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).