I’ve heard that, when people lose a lot of weight, it’s just water weight. Is that true?

When people lose weight, they lose a combination of fat, water (body fluid) and some muscle tissue. This is a normal and necessary process. Whether the weight is lost through low calorie dieting or vigorous exercise, the initial weight loss is composed of a higher percentage of water, particularly in the first week. One study showed that, on the first three days of a diet, 70% of weight loss is from water. But that changes quickly, accounting for the slowed weight loss after the first week or two. By day 24, 85% of the weight loss is from fat.