Powerade.
Sports foods: Clif Z Fruit (i.e., ropes), fruit snacks (e.g., Motts), GoGo Squeez applesauce pouches, Gu Energy gels, Honey Stinger (chews/waffles), Sport Beans.
How is fructose absorbed?
Think of different types of carbs as being unique keys for locked doors. In the small intestine, the glucose “key” unlocks the SGTL-1 “door” whereas the fructose “key” unlocks the GLUT-5 “door”.
Once fructose is present in the small intestine, it can be absorbed in a limited capacity through the small intestinal wall via glucose transporter 5 (GLUT-5). For endurance athletes this means that as glucose transporters (SGTL-1) become saturated with glucose, fructose can simultaneously enter the blood stream through the GLUT-5 door. Thus, consuming multiple transportable carbohydrates (e.g., glucose plus fructose) results in a higher total carbohydrate load being absorbed.
For dosing, this means 60 grams per hour glucose and up to 30 grams per hour of fructose.
The benefits of maximizing multiple transportable carbohydrates for endurance athletes includes increased hourly total carbohydrate intestinal absorption, less intestinal distress, higher oxidation rates, reduced perception of exertion and fatigue, greater ability to maintain cadence five hours into a ride, improved performance (context: mountain biking), and improved time trial (context: 2 hours of cycling at a distance of 100 kilometres).
What are the symptoms of fructose intolerance?
If you’re intolerant to fructose, this could mean one of two things (or both):
You have poorly-functioning GLUT-5.
You don’t have enough GLUT-5 to handle the amount of fructose present.
In the gut of an intolerant athlete, excess fructose moves past the saturated and/or poorly-functioning GLUT-5 and into the large intestine. There, bacteria feast on the fructose (a process called fermentation) and symptoms result.
Similar to the carbohydrate intolerance of lactose, symptoms include abdominal pain, bloating, and gassiness. Fructose in the large intestine has an osmotic effect, pulling water in, and resulting in crampy, loose stools (diarrhea).
How is fructose intolerance diagnosed?
Step 1. (Possible) Medical intervention: A doctor would order a breath test to evaluate if malabsorption is present. Sometimes, a breath test isn’t pursued, since the intolerance:
Isn’t a medical diagnosis.
Isn’t inflammatory.
Doesn’t result in damage to the gut.
Step 2. Dietary Intervention: A dietitian should recommend an athlete complete a multi-day food, symptom, and timing record that includes:
What’s being eaten and drank: Including brand names and quantities.
The timing of consumption.
The timing, type, and severity of symptoms.
With fructose intolerance, symptoms aren’t immediate and may occur 6-8 hours after consuming the triggering item. When your body senses a meal being consumed in the mouth, a signal is sent to the rest of the gut to move content along as to make room for the current meal. As the previous meal is pushed into and further along the large intestine, resident bacteria feast on the undigested fructose. Symptoms will now present
If an athlete reports that they ate a meal and twenty minutes later had symptoms, it would be incorrect to look for a culprit within the current meal. When symptoms occur, the dietitian needs to consider what was consumed 6-8 hours prior to symptoms occurring.
If an athlete’s meals and snacks change on a daily basis or their symptoms seem erratic and don’t follow any pattern, an elimination diet would be a better starting point. Since this is a more restrictive route, an athlete needs to work with a dietitian, since elimination diets are restrictive, difficult to follow, could result in too few calories and/or nutrients being consumed, and are high-risk for athletes with current or past disordered eating or clinically-diagnosed eating disorders. Always look for a professional with the “registered dietitian” or “RD” credential.
Take-home Messages: How to Fuel Your Athlete
For a normally-functioning gut, 30 grams of fructose per hour seems to be properly absorbed. For someone with an intolerance, all we know is that 30 grams per hour is likely too much. Again, an intolerant individual either has poorly-functioning and/or an inadequate supply of transporters. This means that maybe an athlete can only tolerate a bolus dose of 20 grams fructose or they can tolerate 30 grams per hour, but in split doses (e.g., 10 grams every 15-20 minutes).
Simply because a food contains fructose, doesn’t mean it needs to be avoided. Identical to another carbohydrate intolerance—lactose intolerance—some people can tolerate a half cup of high-lactose cow’s milk, but two cups would result in an upset stomach or diarrhea. Thus, evaluating what fructose dose and timing works for the athlete is needed.
The Monash University FODMAP Diet app allows for filtering foods and beverages for fructose, which helps the athlete and practitioner learn what foods contain fructose and in what doses. Items may also be categorized by country.
For sport foods and beverages, review my two free downloadable handouts for fermentable carbohydrates found in common items (see the "Further Reading" section below). For example, an athlete may not tolerate Powerade and would need to swap for Gatorade.
Lastly, fructose intolerance is present 24/7. The athlete's daily diet plus training nutrition plan will need to managed.
Further Reading:
Lists out commonly-found ingredients and includes their low-FODMAP serving size and what FODMAP(s) the ingredient is high in.
Categorizes fuel into 5 categories: (1) pre- and during-exercise beverages; (2) pre- and during-exercise gels, chews, and waffles; (3) post-exercise beverages; (4) protein powders; and (5) snacks, energy bars, and nut butters. Plus, an explanation is provided as to why each high-FODMAP item is considered high.
The university that penned the term FODMAPs, and continues to lead research on the topic, created an app that allows users to filter by FODMAP subgroup—making it helpful during both the challenge and modified diet stages. The app has a one-time fee and is a tool I use when counseling.
A fabulous book outlining the many different ways patients describe their bloating and symptoms, which helps the dietitian working with a doctor arrive at a diagnosis and nutritional, supplementation, and medication plan.
Jeukendrup, A.E. (2013). SSE,26(108):1-5.
Fedewa, A., & Rao, S.S.C. (2014). Curr Gastroenterol Rep,16(1):370.
Changes in blood flow during exercise can increase gut symptoms in athletes. Add on improper fluid and/or food intake and the results can be disastrous.
Blog Article - FODMAPs, Part 1: What Are They?
This post covers what FODMAPs are, why they can be a problem for some athletes, and why one would follow a low FODMAP diet.
Blog Article - FODMAPs, Part 2: Applying the Diet
Learn more about where to start and what plan works best for your athlete.
Blog Article - FODMAPs, Part 3: What the Practitioner Needs to Know
The diet's nuances are plentiful, so dig in for a behind-the-scenes look. Topics include why eliminating gluten may not be the answer, what country of origin FODMAP cut-off values arise from, and what items to use during the challenge phase.
Blog Article - FODMAPs, Part 4: Fueling the Athlete
Foods and beverages marketed to athletes contain FODMAP ingredients. Find out what products contain and are free of FODMAPs.
Kate is a leading dietitian and researcher in the area of FODMAPs. Her free resources include low FODMAPs grocery and a high- and low-foods lists. Follow her on Instagram for FODMAP-friendly recipes and products.
“The Complete Low-FODMAP Diet” by Sue Shepherd, PhD and Peter Gibson, MD.
The book includes lists of low-, medium-, and high-FODMAP options, which is helpful during the challenge and modified diet stages. It also includes a library of recipes and is my go-to book when counseling symptomatic athletes.
