Final answer:
Yes, when a person goes on a diet and loses weight, they lose mass because weight loss involves the body breaking down its stored energy sources like fats, which reduces body mass.
Step-by-step explanation:
When a person goes on a diet and experiences weight loss, they do indeed lose mass. Mass is a measure of the amount of matter in an object, which includes the body. Weight is the force that gravity exerts on that mass. Weight loss occurs when the body uses up more energy than it takes in, leading to a deficit that the body compensates for by breaking down its stored energy sources, like fats and carbohydrates, which reduces the body's mass.
Fundamentally, weight loss is about the balance of calories consumed versus calories expended. In order to lose weight, a person must consume fewer calories than they use. This leads to the body metabolizing fat stores for energy, causing a reduction in adipose tissue mass - and therefore a reduction in overall body mass. Muscle mass can also decrease during weight loss, especially if the diet is not well managed or if the person is not engaging in strength training.