15 Warning Signs You Might Be Underfueling Your Workouts

When your car is low on gas, you have two clear warning signs. First, your gauge edges ever closer to “E.” Then, if you keep pushing, you’ll stall.

But our bodies are much more intricate machines. When you underfuel your workouts—taking in less energy than you’re expending—you’ll get indicators that are a lot more vague and subtle, but also widespread. “How systemic it is is surprising—how deeply it can affect your whole body,” sports dietitian Holley Samuel, RD, CSSD, tells SELF.

Before we get into the signs and symptoms you can experience when you’re not eating enough, let’s take a step back to delve into what “underfueling” your fitness routine actually means—and why the sooner you catch it, the better for your health and performance.

“Underfueling” is more common than you may think—and you might not know you’re doing it either.

Despite constant messages to eat less and move more, researchers now know consuming too little for the amount of exercise you’re doing can cause its own set of problems. Failing to match your intake to your output may result in what’s called low energy availability, where your body runs short on the fuel it needs to power all your normal biological functions, let alone reach any of your performance goals, sports dietitian Rebecca McConville, RD, CSSD, author of the book Finding Your Sweet Spot, tells SELF.

Over time, this can progress into a condition called relative energy deficiency in sport, or REDs. And that can wreak havoc on just about every system in your body, from your brain function to your heart health to your reproductive system, as your body runs out of resources to keep things humming along smoothly, Samuel says.

Don’t let the word “sport” in the name throw you off. While these conditions are common among elite and pro athletes, everyday exercisers can absolutely develop them. For example, a study published last year in the British Journal of Sports Medicine found that out of 1,000 nonelite participants in the Boston Marathon, 42% of women and 17% of men had signs of low energy availability. Even far more casual runners, cyclists, and gym-goers—anyone who’s active—can underfuel too, McConville says. And this holds true for folks of all body sizes.

“Underfueling” doesn’t necessarily have to be intentional either. While eating disorders or disordered eating can cause low energy availability and REDs when you purposely restrict how much you take in or digest, it’s entirely possible to underfuel without knowing or realizing it. And it isn’t just about total intake—increasingly, researchers realize that slacking on carbs to fuel exercise can cause further problems, either with or without an overall deficit. (That’s one reason mid-run fueling is so helpful for runners—it can help you stay on top of your carb needs.)

To understand why, think of your body more like a bank account than a car, McConville says. Each time you eat, it’s an investment. Everything you do throughout the day, including your workouts, requires a withdrawal. Because unexpected energy expenses often arise—dashing to catch the elevator, fighting off a minor illness, eking out those few extra reps—it’s all too easy to create an unplanned gap between your intake and output.

Not only can this cause issues in the moment, it also raises your risk of longer-term health problems, including depression, osteoporosis, and heart damage. That’s why knowing underfueling is possible, and staying alert for red flags, is important.

If you have a menstrual cycle and aren’t on hormonal birth control, irregular or missing periods are a frequently cited sign. But that doesn’t apply to everyone, and there are ample red flags and harmful effects that might appear even before you notice changes in your cycle. Here are 15 ways to know you might be running on low.

1. Your mood is tanking, and you’re not sure why.

A few minutes of doomscrolling these days can easily make you anxious or sad. But if you’re generally an upbeat person and suddenly find yourself moody and lashing out, that could be a red flag.

“If we are hungry, we are cranky,” a.k.a. hangry, Stephanie Roth-Goldberg, LCSW, a New York City–based therapist specializing in sports psychology and eating disorder recovery, tells SELF. “If you take hangry and you make it more long-standing, it turns into being grouchy and cranky and just generally having a low mood.”

2. You think about food a lot—but often can’t make up your mind about what to eat.

When your body feels deprived, it’s easy for your mind to become preoccupied with thoughts of food, Roth-Goldberg says. By the same token, it can feel hard to think clearly about how to refuel when your tank is nearly empty. “Feeling hungry but unable to decide what you want, and then getting into a conflict—nothing sounds good, nothing feels good—can also be a sign,” she says.

