What is the main reason why your body weight might fluctuate from day to day?

The primary reason for daily fluctuations in body weight is often related to changes in body water. This can occur due to various factors, including hydration levels, sodium intake, physical activity, and hormonal changes. For instance, consuming a salty meal can lead to water retention as the body holds onto fluid in response to increased sodium levels. Similarly, fluctuations in hydration from sweating during exercise or changes in fluid intake throughout the day can also significantly impact body weight.

While changes in body fat, muscle mass, and food intake can influence weight over a longer period, they typically do not cause the rapid daily variations seen with water weight. Body fat changes occur gradually due to consistent caloric surplus or deficit, and muscle mass takes time to change significantly. Food intake can affect weight, but the immediate impact is often due to the weight of the food and fluid content consumed rather than its energetic value being translated into weight gain or loss on a day-to-day basis. Therefore, changes in body water stand out as the leading cause of daily weight fluctuations.