Are you getting enough nutrition to fill your tank? Nutrition deficiencies are stereotypically thought of as affecting young, skinny female runners. You may have even heard the term “female athlete triad syndrome,” which describes female athletes who have disordered eating, loss of their period, and bone density loss (stress fractures). However, it is essential to know that nutritional deficiency can affect anyone! Male or female. Young or old. Novice or elite. If you do not take in as many calories as you burn, you are at risk.
This does not mean you have to have disordered eating. You might just be simply unaware that you are not taking in as many calories as you need or not getting the balance of nutrients that you need (protein, fat, carbohydrate, vitamins and minerals). This is why medical professionals, nutritionists, and mental health counselors are now using the term RED-S. RED-S stands for “relative energy deficiency in sport.” It does not limit the condition to the more severe cases of the female athlete triad. We no longer have to wait until you’re depleted enough to have stress fractures and loss of period (or depleted testosterone for males) to raise the alarm about nutritional deficiencies.
In this article, we’ll talk about the common signs of RED-S and what you can do to prevent or treat RED-S.
Common Signs of RED-S
🧠 Nervous System (Brain & Mood)
-
Constant fatigue or “brain fog”
-
Trouble concentrating
-
Irritability, mood swings, or feeling down more often
-
Sleep disturbances
🩸 Hormonal / Endocrine System
-
Women: irregular, light, or missing menstrual cycles
-
Men: low libido, reduced morning erections
-
Low testosterone or estrogen levels (if tested)
-
Feeling unusually cold (low thyroid activity)
💪 Musculoskeletal System
-
Frequent or lingering injuries
-
Stress fractures or shin pain that won’t go away
-
Muscle weakness or loss of strength
-
Slower healing after workouts or races
❤️ Cardiovascular System
-
Slower heart rate than usual (not just “athlete’s heart”)
-
Dizziness or lightheadedness, especially when standing up
-
Feeling “flat” during runs even with good training
🍽️ Gastrointestinal System
-
Bloating, constipation, or stomach discomfort
-
Loss of appetite (even when training hard)
-
Feeling full quickly
🛡️ Immune System
-
Getting sick more often (colds, sore throats)
-
Infections that linger or take longer to clear
🧬 Metabolic System
-
Difficulty maintaining weight despite eating normally
-
Plateau or drop in performance
-
Feeling cold all the time
-
Slower recovery between workouts
The big picture is that RED-S isn’t just about being “tired.” It’s your whole body signaling it doesn’t have enough fuel to cover both daily life and training demands.
Causes of RED-S
-
Not eating enough to match training load: Skipping meals, underestimating calories burned, or being “too busy to refuel” after runs.
-
Restrictive eating patterns: Following diets that cut out major food groups, being overly focused on “clean eating,” or trying to lose weight for performance or appearance.
-
High training volume without proper recovery: Back-to-back long runs, heavy mileage weeks, or lots of intensity without balancing rest and fuel.
-
Intentional or unintentional weight loss: Chasing a certain race weight or body composition goal, which can leave energy availability too low.
-
Frequent fasted or low-carb training: Doing lots of runs without pre-fueling, or consistently limiting carbs, can leave energy stores too depleted for the body’s needs.
Practical, Positive Steps for Runners to Keep the Tank Full
Make Energy Balance a Priority
-
Track your fuel: If you’re consistently feeling wiped out or not bouncing back after runs, take that as a cue.
-
Use apps or a food log to get a rough sense of intake versus output.
-
Aim for high-quality, energy-dense snacks, like nuts, whole-grain toast with nut butter, or smoothies after runs.
Fuel Timing Is Key
-
Pre-run: Have a banana, toast with jam, or a small energy bar.
-
Post-run: Reach for carbs + protein—think Greek yogurt with fruit or a peanut butter and honey wrap.
-
Throughout the day: Have mini-meals or snacks ready—especially on big mileage days.
Re-Evaluate Training Load
-
Add in rest days or easy runs if you’re feeling chronically tired.
-
Avoid too many fasted runs—unless you’re experimenting safely and know you’re fueling elsewhere.
-
Balance training phases with intentional fueling windows, especially when increasing mileage or intensity.
Monitor Body Signals (Not Just the Scale)
- For women not on birth control: track your menstrual cycle. Missed, light, or irregular periods can be one of the earliest signs of low energy availability.
- For women on hormonal birth control: cycles may not reflect hormonal health, so instead, watch for changes in sleep quality, mood swings, unusual fatigue, or recovery that feels slower than it should.
- For men: hormonal shifts can show up as low libido, fewer or weaker morning erections, or difficulty building/maintaining muscle.
- Non-hormonal signals for everyone: frequent injuries (especially stress fractures), lingering muscle soreness, brain fog, feeling cold all the time, getting sick often, or just feeling “flat” in training.
- Want to go deeper? Some athletes track basal body temperature, heart rate variability, or even mood journaling to spot early changes in recovery and hormone balance.
Build a Support Squad—and Normalize Self-Care
-
Work with a sensible team: Ideally, connect with a sports dietitian, a coach who values health first, and if needed, a sports-friendly counselor or mental health provider.
-
Educate yourself and your training circle: Know that RED-S is common and treatable; fuel is an essential part of training—not an indulgence.
-
Be patient through recovery: Some weight gain, slower paces, or rest may be needed—but they pave the way for long-term strength.
Conclusion
At the end of the day, RED-S is not about weakness—it’s about awareness. Your body is always talking to you, whether through your energy, mood, recovery, or hormonal health, and learning to listen is one of the most powerful skills a runner can develop. By fueling enough, respecting rest, and paying attention to signals beyond the scale, you give yourself the best chance to stay healthy, strong, and joyful in your running. Remember, performance thrives on balance—not restriction. If any of these signs feel familiar, know that small, positive changes can make a big difference, and you don’t have to figure it out alone. With the proper fueling and support, you can keep running toward your goals with confidence, strength, and longevity.
