Signs You're Underfueling as an Athlete (And How to Fix It)
Nora FiermanHere's something most endurance athletes don't want to hear: the thing holding back your performance probably isn't your training plan, your gear, or your genetics. It's that you're not eating enough.
Underfueling is one of the most widespread and least discussed performance problems in endurance sports. Research in the area has shown up to 70% of athletes are not eating enough to meet the demands of their training, and most of them have no idea. The symptoms don't announce themselves. They show up as fatigue, slower times, constant illness, and the creeping feeling that your fitness is going backward despite consistent training.
Here's how to recognize underfueling before it derails your season and what to do about it.
What Underfueling Actually Means
Underfueling doesn't always mean eating too little overall. For endurance athletes, it often means eating too little relative to the energy demands of training, particularly carbohydrates, which are the primary fuel source for moderate-to-high intensity exercise.
You can eat three meals a day and still be underfueling if those meals don't account for a 90-minute run, a two-hour ride, or back-to-back training days. The mismatch between energy in and energy expended is what creates the problem and it compounds quickly when training volume is high.
The clinical term for chronic underfueling in athletes is Relative Energy Deficiency in Sport (RED-S). In one study of elite and pre-elite female athletes, 80 percent showed signs of RED-S, a number that should make every endurance athlete take this seriously. But RED-S exists on a spectrum. You don't have to be at the severe end to experience performance consequences.
8 Signs You're Underfueling as an Athlete
1. You're tired all the time, even when you're sleeping enough
This is the most commonly reported underfueling symptom: "I slept eight hours and I still feel like I've been hit by a truck." When your body doesn't have enough energy coming in, it starts rationing. Basic functions, including recovery from training, get deprioritized. The result is a bone-deep fatigue that no amount of sleep fully fixes.
If you wake up unrefreshed consistently despite adequate sleep and regular training, low energy availability is the first thing worth investigating, not your sleep habits.
2. Your performance is declining despite consistent training
If a pace that used to feel comfortable now feels like a grind, if you're struggling to finish workouts you used to complete easily, or if your heart rate is higher than normal at the same effort level, underfueling is a likely culprit.
Carbohydrates are your body's primary fuel source for moderate-to-high intensity exercise. When you're not eating enough of them, your body simply doesn't have the resources to perform at the level your training should support.
3. You're getting sick or injured more than usual
Chronic underfueling suppresses immune function and compromises bone density over time. If you're catching every cold that comes through your training group, dealing with recurring soft tissue injuries, or experiencing stress fractures, your body may be diverting resources away from immune function and tissue repair to cover basic energy needs.
Under-fueling can impact your body's ability to repair and strengthen muscles, making you more prone to injuries. If you're experiencing frequent strains, sprains, or other musculoskeletal issues, it could be a result of insufficient nutrition.
4. Your recovery is slow
You finish a hard workout and you still feel it three days later. Your legs feel heavy going into sessions that should feel fresh. Underfueling can delay muscle glycogen replacement and repair of muscle tissue, hindering training.
Recovery happens between sessions, not during them, and recovery requires energy. When that energy isn't available, your body can't complete the repair process that makes training adaptations possible.
5. You're thinking about food constantly
Food preoccupation is one of the most under recognized underfueling symptoms in athletes. When you're not eating enough, your brain becomes hyper-focused on food as a survival mechanism. It's not a character flaw. It's biology. If you find yourself constantly thinking about what you're going to eat next, planning meals obsessively, or spending significant mental energy on food throughout the day, your body is telling you something. It's asking for more.
6. Your mood and motivation have shifted
Feeling lethargic, weak, and mood changes are signs of improper fueling. Underfueling affects neurotransmitter function, the same systems that regulate mood, motivation, and mental sharpness. If training feels like a chore when it used to feel like something you looked forward to, and nothing in your life has changed to explain it, energy availability is worth examining.
7. You're sleeping poorly
Difficulty falling asleep, restless nights, or waking frequently can all be signs that your body isn't getting enough energy. Poor sleep then further impairs recovery and performance, creating a cycle that often continues until fueling is addressed.
