By: Jennifer A. Kurtz, PhD, CSCS, CISSN, EP-C

Context Matters: Carbohydrate Intake and Metabolic Health in Athletes

Recent conversations among endurance athletes have highlighted a growing concern: is long-term, continuous carbohydrate use—especially from engineered products like gels, chews, and high-sugar drinks—potentially harmful, even for trained individuals? While it’s well-established that sedentary individuals consuming excess carbohydrates, particularly added sugars, are at higher risk for insulin resistance, Type 2 Diabetes, and cardiovascular disease (Ludwig et al., 2018), the application of this concern to athletic populations is more nuanced.

For endurance athletes, high carbohydrate intake is often paired with high energy expenditure, and chronic aerobic training enhances mitochondrial biogenesis, glucose translocation, and skeletal muscle insulin sensitivity (Hawley & Lessard, 2008). As a result, trained individuals exhibit greater capacity to handle glucose loads without the same adverse metabolic effects seen in sedentary individuals. For example, research has shown that endurance athletes display lower fasting insulin and improved glucose disposal, despite high-carbohydrate diets (Phieliex et al., 2019).

That said, the protective metabolic effects of exercise don’t entirely eliminate the risks of excessive carbohydrate intake. Athletes in off-season periods, undergoing injury recovery, or engaging in insufficient fueling periodization may still accumulate metabolic strain if carbohydrate intake is disproportionately high relative to energy expenditure (Achten & Jeukendrup, 2004). This highlights the importance of matching fueling strategies to training load and using biomarker monitoring to detect subtle signs of dysregulation.

Furthermore, not all carbohydrate strategies are equally beneficial. Research increasingly supports the need to examine the types, timing, and total load of carbohydrates consumed, particularly when they are introduced in engineered, rapidly absorbed forms like fructose or high-glucose gels over long periods of time.

Fructose vs. Glucose: Are All Carbs Created Equal?

The debate over the best type of carbohydrate for fueling exercise has brought increased scrutiny to fructose, a simple sugar commonly used in sports nutrition products. Unlike glucose, which is absorbed via SGLT1 transporters and can be used directly by muscles, fructose is absorbed through GLUT5 and must first be metabolized by the liver (Rizkalla, 2010). In excess, fructose metabolism increases hepatic de novo lipogenesis, triglyceride formation, and uric acid production, which may contribute to non-alcoholic fatty liver disease and systemic inflammation, even in otherwise healthy individuals.

In exercise settings, however, fructose may play a beneficial role. Studies have shown that combining glucose and fructose during endurance activities can enhance total carbohydrate oxidation rates beyond what is possible with glucose alone (Jeukendrup & Jentjens, 2005). This dual-pathway strategy improves gut absorption and reduces the risk of gastrointestinal discomfort, which is particularly important in ultra-endurance events, where gut distress is common. Therefore, small-to-moderate amounts of fructose during exercise are not inherently harmful, and can actually be performance-enhancing.

Problems arise when fructose intake is high during non-exercise periods or consistently elevated across training cycles without a sufficient match in energy demand. Chronic high-fructose intake outside of training has been associated with increased fasting triglycerides, visceral adiposity, and markers of insulin resistance, even in lean individuals (Stanhope et al., 2009). Athletes who consume high-fructose recovery drinks or rely on fructose-heavy products on rest days may unintentionally increase long-term metabolic risk.

Given these findings, a strategic approach is recommended: use fructose strategically during long-duration training or competition (especially in mixed blends with glucose) and minimize intake during sedentary periods or non-training windows. One possible exception to this rule are natural fructose sources like fruit, which, because they are paired with fiber and polyphenols, pose less risk and offer additional health benefits.

Monitoring Long-Term Effects: Biomarkers and Metabolic Trends

To mitigate potential risks from long-term carbohydrate fueling, many athletes and practitioners are now turning to biomarkers for early detection of metabolic shifts. Tracking continuous glucose monitoring (Flockhart & Larsen et al., 2023), fasting insulin, HbA1c (a measure of average blood sugar over 2–3 months), and lipid panels can offer valuable insights into cardiometabolic health and how the body is handling carbohydrates—even in highly-trained populations.

Fasting insulin is particularly informative, as it provides a snapshot of baseline insulin sensitivity. Elevated fasting insulin, even in the presence of normal glucose, can suggest compensatory hyperinsulinemia, a common early marker of insulin resistance (Reaven, 1993). HbA1c is also helpful for detecting chronic glycemic load issues by detecting subtle increases that may be missed by single-point glucose tests. In athletes, quarterly testing during heavy training and off-season periods is advisable.

