Since we all store fat in different areas of the body, this method cannot be considered perfect, but it will provide you with a point of reference and a general idea of your body composition. To the contrary, losing fat generally means that you must use more energy each day than you are eating, putting you in a negative caloric balance. In addition to burning body fat, dieting also causes the body to use some of its protein for energy, which can result in a sizable breakdown of muscle protein. So, while you may lose some excess body fat by restricting your food intake, you can also lose muscle tissue, leading to a decline of muscle strength and swimming performance. Consequently, if you intend to lose weight to improve your swimming, make sure your dieting is done during periods of the year when your training can be at relatively low intensity and when you don't intend to compete.
Body composition is only one of the considerations for optimal swimming performance.
Dr. David Costill is the director of Ball State University's Human Performance Laboratory and is current chairman of the USMS Sports Medicine, Health and Safety Committee. A Masters swimmer for the past 12 years, he has won numerous national freestyle and individual medley championships.