When people picture an athlete dealing with a nutrition deficiency, the image that usually comes to mind is a lean, young female runner. That stereotype comes from a real and well-documented condition known as “female athlete triad syndrome,” a combination of disordered eating, missed menstrual cycles, and weakened bones that shows up as stress fractures. But the truth is broader and more important: an energy deficit can affect any athlete, regardless of sex, age, or experience level. If your calorie intake consistently falls short of what your training burns, you’re at risk.

That doesn’t mean disordered eating is the only path to this problem. Plenty of runners simply don’t realize they’re under-fueling, or that they’re missing the right balance of protein, fat, carbohydrates, vitamins, and minerals. That’s part of why clinicians, dietitians, and mental health professionals have shifted toward a broader term: RED-S, or Relative Energy Deficiency in Sport. Unlike the female athlete triad, RED-S doesn’t wait for the most severe warning signs — stress fractures or a missed period in women, or dropping testosterone in men — before it’s taken seriously.

Below, we’ll walk through the most common signs of RED-S and what you can actually do about it.

Recognizing the Signs of RED-S

RED-S can show up almost anywhere in the body. Here’s what to watch for, system by system.

Brain and Mood (Nervous System)

• Ongoing fatigue or mental fog
• Difficulty focusing
• Increased irritability, mood swings, or low mood
• Disrupted sleep

Hormones (Endocrine System)

• In women: irregular, lighter, or absent periods
• In men: reduced libido or fewer morning erections
• Lower-than-normal testosterone or estrogen, if tested
• Feeling cold more often than usual (a sign of slowed thyroid function)

Muscles, Bones, and Joints (Musculoskeletal System)

• Injuries that keep coming back or won’t fully resolve
• Stress fractures or persistent shin pain
• Loss of strength or muscle weakness
• Slower recovery after training or races

Heart and Circulation (Cardiovascular System)

• A resting heart rate lower than expected (beyond typical “athlete’s heart”)
• Lightheadedness or dizziness, particularly when standing
• A “flat” feeling during runs despite solid training

Digestion (Gastrointestinal System)

• Bloating, constipation, or general stomach discomfort
• Reduced appetite even during heavy training
• Feeling full after only a small amount of food

Immunity

• Catching colds or sore throats more frequently
• Illnesses that linger longer than they should

Metabolism

• Trouble maintaining weight despite eating what feels normal
• A performance plateau or decline
• Persistent feeling of being cold
• Recovery between sessions that keeps getting slower

Taken together, these aren’t just signs of being tired — they’re your body telling you it doesn’t have enough fuel to keep up with both everyday life and training.

What Leads to RED-S

• Under-fueling relative to training: skipping meals, underestimating how many calories a workout actually burns, or being too rushed to refuel afterward.
• Restrictive eating habits: diets that eliminate entire food groups, an overly rigid focus on “clean eating,” or eating less in pursuit of a performance or aesthetic goal.
• Heavy training loads without enough recovery: stacking long runs, high-mileage weeks, or intensity work without matching rest and fuel.
• Deliberate or unintentional weight loss: chasing a specific race weight or physique that pushes energy availability too low.
• Regular fasted or low-carb training: frequently running without fueling beforehand, or consistently restricting carbohydrates, which can leave the body without enough stored energy to meet its needs.

Practical Ways to Keep Your Tank Full

Prioritize Energy Balance

• Pay attention to how you feel: chronic exhaustion or a slow bounce-back after runs is a signal worth acting on.
• A food-tracking app or simple log can give you a rough read on intake versus output.
• Lean on energy-dense, nutrient-rich snacks — nuts, whole-grain toast with nut butter, or a post-run smoothie.

Time Your Fuel Well

• Before a run: something light like a banana, toast with jam, or a small energy bar.
• After a run: pair carbs with protein — Greek yogurt and fruit, or a peanut butter and honey wrap.
• Across the day: keep mini-meals or snacks on hand, especially on high-mileage days.

Reassess Your Training Load

• Build in rest days or easy runs if fatigue is becoming chronic.
• Be cautious with frequent fasted training unless you’re deliberately experimenting and fueling well the rest of the time.
• Pair heavier training blocks with intentional fueling, especially as mileage or intensity climbs.

Watch the Signals, Not Just the Scale

• Women not using hormonal birth control: keep an eye on your cycle. Missed, light, or irregular periods are often one of the earliest warning signs of low energy availability.
• Women on hormonal birth control: your cycle may not reflect what’s happening hormonally, so instead watch sleep quality, mood, and how quickly you recover.
• Men: watch for reduced libido, fewer or weaker morning erections, or trouble building or holding onto muscle.
• Everyone: recurring injuries (stress fractures especially), soreness that lingers, brain fog, feeling cold, getting sick often, or a training session that just feels flat.

Want more detail? Tracking basal body temperature, heart rate variability, or even a simple mood journal can help surface early changes in recovery and hormonal health.

Build a Support System and Normalize Taking Care of Yourself

• Assemble a team that takes health seriously: a sports dietitian, a coach who prioritizes long-term health over short-term numbers, and, if it would help, a mental health provider familiar with athletes.
• Learn about RED-S, and share what you learn: it’s common, it’s treatable, and fueling your body properly isn’t optional — it’s part of training.
• Give recovery time: some weight gain, slower training paces, or extra rest may be part of the process, but they’re what make long-term strength possible.

The Bottom Line

RED-S isn’t a sign of weakness — it’s a sign that it’s time to pay closer attention. Your body communicates constantly, through your energy levels, your mood, how you recover, and your hormonal health, and learning to read those signals is one of the most valuable skills a runner can build. Fueling consistently, respecting recovery, and watching for signs beyond just the number on the scale gives you the best shot at staying healthy, strong, and genuinely enjoying your training.

Performance is built on balance, not restriction. If any of this sounds familiar, small changes can add up quickly, and you don’t have to sort it out solo. With the right fuel and the right support behind you, you can keep working toward your goals while staying strong for the long run.