Researchers looked at 13 studies involving 145 athletes to understand what happens inside the body when people exercise but don’t eat enough food to fuel their activity. They found that when athletes don’t consume enough calories, their bodies quickly make changes to hormones and bone health—particularly affecting a hormone called leptin and testosterone levels. The research shows that bone breakdown increased in about half the studies, while other important markers like calcium and inflammation stayed relatively stable. These findings suggest that athletes and coaches should pay close attention to eating enough, especially during intense training periods.
The Quick Take
- What they studied: What happens to athletes’ bodies when they don’t eat enough calories to match their exercise and daily needs
- Who participated: 145 athletes across 13 different research studies who participated in controlled experiments lasting at least 24 hours
- Key finding: When athletes don’t eat enough, their bodies quickly change hormone levels and bone metabolism—especially leptin (a hunger hormone) which dropped consistently, and testosterone which fell in half the studies
- What it means for you: If you’re an athlete, not eating enough food can affect your hormones and bones faster than you might think. This suggests monitoring your food intake is important, though more research is needed to understand long-term effects
The Research Details
This is a systematic review, which means researchers searched through published scientific studies to find all the high-quality experiments that tested what happens when athletes don’t eat enough food. They looked in three major scientific databases and found 13 studies that met their strict criteria. Each study had to be an experiment where researchers deliberately gave athletes less food than they needed and measured changes in their blood and body markers before and after. The studies lasted at least 24 hours, allowing researchers to see quick changes in the body’s chemistry.
The researchers looked at many different body markers including bone health indicators, calcium levels, hormones that control hunger and energy, inflammation markers, iron status, sex hormones, and thyroid function. By combining results from all 13 studies, they could see which changes happened consistently across different groups of athletes and which changes were less reliable.
This approach is important because it shows what happens to athletes’ bodies in the short term when they don’t eat enough. Rather than relying on one small study, combining 13 studies gives us a clearer picture of what’s really happening. Understanding these quick changes helps athletes, coaches, and doctors recognize problems early before they become serious health issues.
This review followed strict scientific guidelines (PRISMA standards) for combining research studies. The studies included were experiments where researchers controlled the conditions, which is stronger evidence than just observing what happens naturally. However, the total number of participants (145) is relatively small when spread across 13 studies, and the studies only lasted at least 24 hours, so we don’t know what happens over weeks or months. The researchers noted that future studies should include more women and use consistent methods so results can be compared fairly.
What the Results Show
The most consistent finding was that leptin—a hormone that tells your brain you’re full—dropped significantly whenever athletes didn’t eat enough. This makes sense because your body produces less leptin when food is scarce, which can make you feel hungrier.
Bone health showed interesting changes in about half the studies. Markers that indicate bone breakdown (called βCTX-1) increased, while markers showing new bone formation (P1NP) decreased. This suggests that when you don’t eat enough, your body breaks down bone faster than it builds new bone, which could be a problem if it continues for a long time.
Testosterone levels fell in about half of the studies examined. This is important because testosterone helps with muscle building and overall athletic performance in both men and women. However, other sex hormones like estradiol and progesterone didn’t show consistent changes across studies.
Surprisingly, many other markers didn’t change much in the short term. Thyroid hormones, IGF-1 (a growth hormone), calcium levels, inflammation markers, and iron status mostly stayed stable. This suggests these systems take longer to respond to not eating enough.
The research revealed that the effects of not eating enough are complex and may depend on other factors. For example, many of the studies also involved low carbohydrate intake, which might have made the effects stronger or different than if only calories were reduced. This means we can’t always tell if the changes are from low calories alone or from the combination of low calories and low carbs. The researchers also noted that results sometimes differed between men and women, suggesting that sex matters when studying these effects.
This review builds on earlier research showing that long-term energy deficiency in athletes (called RED-S) causes serious health problems. What’s new here is documenting that some of these changes happen very quickly—within 24 hours to a few days. This is important because it shows the body responds fast to not eating enough, which means problems can develop sooner than previously thought. However, the short-term changes observed here don’t necessarily predict what happens over weeks or months of undereating.
The biggest limitation is that these studies only lasted at least 24 hours, so we don’t know if these changes continue, get worse, or stabilize over longer periods. The total number of athletes studied (145) is relatively small when divided among 13 different studies. Most studies included mostly men or didn’t clearly report the sex of participants, so we can’t be sure if women respond the same way. Additionally, the studies used different methods and definitions of ’not eating enough,’ making it harder to compare results directly. Finally, these were controlled experiments in labs, which may not reflect what happens in real-world athletic training.
The Bottom Line
Athletes should aim to eat enough calories to match their exercise and daily needs. If you’re training hard, track your food intake to make sure you’re eating enough. If you notice signs of undereating (extreme fatigue, loss of menstrual cycle, constant hunger, poor performance), talk to a doctor or sports nutritionist. These recommendations are based on moderate evidence from short-term studies, so they should be part of a broader approach to athletic health.
This matters most for competitive athletes, especially those in sports that emphasize low body weight (like running, gymnastics, or wrestling) or those training intensely. Coaches and athletic trainers should also pay attention because they can help athletes eat appropriately. If you’re a casual exerciser, this is less urgent, but the basic principle—eating enough for your activity level—still applies. Anyone with a history of eating disorders should work with healthcare providers before making changes.
Some changes (like leptin dropping) happen within 24 hours of undereating. Bone changes may take days to weeks to become noticeable. However, serious problems like weakened bones typically develop over months or years of undereating. If you’re concerned about your eating habits, don’t wait—talk to a healthcare provider now rather than waiting to see if problems develop.
Want to Apply This Research?
- Log your daily calorie intake and exercise minutes to calculate your energy balance (calories eaten minus calories burned). Aim for a positive or neutral balance on most days. Track this weekly to spot patterns of undereating.
- Set a daily calorie target based on your activity level and use the app to log meals and snacks. If you consistently fall short, add one extra snack or increase portion sizes at meals. Start with one meal per day and gradually add more if needed.
- Weekly check-in: Review your energy balance trend. Monthly assessment: Note any changes in energy levels, performance, or mood. If you see consistent undereating or notice fatigue and poor performance, flag this for discussion with a coach or nutritionist. Track any menstrual cycle changes (for those who menstruate) as a sign of potential energy deficiency.
This research summary is for educational purposes and should not replace professional medical advice. If you’re an athlete concerned about your eating habits, energy levels, or hormonal health, consult with a doctor, registered dietitian, or sports medicine specialist. This review examined short-term changes in controlled settings and may not apply to all individuals or real-world situations. Anyone with a history of eating disorders should seek professional guidance before making dietary changes.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.