Every guy wants to get leaner. He wants his pecs to stop jiggling and his abs to come out from hiding.

But there comes a point when shedding fat isn’t healthy. According to Pennsylvania State University, the average guy’s body fat is 15-20, 3% of which is “essential,” meaning you’d die without it.

“A body-fat percentage below 5% is regarded as a warning sign of poor health, even in elite athletes,” says Georgie Fear, R.D., author of Lean Habits for Lifelong Weight Loss. “Male bodybuilders typically go below 5% for competition, but don't stay there in the off-season.” That’s because body fat is integral to keeping your entire body—including your cardiovascular, endocrine, reproductive, skeletal, and central nervous systems—in check. Without enough body fat, they all go haywire.

Check out 15 ways your body freaks out when your body-fat percentage drops too low.

1. You become prone to serious heart problems

“Having an extremely low body-fat percentage can affect the cardiovascular system’s ability to function normally,” says cardiologist Kevin Campbell, M.D., fellow of the American College of Cardiology. For example, in one study published in the International Journal of Sports Physiology and Performance, when bodybuilders prepared for competition by lowering their body fat, their heart rate dropped to 27 beats per minute. Too-low heart rates, called bradycardia, can lead to dizziness, passing out, and cardiac arrest. Meanwhile, other electrolyte imbalances due to too-low of a body-fat percentage, not to mention caloric intake, can lead to “cardiac arrhythmias and sudden cardiac death," he says.

2. Your energy levels plummet

“When you are fat-depleted, you have no reserve energy stores and your body can't perform at optimal levels due to the fact that it's starving of energy,” Campbell says. What’s more, low levels of body fat are also linked with slow heart rates as well as drops in thyroid hormone production, which can further contribute to fatigue, Fear says.

3. You’re suddenly always cold

Is it cold in here or is it just you? “Fats play a role in helping your body maintain its temperature and also providing insulation for organs,” says Campbell. “People with ultra-low body fat can complain of being cold all the time.” Thyroid dysfunctions, which can also spring up in guys with too-low body-fat percentages, could also dial down your internal thermostat.

4. Your workouts suck

“After about 20 minutes of exercise, your body can begin to run out of carbohydrates for energy and turn to fat as an energy source. If you have too little fat available for energy, your athletic performance will suffer significantly,” Campbell says.

5. Your muscles don’t recover from exercise

Even if you’re able to get in a good workout with your ultra-lean figure, you’ll probably have difficulty getting much out of them. That’s because when your body-fat percentage is too low, so are your levels of glycogen, or stored carbohydrates in your muscles and liver. That glycogen is vital to helping your body recover from exercise, says exercise physiologist Mike Nelson, Ph.D., C.S.C.S.

6. You’re constantly hungry

The more you live on chicken breast, broccoli, and air, the better pizza sounds. “Dieting down to low body-fat levels decreases circulating levels of leptin, a hormone produced by body-fat cells. Leptin receptors in the hypothalamus sense this drop and increase appetite so that you're hungry all the time, Fear says.

7. Your testosterone levels drop

Low leptin levels can mean even lower testosterone levels. Through a chain of signals in the brain’s hypothalamus and pituitary gland, low leptin levels tell your testicles to produce less testosterone,” says Spencer Nadolsky, D.O., a board-certified family and bariatric medicine physician in Maryland. For instance, in the International Journal of Sports Physiology and Performance study of bodybuilders, during preparation for competition the average guy’s testosterone declined from 9.22 to just 2.27ng/mL.

8. Your muscles become weaker

Between sucky workouts, poor muscle recovery, and super-low testosterone levels, muscle gain is all but impossible in guys with too-low body-fat percentages, Fear says. In that same International Journal of Sports Physiology and Performance study, bodybuilders actually dropped strength and muscle mass in the months before competition. It took more than six months to get their muscle back.

9. Your sperm count—and hard on—drop

Too-low leptin and testosterone levels can result in hypogonadotropic hypogonadism or secondary hypogonadism, in which the reproductive system basically shuts down. “Your body gets the message that it’s time to survive. It’s not time to reproduce,” Nadolsky says. Your production of sperm drops significantly. But, if you’re trying to get your partner pregnant the old-fashioned way, that might be irrelevant since you’re probably suffering from trouble getting it up.

10. You get sick a lot

“Extremely low body fat and energy intake is associated with higher cortisol levels, which hamper the immune system, increasing your rick of bacterial infection or contracting a cold or flu virus,” Fear says.

11. Your bones become brittle

When your body-fat percentage and weight drop too low, you interfere with your body’s ability to properly absorb and use calcium and vitamin D. “Athletes in weight-class sports who strive for extreme levels of leanness have been shown to have lower bone density, in some cases being diagnosed with osteoporosis,” Fear says.

12. You don’t have a life

“Achieving very low levels of body fat requires lifestyle changes that most guys don't want to make,” Fear says. “Avoiding alcohol intake, preparing all of one's food and never eating out, and rigidly tracking each gram of food consumed add up to a more-than-full-time job that doesn't even come with a paycheck."

13. You’re always in a bad mood

If the social isolation and all-consuming calorie-counting that comes with maintaining a low body-fat percentage doesn’t tick you off, the low levels of fats in your brain sure will. “Fats are important for neurologic function. Having low levels of essential fatty acids that your brain badly needs for energy can result in irritability and severe mood swings,” Campbell says.

14. You suffer with brain fog

Low levels of essential fatty acids in the brain can also result in trouble concentrating and mental fatigue, says Campbell. Meanwhile, generally low energy levels don’t help you plow through your office work any better.

15. Your skin looks like crap

“Fats in your diet and your body are important for healthy skin," Campbell says. What’s more, since people who are trying to look extra-lean often aren’t eating a lot of water-storing carbs, dehydration is common. When you’re dehydrated, your body sucks water from your skin, rerouting it to your vital organs. Your skin becomes dry and splotchy.