Unintentional weight loss may result from loss of body fats, loss of body fluids, muscle atrophy, or even a combination of these. It is generally regarded as a medical problem when at least 10% of a person's body weight has been lost in six months or 5% in the last month. Another criterion used for assessing weight that is too low is the body mass index (BMI). However, even lesser amounts of weight loss can be a cause for serious concern in a frail elderly person.
Is a calorie truly a calorie? Not always, because different types of calories can affect your body and your results differently. Dietary fat, for example, is more "fattening" than protein or carbs because it's less likely to be used to build your body. Granted, carbs can potentially make you fat, but they also directly fuel your training. Protein? That's a no-brainer: It builds muscle. Fat does neither, but it isn't useless; moderate amounts of it support vitamin absorption and help manufacture hormones.
At its core, burning fat comes down to the process of lipolysis—the breaking down of fat lipids, explains Seedman. This happens in the mitochondria of the muscles, or the powerhouses of the cells, responsible for generating the energy our cells need to do their jobs. Exercise has been shown to improve mitochondria function, which then promotes fat breakdown, Seedman adds. Plus, working out helps regulate pretty much all the hormones that optimize fat loss.