Eating too much is a widespread problem in America. That’s why more than 70 percent of men and women over the age of 20 are overweight or obese. And while there are plenty of clowns running around blaming individual nutrients or food types for these numbers, the simple fact is that overweight and obesity are caused by eating too much.

Because overeating is so common, little thought is given to the possibility of undereating, which is very real for endurance athletes given the high energy demands of their training and their interest in being lean not just for reasons of aesthetics and health but also for reasons of performance. And rare indeed is the endurance athlete who considers the fact that habitual undereating is more detrimental to performance than is its opposite.

Think about it. Athletes who eat slightly more food than they need every day will tend to feel good and perform well in workouts because they have plenty of fuel available for them, and they will also tend to recover from and adapt well to training because the raw materials that these processes depend on also come from food. The only negative (aside from long-term health issues, which are themselves mitigated by high activity levels) is that they will show up to the start line a few pounds over their ideal racing weight.

In contrast, athletes who habitually eat too little are unlikely to even make it to the start line. Lacking the fuel they need for optimal workout performance and the raw materials they need for recovery and adaptation, they are at high risk of succumbing to overtraining fatigue or injury before race day comes around.

An interesting new study by Danish and Dutch researchers investigated some of the negative effects of within-day energy deficits in endurance athletes. A within-day energy deficit is a short period (we’re talking hours) where the body’s energy needs exceed the supply of energy from recently consumed food. It is possible to experience one or more within-day energy deficits of lesser or greater magnitude even if you get enough to eat over the course of the day as a whole.

Scientists from the University of Agder and the University of Copenhagen looked specifically at associations between within-day energy deficits and suppressed resting metabolism and hormone levels in a group of 31 male cyclists, runners, and triathletes. They found that 20 of the athletes had suppressed resting metabolic rates (RMR), meaning their bodies did not burn as many calories at rest as they should have; the remaining 11 athletes exhibited normal RMR. Interestingly, although all of the athletes ate as many calories as they burned over a period of 24 hours, those with suppressed RMR spent almost double the amount of time in energy deficits exceeding 400 calories (20.9 vs. 10.8 hours, on average). Additionally, higher cortisol levels and lower testosterone levels were found in the athletes who had the largest within-day energy deficits.

What these results tell us is that not eating enough throughout the day as an endurance athlete is a form of self-sabotage. Undereating actually makes it harder to achieve and sustain a lean body composition by reducing resting metabolism. Athletes with suppressed RMR also tend to have trouble marshaling energy for intense workouts. To make matters worse, undereating compromises recovery from and adaptation to training by increasing catabolic hormone levels and reducing anabolic hormone levels. And it’s worth underscoring that the athletes in this study did eat enough to meet their energy needs when the day was considered as a whole. They only fell behind during specific periods during the day.

In summary, being too restrictive with your calories is a great way to hold yourself back as an endurance athlete.