Triglyceride levels and the triglyceride-to-HDL ratio are additional markers worth monitoring, especially in athletes with high sugar intake, or those fueling with fructose-rich products. Elevated triglycerides and low HDL levels are hallmarks of diet-induced dyslipidemia, even in lean individuals. In the context of high-volume exercise, these markers should ideally remain well-controlled; elevations could suggest a mismatch between diet and energy expenditure, or early hepatic overload.

Some athletes may also benefit from measuring liver enzymes (ALT, AST), which can detect early signs of non-alcoholic fatty liver disease or hepatic stress. These tests are relatively low-cost, accessible, and actionable. If values begin to drift from optimal ranges, a reevaluation of carbohydrate type, timing, and quantity is warranted.

Practical Guidelines: Evidence-Based Fueling for Performance and Health

So, what’s the bottom line for athletes trying to fuel effectively without compromising long-term health? First, use carbohydrate blends strategically—glucose-fructose combinations are useful during prolonged endurance exercise for enhancing absorption and oxidation. During shorter bouts or lower-intensity sessions, single-source glucose may suffice.

Second, periodize your carbohydrate intake. Match high-carb intake with periods of high training volume and intensity. During rest days, recovery weeks, or injury recovery, reducing carbohydrate intake and shifting toward more complex carbs (e.g., legumes, whole grains, and vegetables) can help maintain metabolic flexibility and reduce unnecessary glycemic load.

Third, monitor your bloodwork regularly. Even in healthy, high-performing athletes, early signs of metabolic dysregulation can go unnoticed without lab data. Simple panels like fasting insulin, glucose, HbA1c, triglycerides, and liver enzymes can offer a long-term lens into how your body is adapting to fueling strategies.

In addition, consider using continuous glucose monitoring (CGM) to gain real-time insight into how your body responds to training, sleep, and meals. Research shows that athletes often experience large swings in glucose levels — spending unexpected amounts of time in both low (hypoglycemia) and high (hyperglycemia) zones. These individual glucose patterns often go undetected with standard lab tests, but may play a key role in recovery, energy availability, and long-term performance (Flockhart & Laresen, 2024).

Finally, be mindful of your off-season, as the protective metabolic effects of exercise may diminish during periods of reduced activity. Many athletes maintain the high-carb intake habits from their peak training phase during taper or recovery periods without adjusting for the lower energy expenditure. This is a key window for negative metabolic adaptations—adjusting fueling here can prevent long-term health risks.

References

• Achten, J., & Jeukendrup, A. E. (2004). Optimizing fat oxidation through exercise and diet. Nutrition, 20(7-8), 716–727. https://doi.org/10.1016/j.nut.2004.04.005

• Flockhart, M., & Larsen, F. J. (2024). Continuous glucose monitoring in endurance athletes: interpretation and relevance of measurements for improving performance and health. Sports medicine, 54(2), 247-255.

• Hawley, J. A., & Lessard, S. J. (2008). Exercise training-induced improvements in insulin action. Acta Physiologica, 192(1), 127–135. https://doi.org/10.1111/j.1748-1716.2007.01783.x

• Jeukendrup, A. E., & Jentjens, R. L. (2005). Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Medicine, 35(8), 747–763.

• Ludwig, D. S., Hu, F. B., Tappy, L., & Brand-Miller, J. (2018). Dietary carbohydrates: role of quality and quantity in chronic disease. BMJ, 361, k2340. https://doi.org/10.1136/bmj.k2340

• Reaven, G. M. (1993). Role of insulin resistance in human disease. Diabetes, 37(12), 1595–1607.

• Rizkalla, S. W. (2010). Health implications of fructose consumption: A review of recent data. Nutrition & metabolism, 7, 1-17.

• Phielix, E., Begovatz, P., Gancheva, S., Bierwagen, A., Kornips, E., Schaart, G., ... & Roden, M. (2019). Athletes feature greater rates of muscle glucose transport and glycogen synthesis during lipid infusion. JCI insight, 4(21), e127928.

• Stanhope, K. L., Schwarz, J. M., Keim, N. L., Griffen, S. C., Bremer, A. A., Graham, J. L., ... & Havel, P. J. (2009). Consuming fructose-sweetened, not glucose-sweetened, beverages increases visceral adiposity and lipids and decreases insulin sensitivity in overweight/obese humans. Journal of Clinical Investigation, 119(5), 1322–1334.