In Part One of this series on what's the best food for training, we explored the pros and cons of whole foods, bars, chews, gels, and drink mixes in terms of ingestion and digestion. In Part Two, we’ll dive deeper to uncover the differences—if any—among these types of foods when it comes to nutrient absorption and how they are used by the muscles during oxidation.
Absorption
After we eat food and the stomach has done its job, the next step is absorption. The regulation of stomach contents into the small intestine is important because if the stomach does not properly mix, prepare, and regulate the rate of content release into the intestines, it could lead to bloating, gas, and diarrhea.
In the small intestine, layers of folds and tiny finger-like projections called microvilli increase the surface area of the intestinal wall to help with absorption. On these microvilli, there are "cell doors" where carbohydrates and other nutrients are absorbed. As carbohydrates of all forms break down into single sugars, they either enter through a cell door known as SGLT1, responsible for handling glucose and galactose, or GLUT5, which handles fructose (1,2).
Only a limited amount of carbohydrate can be absorbed at a time, so these cell doors become a limiting factor. If too much carbohydrate is consumed, it can cause a "traffic jam." Imagine a highway: it can handle many cars, but if too many try to enter at once, they get backed up. Similarly, as carbohydrate consumption increases, the right ratio of glucose (GLU) to fructose (FRU) is needed to optimize absorption and avoid this traffic jam.
This is where the term "multiple transport carbohydrates" comes in—it refers to the use of varying ratios of GLU and FRU to help you absorb more carbohydrates per hour (2). Researchers study this to determine carbohydrate intake recommendations, and sports nutrition companies use these findings to formulate their products.
Many endurance athletes are now consuming between 100-130 g of carbohydrates per hour, a significant increase from the previously recommended 60-90 g per hour. Advances in sports nutrition products, delivery systems, and a better understanding of multiple transport carbohydrate ratios have contributed to these higher intake rates. Research suggests that as carbohydrate intake increases, the GLU:FRU ratio should shift from 2:1 to 1:1 or 1:0.8 (3).
When fueling needs are high, whole foods and bars may not be practical due to time, usability, or composition. Whole foods can still contribute to an athlete's energy needs, as seen in Grand Tours, where athletes consume rice cakes, paninis, and chef-made bars to avoid flavor fatigue (4). However, as intake increases, using bars, chews, and gels with optimal carbohydrate ratios becomes more important. Athletes may be able to enjoy whole foods early in a training session or race when the pace is lower and digestion is easier, then switch to more advanced fueling sources as the training session or race progresses.
Oxidation
Once carbohydrates are absorbed, the final stage during exercise is oxidation in the muscles. A few studies have examined how different forms of carbohydrates are used by the working muscles in athletes.
Study 1
In one study, eight well-trained cyclists rode for three hours and were given either a bar with water, a drink mix, or just water. Both the bar and the drink mix contained glucose and fructose in a 2:1 ratio at 93 g/hour, with fluids matched across all sessions. Researchers expected the bar to lead to lower carbohydrate oxidation due to its larger particle size and fat and fiber content, which can slow gastric emptying. While oxidation from the bar was about 15% lower at several points during the study, it reached a similar peak oxidation rate, only 7% lower than that of the drink mix. While the drink mix was slightly more effective as fuel, the difference wasn’t statistically significant. This suggests that low-fiber, optimized bars may provide similar fueling rates to drink mixes and likely offer a performance advantage over standard granola bars. This may come as a surprise to those who assume that drink mix would greatly outperform a bar (5).
Study 2
Another study compared chews, gels, and drink mixes at a carbohydrate intake rate of 120 g/hour. Peak oxidation rates were 93.6 g/hour for the drink mix, 94.8 g/hour for the gel, 95.4 g/hour for the chew, and 99.6 g/hour for a combination of all three. Oxidation efficiency ranged from 72% to 75% across the products, with little difference between them. This means that while we may ingest 120 g/hour, the muscle is not able to oxidize that much (6).