3. You tend to go on binges later in the day.

Similarly, underfueling can kick off a vicious cycle: You skimp on fuel through breakfast and lunch. By mid-afternoon or evening, you’re famished enough to demolish your snack drawer. You feel bad, restrict yourself again the next day, then repeat. Not only does this not feel great in the moment, you may worsen some of the other consequences.

“If you give people adequate calories, but it’s spread throughout the day evenly and they’re fueling before and after workouts, they’re going to be in a good energy balance,” Samuel says. Read: They’ll feel pretty good and not notice many of these symptoms. But if they eat the same amount (or sometimes, more) but do it far away from their workouts, they might have symptoms of underfueling, because their body dips into periods of low energy availability.

4. Each workout feels way harder than it should.

Of course, there are a million variables that affect how you feel during any given run, Peloton class, or weight training session—from how much you did the day before to how you slept to whether you had a stressful day at work. But generally, if you’re relatively consistent with fitness, you should be able to keep up your routine with about the same amount of effort or to see signs of progress over time.

If instead you’re regularly struggling with moves or workouts that once felt easy, low energy may be the culprit, Heather Caplan, RD, a registered dietitian, running coach, and cofounder of the Lane 9 Project, tells SELF.

5. And you drag much more than usual afterward.

Yes, hard efforts—think long runs during half or full marathon training—can leave you feeling fatigued. But if you’re ramping up appropriately and fueling properly, you shouldn’t have to crash on the couch the entire rest of the weekend, Samuel says.

“You should be able to finish your long run—and, yeah, sure, be a little tired, but go do stuff with your kids, go run errands, go do what you have to do without feeling like you’re going to fall down,” she says.

6. In fact, you’d rather just bail on the gym altogether.

Regardless of your fitness goals—whether you’re preparing for a big event or working out for stress relief and well-being—it’s normal to feel ebbs and flows in motivation, McConville says. Even the most elite athletes have days when they’d rather not train.

But one sign of underfueling is an unexplained drop in your desire to work out or move, she says. So take note if sessions that once seemed fun to you suddenly lose their appeal, or you’re feeling tempted to skip out entirely more days than not.

“Tune into what your thoughts are saying,” Roth-Goldberg says. “If it’s a constant, ‘This feels so hard. This sucks. Why am I doing this? This is not fun,’ that is a sign of low energy.”

7. Your sleep sucks.

Although being undernourished leads to fatigue, you might struggle to fall asleep, or when you do nod off, you might find your shut-eye constantly disrupted. Episodes of low blood sugar can cause you to wake up feeling anxious in the middle of the night, McConville says. Your sleep quality also suffers, adding to the feeling of dragging; you might be exhausted even though you think you’re sleeping enough.

8. Your gut rumbles way more than it used to.

Underfueling can hamper the way your digestive system works and disrupt your gut bacteria, causing symptoms that include diarrhea, abdominal pain, constipation, and bloating. The experience can resemble irritable bowel syndrome (IBS), McConville says.

This, too, can kick off a harmful cycle. One common reaction to those unpleasant GI symptoms is to avoid particular foods or food groups, such as dairy or gluten, McConville says. But if you start cutting those items out without adding in other sources of nutrients, your energy levels can drop further, digging you deeper into a hole.

9. Meanwhile, your hormones and vitamin levels are all out of whack.

Bloodwork you get at the doctor may also show abnormalities of certain vitamins and minerals. “We get micronutrients from food, and if there’s a shortage of food, you’re going to have a shortage of micronutrients,” Samuel says. And merely popping a multi won’t correct the issue, because underfueling also decreases the absorption of key nutrients.

Take iron: When you’re skimping on fuel, your liver secretes a hormone called hepcidin, which interferes with your ability to soak up iron from foods like meat and greens, or from supplements, McConville says. As a result, your iron tests can come up low.

You may also notice abnormal levels of thyroid-related hormones, including thyroid stimulating hormone (TSH) and free T3. Your doc may flag this as a thyroid problem, but it can actually be one of the first signs of a fueling mismatch—not eating enough interferes with the way your pituitary gland, which produces these hormones, works.