The connection between underfueling and poor sleep is one of the least discussed symptoms and one of the most impactful, because inadequate sleep compounds every other recovery and performance variable.
8. You bonk earlier than you should
Hitting the wall, the sudden, dramatic drop in energy and performance that runners and cyclists experience when glycogen stores are depleted, is often attributed to race-day nutrition. But chronic underfueling means you're starting workouts with lower glycogen stores than you should, which means the wall comes earlier and hits harder than it would for a properly fueled athlete.
If you're bonking on efforts that shouldn't deplete you, the problem may not be what you ate that morning. It may be the cumulative energy deficit you've been running for weeks.
⚡ Eating enough is the foundation. Fueling right is the edge.
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Why Endurance Athletes Underfuel (Without Realizing It)
Understanding why underfueling happens helps explain why it's so common even among serious, experienced athletes.
Appetite suppression after hard training. High-intensity exercise temporarily suppresses hunger hormones. Many athletes finish a hard workout not feeling hungry and take that as a signal to wait before eating. But the recovery window is most effective in the 30 to 45 minutes immediately after finishing, regardless of whether you feel hungry.
Underestimating caloric expenditure. A two-hour trail run in the mountains burns significantly more than most people think, often 800 to 1,200 calories or more depending on intensity, elevation, and body weight. Eating "normally" after that effort doesn't come close to replacing what was spent.
Eating around training without eating enough overall. Many athletes focus on what to eat before and after workouts but don't increase their overall daily intake to match training volume. Pre-workout and post-workout nutrition matter, but they sit within a daily total that needs to support all of it.
Altitude and heat suppressing appetite. For athletes training in the mountains or in hot conditions, appetite suppression from environmental factors compounds the problem. You feel less hungry precisely when your body needs more fuel.
How Much Should Endurance Athletes Actually Eat?
The research on daily carbohydrate needs for endurance athletes is clear: significantly more than general population guidelines suggest. Athletes training 60 minutes or more per day should aim for 8 to 10 grams of carbohydrates per kilogram of body weight on heavy training days. For a 150-pound athlete, that's 545 to 680 grams of carbohydrates daily, a number that surprises most people who've been following standard nutrition advice.
Total caloric needs vary by sport, intensity, body weight, and training volume, but the consistent finding across sports nutrition research is that most recreational endurance athletes are eating less than their training demands.
How to Fix Underfueling
Start with carbohydrates. Carbs are your body's primary fuel for training intensity. If you've been eating low-carb or simply not prioritizing carbohydrate intake around training, this is the single most impactful change you can make.
Don't skip the recovery window. Within 30 to 45 minutes of finishing a hard effort, eat something with both carbohydrates and protein, even if you're not hungry. A 4:1 carb-to-protein ratio is well-supported for this window. Neve's Tart Cherry Cacao covers this with tart cherry for inflammation, plant protein, and the right ratio in a single pouch.
Eat before you're hungry during training. Hunger during exercise is a lagging indicator of glycogen depletion. Start fueling around 45 minutes into efforts over 90 minutes and eat on a schedule, not on sensation.
Increase overall daily intake on hard training days. Pre- and post-workout nutrition matters, but so does the total amount you eat across the day. Heavy training days need more food than rest days, not the same amount.
If symptoms are severe or persistent, work with a sports dietitian. RED-S exists on a spectrum from mild performance consequences to serious health impacts. If you're experiencing multiple symptoms from this list consistently, a registered dietitian who specializes in sports nutrition can help you build a fueling plan specific to your training load.
The Bottom Line
Underfueling is the most common performance problem most athletes never identify because the symptoms don't look like a nutrition problem. They look like overtraining, poor sleep, bad fitness, or a rough patch. But the fix is often simpler than any of those explanations: eat more, eat consistently, and fuel the work you're actually doing.
Your training is only as good as the fuel behind it.
🌿 Fuel the work you're putting in.
Neve smoothie pouches are whole-food endurance fuel with real fruits, vegetables, natural electrolytes, and plant protein designed for athletes who train consistently and want nutrition that keeps up. Real food, real simple.
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Or start with a Variety Pack → to try both flavors.