Based on intravenous glucose infusion studies that bypass digestion and absorption limitations, the maximal carbohydrate oxidation rate is about 1.8 g/min, with an estimated 70–75% efficiency. Consuming 140–150 g/hour of carbohydrates may maximize this, as seen in a study where athletes consuming 144 g/hour (1:1 GLU:FRU) reached a peak oxidation rate of 1.75 g/min—the highest recorded (7). However, this doesn't mean that everyone should aim this high on their next ride or run. Training the gut and other factors still need to be considered when attempting to ingest these high amounts(8).
Conclusion
The question of what's the best food for training is a complicated one. Though we cannot definitively say one product is better than another, we can use some of our digestion knowledge to help the decision-making process. With all the science of digestion and oxidation that we’ve covered, it's worth mentioning that grabbing a cookie or fig bar from the local bakery—if timed properly—can provide valuable fuel alongside your favorite bar, chew, gel, or drink mix. Moreover, these kinds of whole foods can also add some much needed and often underrated 'happiness watts,' especially during those long winter training sessions.
Key Takeaways
●If aiming for high carbohydrate intake per hour, choosing a product with listed GLU:FRU ratios is important.
● Training the gut is essential, as digestive enzymes and processes that aid carbohydrate absorption/oxidation can be influenced by diet.
● Whole foods have their place but are best used early in a session or before the pace increases
● Bars should be paired with fluids and other products for optimal fueling. Look for bars that limit fiber, fat, and protein.
● Chews, gels, and drink mixes are similar in digestion, absorption, and oxidation if carbohydrate ratios and fluid intake are maintained.
● Carb ratios matter more at higher intake levels: 2:1 for 60-90 g/hour and 1:1 or 1:0.8 for 90-120 g/hour.
● Mixing and matching chews, gels, and drinks can optimize fueling and reduce palate fatigue.
● Be critical of product marketing claims (e.g., isotonicity, encapsulation, cyclic dextrins). Do you really need a $4 gel for a 2-hour endurance session?
References
Jeukendrup A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports medicine (Auckland, N.Z.), 44 Suppl 1(Suppl 1), S25–S33. https://doi.org/10.1007/s40279-014-0148-z
Jeukendrup A. E. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current opinion in clinical nutrition and metabolic care, 13(4), 452–457. https://doi.org/10.1097/MCO.0b013e328339de9f
Jeukendrup, A. (2018, March 18). The optimal ratio of carbohydrates during exercise. MySportScience. https://www.mysportscience.com/post/the-optimal-ratio-of-carbohydrates
Strobel, N., Quod, M., Fell, J., Valerio, D., Dunne, D., & Impey, S. (2022). CHO periodisation in cycling: Case study: The application of daily carbohydrate periodisation throughout a cycling Grand Tour.
Pfeiffer, B., Stellingwerff, T., Zaltas, E., & Jeukendrup, A. E. (2010). Oxidation of solid versus liquid CHO sources during exercise. Medicine and science in sports and exercise, 42(11), 2030–2037. https://doi.org/10.1249/MSS.0b013e3181e0efc9
Hearris, M. A., Pugh, J. N., Langan-Evans, C., Mann, S. J., Burke, L., Stellingwerff, T., Gonzalez, J. T., & Morton, J. P. (2022). 13C-glucose-fructose labeling reveals comparable exogenous CHO oxidation during exercise when consuming 120 g/h in fluid, gel, jelly chew, or coingestion. Journal of applied physiology (Bethesda, Md. : 1985), 132(6), 1394–1406. https://doi.org/10.1152/japplphysiol.00091.2022
Jentjens, R. L. P. G., & Jeukendrup, A. E. (2005). High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise. British Journal of Nutrition, 93(4), 485–492. doi:10.1079/BJN20041368
Jeukendrup A. E. (2017). Training the Gut for Athletes. Sports medicine (Auckland, N.Z.), 47(Suppl 1), 101–110. https://doi.org/10.1007/s40279-017-0690-6
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