10. Your libido sinks.

Speaking of hormones: Altered levels of estrogen and testosterone can decrease your sex drive—and that’s on top of the crankiness that may already be affecting your relationship.

If all of a sudden you notice you’re not as interested, that can be your brain trying to conserve energy, McConville says. “That should be viewed as a warning sign.”

Although women appear more sensitive to the negative effects of low energy availability, men are far from immune, Samuel says. Their libido might tank too, and if they usually get morning erections, those may disappear or become less frequent.

11. Your heart rate falls, while your cholesterol levels rise.

Most athletes and regular exercisers have a lower resting heart rate—but underfueling can slow things down even more, sometimes even below 30 beats per minute. Meanwhile, low estrogen levels can raise your levels of total cholesterol along with “bad” types of lipids, LDL and Apo B.

12. You’re gaining weight.

Conventional wisdom holds that eating less = shrinking your body. While some people with REDs have a low body weight, others experience weight gain as their metabolism slows to conserve energy, McConville says.

13. You’re constantly freezing.

Samuel has Raynaud’s syndrome, a condition in which blood vessel spasms cause your hands, feet, or other body parts to feel numb and turn colors in cold weather. She noticed her symptoms were much worse when she wasn’t eating enough, since low energy can interfere with blood vessel function.

In addition, abnormal levels of thyroid hormones could leave you feeling unusually chilly. “Low T3 is a means of dropping the thermostat down to conserve energy being spent on your body temperature,” McConville says. “That’s why people who are underfueling might be cold all the time.”

14. Your hair and skin might suffer.

Altered hormone levels, micronutrient deficiencies, and a lack of protein can also hit you in the head and hands. Some people notice their hair thinning or falling out, your skin might be a mess, and your nails may become brittle. And again, because it’s a bigger overall problem, “no amount of biotin is going to fix that,” Samuel says.

15. You get hurt more often, and healing takes longer.

Bone injuries have long been linked to disordered eating and eating disorders, especially in women. Not only might you not get enough of bone-building nutrients like calcium and vitamin D, low energy levels overall can result in hormone changes that weaken your bones and make them more prone to problems like stress fractures.

But increasingly, sports medicine experts recognize that underfueling can contribute to other issues too. Altered hormone levels can hamper your body’s ability to repair the minor damage caused by exercise, leaving your muscles, tendons, and joints prone to pain and injury. You’ll likely heal more slowly when you do get hurt too. “Always feeling like you are playing injury Whac-A-Mole can be a sign that you’re just not giving yourself enough to actually recover,” Samuel says.

Wait—this makes me realize I might be underfueling! What should I do about it?

Noticing a potential problem is a huge first step. Treatment involves rebalancing by eating more, exercising less, or both. Sounds simple enough in theory, but in practice, it can be tricky both logistically and psychologically—which is why it’s a good idea to loop in some pros if you can.

If it’s within your budget or covered by your insurance, book a visit with a registered dietitian or look for a sports medicine provider who’s knowledgeable about these topics—Caplan’s Lane 9 Project features a directory of them, and McConville also has a list of experts who’ve completed her REDs-informed provider certification program. You can also learn more by seeking out information on sports nutrition produced by people with appropriate credentials: for example, registered dietitians trained in sports nutrition (they’ll often have the initials CSSD after their names), Caplan says.

Depending on how long and how much you’ve been underfueling, it may take time, help from experts like dietitians and psychologists, and some extra intake to rebalance your accounts—think of it like paying off interest on a loan, McConville says. But although it can be hard to break existing habits, the payoff is more than worth it. After all, the effects of eating enough benefit just about every part of your mind and body too, Samuel says—for instance, strong workouts, good sleep, stable energy levels, and just generally being a happier and healthier person.

Related:

• How to Get Help for Disordered Eating, According to Experts
• You Might Not Be Hungry After a Workout—But You Should Still Eat Something
• My Mom Was a Magazine Editor in the ’90s. We’re Finally Talking About What It Did to Our Body Image

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