Nutrition directly determines how much energy you have during exercise, how quickly you recover afterward, and how well your body adapts to training over time. Elite endurance athletes routinely consume 3,000 to 5,000 calories per day during peak training, but total calories are only part of the equation. The type of fuel, the timing, and the micronutrients behind the scenes all shape whether your body performs at its ceiling or well below it.
Carbohydrates Are Your Primary Fuel Source
Carbohydrates stored as glycogen in your muscles and liver are the dominant energy source for anything above a moderate pace. When those stores run low, you hit a wall: pace drops, concentration fades, and effort feels dramatically harder. Athletes need anywhere from 3 to 12 grams of carbohydrate per kilogram of body weight per day, depending on training volume and intensity. A 70-kilogram runner on an easy recovery day might need around 210 grams, while the same runner during a heavy training block could need 700 grams or more.
Your body can burn fat efficiently at moderate intensities, up to roughly 60 to 65% of maximum effort in trained individuals. Above that threshold, fat burning drops sharply and is nearly zero above 90% effort. This is why carbohydrates matter most for high-intensity and competitive performance. Athletes who follow low-carb, high-fat diets can adapt to burn fat at higher rates (up to about 1.5 grams per minute at 70% effort), and this may help in ultra-endurance events where pacing stays moderate. But the trade-off is real: elevated blood levels of certain byproducts during exercise can contribute to earlier mental fatigue, and the ability to sustain high-intensity surges suffers when glycogen stores are chronically low.
Protein Needs for Active People
Protein repairs damaged muscle fibers and drives the adaptations that make you stronger and more resilient. The International Society of Sports Nutrition recommends that exercising individuals consume 1.4 to 2.0 grams of protein per kilogram of body weight daily. Where you fall in that range depends on your sport. Endurance athletes do well at the lower end (1.0 to 1.6 grams per kilogram), while strength and power athletes benefit from the upper end (1.6 to 2.0 grams per kilogram). Athletes in intermittent sports like soccer or basketball typically land in the middle.
Interestingly, protein requirements may actually decrease as you become more trained, because your body gets better at retaining and recycling the protein it already has. But during phases when you sharply increase training volume or start a new program, your needs temporarily spike. Spreading protein intake across meals throughout the day, rather than loading it into one sitting, supports a more consistent rate of muscle repair.
When You Eat Matters for Recovery
After hard exercise, your muscles are primed to absorb fuel and begin rebuilding. Consuming carbohydrates and protein together in a roughly 4-to-1 ratio significantly improves the rate at which your muscles restore their glycogen. A practical target is 0.8 grams of carbohydrate and 0.2 grams of protein per kilogram of body weight, consumed shortly after exercise and again about two hours later. For a 70-kilogram athlete, that works out to about 56 grams of carbohydrate and 14 grams of protein per serving, roughly equivalent to a large banana with a cup of chocolate milk.
This window matters most when you have another training session or competition within 24 hours. If you have a full day or more before your next hard effort, total daily intake becomes more important than precise timing.
Dehydration Costs More Than You Think
Losing just 2% of your body weight through sweat, about 1.4 kilograms for a 70-kilogram person, measurably increases physiological strain and decreases performance during intense exercise in the heat. Heart rate climbs, core temperature rises faster, and perceived effort increases at the same pace. Cognitive function also suffers, which matters in sports that require quick decisions.
The challenge is that thirst is a lagging indicator. By the time you feel noticeably thirsty, you may already be past that 2% threshold. Weighing yourself before and after training sessions gives you a practical read on your sweat rate, which can then guide how much you drink during similar efforts in the future.
Iron and Vitamin D: The Hidden Performance Factors
Two micronutrients deserve special attention because deficiencies are common in athletes and the performance consequences are significant.
Iron-deficient athletes, even those who aren’t technically anemic, show endurance performance 3 to 19% worse than athletes with adequate iron stores. In trained athletes, iron-sufficient individuals completed time trials 1 to 19% faster than their iron-deficient counterparts. Power output dropped 6 to 10% in iron-deficient athletes, and isokinetic strength was impaired by up to 23% in some assessments. Female athletes are particularly vulnerable due to menstrual iron losses combined with the demands of training. The tricky part is that iron deficiency without full-blown anemia often goes undetected because standard blood tests may come back in the “normal” range.
Vitamin D plays a direct role in muscle force production and bone integrity. Professional soccer players with severely low vitamin D levels produced less peak muscle force than teammates with adequate levels. The stress fracture risk is striking: athletes with very low vitamin D were 5.1 times more likely to suffer a specific foot stress fracture compared to those with normal levels, and at moderately low levels, the risk was still 2.9 times higher. Athletes with low vitamin D had a 12% higher rate of stress fractures overall. The recommended blood level for athletes is above 32 ng/mL, with 40 ng/mL or higher considered ideal.
Undereating Disrupts Far More Than Energy
Chronically consuming too few calories relative to your training load triggers a cascade of problems known as Relative Energy Deficiency in Sport (RED-S). This goes well beyond feeling tired. Low energy availability suppresses reproductive hormones, which in female athletes can lead to missed periods, and in male athletes to drops in testosterone. It disrupts metabolic hormones that regulate hunger, blood sugar, thyroid function, and stress response. Bone density declines, and once bone health deteriorates, recovery takes months to years even after calorie intake is corrected.
Warning signs include chronic fatigue, repeated stress fractures, significant changes in body weight or composition over short periods, loss of menstrual regularity, decreased libido, and inconsistent training due to injury or illness. Athletes in weight-class sports, aesthetic sports, and endurance disciplines are at highest risk, but RED-S occurs across all sports and affects both men and women.
Preventing Gut Problems During Exercise
Stomach cramps, nausea, and other digestive issues during competition are among the most common nutrition-related performance problems, and they’re largely preventable with the right strategy.
Gut training, the practice of deliberately consuming carbohydrates and fluids during training runs or rides, reduces symptoms over time. Two weeks of purposeful carbohydrate intake during running lowered both the severity of symptoms and signs of carbohydrate malabsorption. Similarly, gradually increasing fluid intake during training helped athletes tolerate higher volumes with fewer issues.
The form of carbohydrate also matters. Liquid sources generally cause fewer problems than solid bars or concentrated gels. Using a mixture of glucose and fructose (in roughly a 1.2-to-1 ratio) improves absorption because the two sugars use different transport pathways in the gut, reducing the chance of a bottleneck that leads to bloating or cramping. For athletes who still struggle, a short-term low-FODMAP diet in the days before competition can reduce symptoms. Rather than eliminating entire food groups, the more targeted approach is to identify which specific carbohydrate triggers your symptoms, whether it’s fructose, lactose, or another fermentable sugar, and reduce just that one around intense efforts.
Putting It All Together
Nutrition affects every system your body relies on during exercise: energy production, muscle repair, hydration balance, hormone regulation, bone strength, and even gut comfort. The athletes who gain the biggest edge aren’t following exotic diets. They’re consistently matching their carbohydrate intake to their training demands, eating enough total calories, distributing protein across the day, staying on top of hydration, and catching micronutrient gaps before they become performance problems. Small, sustained habits in these areas compound into measurable differences in how you train, recover, and compete.