When we want to shed pounds, we usually think in terms of calories. After all, the many calorie-counting apps out there would have us believe that slashing our intake is the only way to make weight loss happen. But cutting too many calories can actually have some dire consequences—and it’s easier to do than you might think.

If you’re eating too few calories for your body and lifestyle, though, your body will send you some major signals (long-term weight-gain, included!) that you need more fuel, says sports dietitian Kimberly Feeney M.S., R.D., C.S.S.D., L.D., C.S.C.S. The following six signs indicate your body is undernourished and begging for more calories.

1. You Feel Like A Sloth

If you notice a slip in your overall energy level and declining performance during your workouts, it could mean your metabolism is slowing down because you’re not eating enough calories, says Jenny Mahoney, R.D., L.D., of Nutriformance. We all have a baseline number of calories our body needs in order to maintain basic functions like making our heart beat, brain work, and lungs pump oxygen. (This is known as our ‘basal metabolic rate.’)

To do anything beyond just staying alive—like move or work out—our body needs additional calories. So when we cut calories too close to that basal metabolic rate, our metabolism slows down so we can survive off the little energy we do get, Mahoney explains. “Even if cutting calories is a choice we make in an effort to lose weight, our body still treats it as a famine and begins slowing down metabolic processes to preserve fuel,” she says. And so we feel tired and slow.

2. You Can’t Focus

If you find yourself zoning out even outside of boring meetings, insufficient calories may be to blame. That’s because your brain demands a constant supply of fuel—particularly glucose (a.k.a. sugar), says Megan Casper, M.S., R.D.N., owner of Megan Casper Nutrition. In fact, up to 20 percent of our daily calories and half our available sugar goes to our brain, according to Harvard Medical School.

If you don’t take in enough calories, your blood sugar drops, impacting your brain function and messing with your memory and ability to pay attention, according to Casper. A surefire way to tell if your brain fog is because of low blood sugar: Drink a small glass of orange juice, which contains easily-digestible sugars, and note whether your brain power perks up. Feel more awake and productive? You’re likely not eating often enough, not eating enough overall, or both.

3. You’re Sore ALL The Time

In addition to feeling sluggish during your workouts, you may also find it harder to recover from exercise if your calorie consumption is too low. While some soreness is normal after a tough workout, consider it a red flag if it persists for close to a week, says Feeney. Same goes if you’re a regular exerciser and feel sore when you normally wouldn’t.

“When we consume too few calories below our total daily needs, our body prioritizes what it uses that energy for,” Feeney says. And healing is one of the first things to get the boot. Long-term, exercising regularly while falling short on fuel puts you at greater risk for injury—particularly for stress fractures.

4. You’re Not Making Muscle Gains

If you notice your muscle tone stall or even start to decline, consider it yet another sign that you may not be eating enough calories to fuel your workouts and build muscle—even if you’re strength training, says dietitian and personal trainer Lauren Manganiello M.S., R.D.N., C.D.N., C.P.T. “When we don’t get enough calories, muscle begins to break down because our body is looking for sources of energy,” Manganiello explains. Our body stores carbs as glycogen in our muscles to use as energy later—but when we don’t have enough glycogen stored, our body may break down the protein in our muscles for fuel. So if you’re not getting stronger, struggling through your strength training, or even feeling a little flabbier than usual, there’s a chance you’re not eating the calories your body needs to make progress.

5. You’re Eternally Grouchy

It’s probably no shock that eating too few calories can leave you ‘hangry.’ In fact, mood swings are one of the top signs you’re not taking in enough calories because they’re caused by dips in blood sugar, says Manganiello. Get this: research out of Florida State University found that our self-control itself requires energy, and we’re more likely to snap or lash our when our blood sugar is low and we’ve been dieting hard.

Even just monitoring our calories spikes how stressed we feel, and actually restricting them boosts our production of the stress hormone cortisol, according to research published in Psychosomatic Medicine.

6. You Can’t Sleep

A whacked-out sleep schedule is another major red flag that you’re not eating enough calories. If you feel hungry enough at bedtime or overnight that you have trouble sleeping, your calories are too low, says Manganiello. “Hunger is our body’s way of telling us that we need energy,” she says.

Related: Try adding a casein supplement to your routine to fuel muscle gains in your sleep.

How To Make Sure You’re Getting Enough Calories

If any (or all) of these struggles hits close to home, it’s time to up how many calories you’re eating each day. Your caloric needs depend on your height, weight, activity level, and body composition (how much of your weight is lean mass, like muscle, versus fat), so meeting with a dietitian is one of the most accurate ways to figure out your daily calorie target. But a reputable online tool, like the USDA’s MyPlate Super Tracker, or some quick math can provide a ballpark estimate of your calorie needs. Try this simple formula: Multiply your weight in kilograms (one kilogram is 2.2 pounds) by 20 to estimate the low end of your calorie range and by 25 to estimate the high end, says Casper.

How many calories you can cut healthily depends on how many calories total you’re starting with, but the average person can safely lose about a pound a week by cutting 500 calories per day, says Feeney.  And as a general rule, though, women should never eat fewer than 1,200 calories per day, while men should never eat fewer than 1,800.

If you’re too deep in the calorie-cutting trenches, you’ll need to gradually up your calorie intake until you’re meeting your calorie needs. If you need to up your intake by hundreds of calories, add about 100 calories to your total intake every few days to ease your body into consuming more energy, Mahoney recommends. If you only need to add about 200 calories or so, though, just go for it. Just remember that the quality of the calories you’re adding matters, and focus on eating more produce and whole-grain carbs instead of processed foods, Mahoney says.

Casper also recommends adding light snacks in between meals, or eating smaller, more frequent meals throughout the day to keep your blood sugar and energy stable. And make sure to include protein, fiber, and some healthy fats in every meal or snack to keep your belly satisfied, which can help you maintain or lose weight over time.

Related: 6 Tips For Losing Weight Without Counting Calories