When you are trying to improve your fitness level, the most desirable weight loss is of fat. However, to lose fat takes time and effort. When weight is lost quickly most of the decrease is water loss. Some of the lost weight will be due to the utilizing of carbohydrates stored in the body, and some will be lean body mass tissue or muscle. Sweat is just water loss; it's not fat reduction.
Water Loss
Water makes up about 60 percent of your total body weight. That means that if you weigh 160 pounds your body contains about 12 gallons of water. During the course of a typical day you will lose and replace about 2.5 liters of water. However, this is significantly increased during exercise to the point where as much as 12 liters of water may be lost per day. If we estimate that for every 2-pound weight reduction is a loss of 1 liter of water, it's easy to see how rapid weight loss is really nothing more than fluid loss.
Fat Loss
A pound of fat contains 3,500 calories. It is recommended that a healthy weight-reduction plan requires losing between one and two pounds of fat per week. This means that to lose two pounds of fat per week you will have to burn off 7,000 calories in excess of what you are consuming through meals. To put that in perspective, someone who weighs about 180 pounds will burn off about 1,100 calories per hour when running at eight miles per hour. In order to burn off two pounds of fat they would have to run for almost seven hours.
Energy Stores
Energy is stored in the body as glycogen in the blood, liver, muscles and as fat. When exercising, this stored glycogen is the first source of energy converted by the body into energy. The higher the intensity of the exercise, the faster you use these energy stores. Unfortunately it is replaced with lactic acid, which will eventually prevent muscle contraction. These energy stores contribute about 1,200 calories, or about half a pound, to your total body weight. To actually burn fat from exericse requires a longer duration with a reduced intesity level.
Muscle Loss
Despite all the information available, when people want to lose weight they still tend to fast or dramatically reduce their caloric intake. Fasting will lead to weight loss, although the decrease will be lean muscle tissue. The reason for this is that when you severely limit calories, the body switches to starvation mode, meaning it will begin to use protein for energy. Since muscle is composed of protein, the body will convert the muscle tissue to energy. Rapid weight loss will be fluid loss, glycogen utilization and lean muscle tissue conversion. Fat loss requires a prolonged commitment to a fitness lifestyle.
References
- Nutrition Now; Judith Brown; 2002
- Fitness and Health; Brian Sharkey; 2002
- Mayo Clinic: Fitness For Everyone; Diane Dahm and Jay Smith; 2005
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