You’ve heard it a million times before: Weight loss comes down to the simple equation of ‘calories in versus calories out.’ Burn more calories than you take in—usually by eating less and working out more—and watch the pounds melt off, right?

“In theory, if you consume fewer calories than you expend, you should lose weight; and if you do the opposite, you should gain weight” says David Greuner M.D., of NYC Surgical Associates. Many people, though, make a calorie-cutting mistake that actually sabotages their weight loss—and that’s restricting calories too much.

Why Eating Fewer Calories Doesn’t Mean Shedding More Pounds

We all have a unique metabolic rate (the number of calories our bodies need throughout the day), which is influenced by factors like gender, age, activity level, and muscle mass.

“The higher your metabolic rate, the more calories you’re burning,” says Leah Kaufman, M.S., R.D.N., dietitian for NYU Langone Health’s weight management program. (Even when you’re doing nothing!) When you restrict calories consistently, though, your metabolic rate drops—and the more drastic the calorie restriction, the more drastic the metabolic spiral, she says.

This incredibly frustrating cause-and-effect actually stems from our caveman days, says Deepa Iyengar, M.D., associate professor at the McGovern Medical School at The University of Texas Health Science Center at Houston. Basically, when you don’t eat enough, your brain thinks you’re starving, and your body holds onto every calorie it’s given, she says. (This came in handy when our cavemen ancestors couldn’t hunt or gather enough food.) Your metabolism slows down to a sluggish rate, and even though you’re trying to lose weight, your results screech to a halt. You may even start to break down muscle for fuel.

Related: 5 Myths About Your Metabolism—Busted

And if your extreme calorie-cutting is also paired with lots of intense exercise, you put yourself at risk for a scary condition called rhabdomyolysis, in which your muscle breaks down so rapidly that you’re left with severe muscle pain, weakness, vomiting and confusion, and potential kidney failure, says Greuner.

How to Tell if You’re Cutting Too Many Calories

As great as losing a few pounds sounds, going in to ‘starvation mode’ or risking your health isn’t so hot. If you’re going too far with calorie-slashing, the first signs you’ll notice are low energy, headaches, and fatigue, says Kaufman. Your mood may also take a hit, so you may feel irritable or depressed, or have trouble concentrating, adds Greuner.

And, of course, you’ll probably feel hungry all the dang time, because your calorie shortage causes your body to release hormones like ghrelin, which signal to the brain that you need some nourishment, pronto.

You can start to experience these symptoms as soon as you cut anything more than 500 calories per day, Iyengar says. But if your caloric intake dips below 1,000 calories a day, you enter into a real danger zone and risk damaging organs like your heart and kidneys, she adds.

Get Your Calories Back in the Safe Zone

Understanding the base number of calories your body needs to function (even if you lie in bed all day) can help you quit your extreme calorie-cutting ways. A qualified health professional can help you calculate your exact minimum needs with a machine that measures your oxygen consumption, which indicates your metabolic rate, says Kaufman.

Otherwise, you can use an online calculator from a medical or health organization (MyFitnessPal has an easy and free one) to estimate your daily calorie needs. Just keep in mind that this is the base number of calories your body needs to stay alive and do nothing else—not how many you should eat to lose weight. You’ll need additional calories to fuel daily activities and exercise. (A dietitian or doc can help you figure out the exact number.)

Men generally need more calories than women because they have more muscle mass, and therefore higher metabolisms, says Iyengar. Active men under age 55 who exercise for about 45 minutes four times a week should start with a baseline of 2,500 calories per day, while active women under 55 should start out at 2,000, she recommends. From there, if you want to lose weight (at about one pound per week) you can reduce your daily consumption by up to 500 calories, but not more than that, says Kaufman. Keep a food journal or use a food-tracking app to make sure you’re getting what you need, she suggests.

Taking this more moderate approach will help you lose weight safely—and sustain it. “You cannot survive on 800 calories a day for the rest of your life. It’s just not possible.” Getting enough calories will keep your body nourished so that you feel strong (instead of totally drained) when you exercise—which is a key piece of any sustainable weight-loss plan, says Greuner.

And one final tip for the road: When you’re in a (healthy) calorie deficit, it’s also important to consume enough protein to support your muscles and eat a variety of fruits and vegetables to ensure you’re getting all of the vitamins and minerals you need, says Kaufman.

Related: Shop multivitamins and minerals to make sure your nutritional bases are covered.