What primarily accounts for the initial weight loss when experiencing significant caloric restriction?

The initial weight loss experienced during significant caloric restriction is largely attributed to the reduction of total body water and the depletion of glycogen stores. When an individual reduces caloric intake substantially, the body first depletes its glycogen reserves, which are stored in the liver and muscles. Glycogen is bound with water; therefore, as glycogen is utilized for energy, water is released and excreted. This leads to a noticeable drop in body weight due to the loss of water weight and glycogen.

As the body continues to adapt to the caloric deficit, later stages may involve fat loss and, to a lesser extent, muscle loss, but the very first phase of weight reduction primarily consists of this initial water loss associated with glycogen depletion. Thus, focusing on the interplay between glycogen stores and total body water provides a clear understanding of why this phase of weight loss occurs and establishes the groundwork for understanding metabolic adaptations during longer-term weight management strategies.