Function

Weight loss is basically a product of supplying your body with too few calories to meet its energy demands. This forces your body to draw from its stores of fat, glycogen and muscle protein to make up for the shortfall, which leads to weight loss.

Types

Weight loss is the result of a drop in your body's fat, water or muscle mass. Often times it's some combination of the three.

Misconceptions

It's often wrongly assumed that any weight lost is good weight loss. Fitness is usually improved as a result of reductions in your body fat mass. But weight loss as a result of the loss of muscle mass is usually not healthy.

Timeframe

According to fitness experts at the American Council on Exercise, weight loss should proceed at a pace of about 1 to 2 pounds per week. In 40 days this amounts to between 6 to 11 pounds of weight loss. This slow but steady rate of weight reduction is the best way to ensure that most of the weight you're losing is coming from fat and not muscle or water.

Alternatives

With a very rigid diet and/or extreme exercise program it is possible to lose more than two pounds per week, but much of that loss will likely come from water and muscle mass. Losing muscle mass actually makes it easier for you to gain more fatty weight.

Source:

UC: Healthy Weight Loss

FamilyDoctor.org

Resource:

American Council on Exercise