Answer:
The majority of weight loss in the first week of fasting or crash dieting is due to water weight loss rather than fat loss. When a person consumes fewer calories than their body needs, the body starts using up its glycogen stores, which are made up of glucose and water. Each gram of glycogen is stored with approximately 3-4 grams of water. As the glycogen stores are depleted, the water that was stored with them is excreted from the body, leading to a rapid decrease in weight.
However, it is important to note that this initial weight loss is not sustainable and does not reflect actual fat loss. In fact, rapid weight loss through fasting or crash dieting can cause the body to go into starvation mode, where it conserves energy by slowing down metabolism and burning muscle tissue instead of fat. This can lead to a loss of muscle mass, which can have negative effects on overall health and weight management in the long term.