What Happens To Your Muscles When You Don't Eat Enough Calories
The basics of weight loss are simple on the surface. You burn off more calories than you consume. That might mean skipping that post-lunch brownie and hitting the treadmill for about 30 minutes after work. You'll need a deficit of 3,500 calories to lose a pound of fat. Although everyone's bodies are different, you could lose up to a pound a week if you create a calorie deficit of 500 calories a day.
Say you want to lose that weight more quickly. You might be tempted to cut up to 1,000 calories a day, or cut 500 calories and do a hard spin class every day. Or you might be tempted to try some types of intermittent fasting that restrict you to 500 daily calories a few days a week. But your body is much smarter than you think.
According to the American Council on Exercise, your body needs a certain amount of energy from calories to sustain functions such as digestion and circulation. If you don't eat enough calories to sustain these functions, your body looks elsewhere for energy. Sure, it will draw some fat, but it will also pull apart some of your muscle that helps boost your metabolism.
Understanding your basal metabolic rate
Your basal metabolic rate (BMR) is how many calories your body needs to keep your body functioning. Everyone's BMR is different, depending on your body size, age, and sex. Although you can estimate your BMR or use a wearable tracker to detect it, women shouldn't reduce their calories below 1,200 calories. Men should consume at least 1,800 calories (per American Council on Exercise).
When your daily calories dip below your BMR, your body knows how to respond. Although it will revert to its glycogen stores for energy the first few days of a diet, eventually you'll burn fat for energy. Yet your brain will also need energy, and it's typically fueled by glucose rather than fat. That's when your body will begin to break down the stored protein in your muscles for energy, and you lose muscle mass in the process.
Over time, your body adapts to this calorie restriction by using less energy for your body's functioning. That can stall your weight loss, according to The Conversation. What's worse is that losing muscle also slows your BMR, which means you'll be burning fewer calories at rest once you return to your normal eating patterns (per Better Health Channel).
Lose the fat, keep the muscle
Even if you have more fat to lose, you're still as susceptible to losing muscle at the same rate as someone who needs to shed a few pounds. If you want to lose weight without losing muscle, you'll need to strength train and add 1.2 to 1.5 grams of protein per kilogram of body weight to support muscle growth (per The Conversation).
If you're into intermittent fasting, you also might be losing muscle, according to Jefferson Health. It depends on what type of intermittent fasting you do. Restricting to 500 calories for two days a week or having an eating window of less than eight hours could cause you to lose muscle as well as fat. Strength training at least two days a week can help you preserve some of your muscle while intermittent fasting.
Healthline suggests avoiding drastic weight loss methods. You're much more likely to lose more muscle if you're trying to lose weight too quickly. It's best to understand the fine line between having enough calories to sustain your body's essential functioning while keeping the muscle and cutting enough calories to see results. Exercise is an important component of weight loss, but exercising too much can wreck your body and zap your energy for other times of the day.