Athletes who don’t eat enough calories can face serious health problems, especially with their bones. This review looked at research showing how low energy availability—basically not consuming enough food for your activity level—affects how fast your body burns calories and weakens your bones. The study found that athletes in endurance sports and sports focused on appearance are most at risk. The good news is that doctors can now screen for these problems early by checking nutrition, hormones, and bone health together. Understanding this connection helps athletes, coaches, and doctors prevent injuries and keep athletes healthy while training hard.

The Quick Take

  • What they studied: How not eating enough calories affects athletes’ bone strength and how their bodies burn energy
  • Who participated: This was a review of many different studies about athletes, especially those in endurance sports (like running and cycling) and appearance-focused sports (like gymnastics and figure skating)
  • Key finding: Athletes who don’t eat enough calories have weaker bones and their bodies adapt by burning fewer calories at rest. This is especially true for endurance and aesthetic athletes, though the effect varies based on the sport and individual differences.
  • What it means for you: If you’re an athlete, eating enough food for your training level is crucial for bone health. Talk to a sports nutritionist or doctor if you’re restricting calories, and get regular bone health checks if you’re in high-risk sports. This is particularly important for young athletes whose bones are still developing.

The Research Details

This is a narrative review, which means researchers looked at many existing studies on the topic and summarized what they found. Instead of doing one new experiment, they examined the current scientific evidence about how low energy availability (not eating enough calories for your activity level) affects athletes’ bones and metabolism. The researchers focused on three main areas: how much energy athletes have available, their resting metabolic rate (how many calories they burn at rest), and their bone mineral density (how strong their bones are). They also looked at a condition called Relative Energy Deficiency in Sport (RED-S), which happens when athletes don’t eat enough for their training demands.

This type of review is important because it brings together information from many different studies to show the big picture. Since individual studies sometimes disagree or use different methods, a review helps identify patterns and common findings. Understanding how low energy availability affects bones is critical because weak bones can lead to stress fractures and long-term health problems for athletes.

As a narrative review, this study summarizes existing research rather than conducting new experiments. This means the findings are only as strong as the studies being reviewed. The researchers noted that many studies use different methods to measure energy availability and bone health, which makes it hard to compare results directly. The review highlights the need for better standardization in how scientists measure these factors in future research.

What the Results Show

The review found clear evidence that when athletes don’t eat enough calories, their bodies respond by burning fewer calories at rest—a process called metabolic adaptation. This happens because the body tries to conserve energy when it’s not getting enough fuel. Along with this change, athletes experience hormonal changes typical of RED-S, a condition affecting overall health. The research shows that low energy availability is connected to weaker bones, with athletes showing lower bone density scores. This effect appears strongest in endurance athletes (runners, cyclists, swimmers) and athletes in appearance-focused sports (gymnasts, figure skaters, dancers). However, the impact on bone health isn’t the same for everyone—it depends on the type of sport, how much impact and stress the sport puts on bones, and individual factors like genetics and age.

The review also found that bone health problems from low energy availability may develop over time with prolonged calorie restriction. Different sports affect bones differently—for example, weight-bearing sports like running stress bones differently than swimming. The research suggests that screening for early warning signs is important, including checking nutrition intake, hormone levels, metabolic rate, and bone density together rather than looking at just one factor.

This review confirms what previous research has suggested: that energy availability is a key factor in athlete health. It builds on earlier work by emphasizing that the relationship between not eating enough and bone problems is complex and influenced by many factors. The review also highlights that RED-S is a real and serious condition affecting many athletes, supporting growing awareness in sports medicine about the importance of adequate nutrition.

The biggest limitation is that different studies measured energy availability, metabolic rate, and bone density in different ways, making it hard to compare results directly. The review is based on existing studies rather than new research, so it can only be as accurate as those studies. Many studies were small or short-term, so we don’t know as much about long-term effects. The review also notes that most research focuses on certain types of athletes, so findings may not apply equally to all sports.

The Bottom Line

Athletes should work with sports nutritionists to ensure they’re eating enough calories for their training level (moderate to high confidence). Young athletes and those in endurance or appearance-focused sports should have regular bone health screenings (moderate confidence). Coaches and medical staff should watch for signs of RED-S, including unusual fatigue, frequent injuries, or hormonal changes (moderate to high confidence). If you restrict calories or train intensely, get professional nutritional assessment (high confidence).

This is most important for competitive athletes, especially those in endurance sports (distance running, cycling, swimming) and appearance-focused sports (gymnastics, figure skating, dance, diving). Young athletes are particularly at risk because their bones are still developing. Coaches, athletic trainers, and sports medicine doctors should also pay attention to this research. If you’re a casual exerciser eating a normal diet, this is less of a concern, but the principles still apply—eat enough for your activity level.

Bone health problems from low energy availability develop gradually over weeks to months of inadequate nutrition. You might notice fatigue or frequent injuries before bone problems show up. Improvements in bone health after increasing calorie intake typically take months to years, depending on how long the problem existed. Young athletes may recover faster than older athletes.

Want to Apply This Research?

  • Track daily calorie intake versus estimated calorie burn from training. Use the app to calculate your energy availability score (calories consumed minus exercise calories burned). Aim for at least 30 calories per kilogram of body weight per day for adequate energy availability.
  • Set a daily calorie intake goal based on your training volume and body weight. Use the app to log meals and workouts together so you can see if you’re eating enough for your activity. Set reminders to eat adequate meals and snacks around training sessions.
  • Weekly: Review your energy availability score and adjust meals if needed. Monthly: Track energy levels, injury frequency, and menstrual cycle changes (if applicable). Quarterly: Share data with a sports nutritionist or doctor to assess bone health risk factors and adjust nutrition plan accordingly.

This review summarizes scientific research but is not medical advice. If you’re an athlete concerned about energy availability, bone health, or RED-S, consult with a sports medicine doctor, registered dietitian, or sports nutritionist for personalized assessment and recommendations. This is especially important for young athletes, those with a history of eating disorders, or athletes experiencing frequent injuries or hormonal changes. Do not make significant changes to your diet or training without professional guidance.