In a calorie deficit diet, not only do you lose fat, but you also lose muscle, which can have serious consequences for your health. When you start a diet, your body initially uses up its glycogen stores, which releases water and leads to early weight loss. Once these stores are depleted, the body turns to metabolizing fat for energy. However, the body cannot use fat for all tissues, including the brain, so it starts breaking down muscle tissue for energy. This muscle loss can slow down your metabolism and contribute to weight regain. While exercise and a high-protein diet can help preserve muscle mass during weight loss, excessive protein intake can have negative effects on metabolism and organ health. Ultimately, sustainable diet and lifestyle changes are crucial for long-term weight management.
Source: https://theconversation.com/weight-loss-why-you-dont-just-lose-fat-when-youre-on-a-diet-209258