The Biggest Diet Mistake That Can Wreck Your Metabolism Isn't What You'd Expect

Social media offers plenty of conflicting diet advice that can easily leave you confused. One TikTok influencer could tell you to fast once a week for metabolic health, while someone on Instagram swears by eating several small meals a day to keep your metabolism stoked. You might blame your genes or hormones for your metabolism.

Whether you're following an intermittent fasting diet, low-carb diet, or paleo diet, you'll need to restrict your daily calories to some degree to lose weight. But don't restrict your daily calories by too much. One of the biggest diet mistakes you can make is not eating enough. Consistently eating fewer calories than your body needs to function can slow down your metabolism. That means your body gets used to that low number of calories, so you don't lose as much weight. For example, you might have lost weight quickly by eating less than 1,000 calories a day, but eventually you'll see your weight loss stall as your metabolism adjusts to this number of calories. Not only that, but excessive calorie restriction can lead to chronic health issues.

If you want to lose a pound a week, you'll have to shave 3,500 calories from your weekly calorie intake. You could shave about 500 calories a day or 700 calories on the weekdays. If you're aiming to lose weight more quickly, you might run into some metabolism problems if you're dipping below your basal metabolic rate (BMR). Your BMR is how many calories your body needs to maintain your breathing, digestion, cellular functioning, and circulation.

If your BMR is 1,300 calories and you consistently dip below that, your body has less energy to maintain its healthy functioning. You might also notice your energy levels significantly drop, making it harder for you to find the motivation to exercise. You're also more susceptible to stress because extreme calorie restriction releases more cortisol, which cues your body to conserve energy by storing more fat.

That slower metabolism might persist longer than your diet. Although your BMR will be lower when you lose weight, your metabolism might have a tough time speeding up again. Contestants on "The Biggest Loser" might inspire you to go to extremes, but some of them had a hard time keeping the weight off because their metabolism remained stalled. According to a 2016 article in Obesity, "The Biggest Loser" contestants had a much slower metabolism compared to what might be expected for their new weight and age.

Most people associate metabolism with weight loss, but your metabolic health looks more widely at your risk of chronic diseases such as heart disease, type 2 diabetes, and stroke. Metabolic syndrome considers your blood pressure, blood sugar, waist circumference, and cholesterol levels, so it is linked to your diet, activity level, and weight. Not eating enough calories for your body's necessary functions can also weaken your immune system.

A moderate calorie restriction can improve your cognition, but fasting or severe calorie restriction can affect your brain's ability to process information quickly, according to a 2025 article in Nutrition Reviews. This includes people who practice 5:2 intermittent fasting or those who will fast for 20 hours. Fasting inhibits your ability to shift your focus between tasks and slows your reaction times.

Cutting too many calories can also wreck your gut health. A 2021 article in Nature showed that low-calorie diets made laboratory mice lose weight, but they also disrupted the balance of their gut microbiome. One type of gut bacteria, Clostridioides difficile, played a role in weight loss, but it also made it difficult for the mice to absorb necessary nutrients from food.