If you're very active, watch the pounds you shed

It's true that it doesn't take too much extra weight to impede physical performance. Try running with a 10-pound sack of potatoes strapped to your back and see how you fare. But as we strive for optimal physical fitness, it's also true that you can get too caught up in the national obsession with body fat.

Fat is an essential component of the human body. Indeed, fat, specifically lipids, forms the membrane of every human cell. It naturally follows that too little body fat can have health consequences just as too much can, and recent studies are uncovering some of those downsides.

Dr. Bob Ross, an associate professor in Queen's University's School of Physical and Health Education, and a member of the Canadian Society of Exercise Physiologists, is conducting research into body fat in conjunction with physiologists in Australia. He has been studying both elite male marathon runners, who sometimes drop as low as four per cent body fat (compared to 16 per cent for an average male) and women distance runners, who often hover around 15 per cent (compared to 20 per cent).

It's been known for some time now that these ultra-thin athletes often compromise their immune systems, and that low body fat can affect women's reproductive abilities. "We now know that body fat has a role to play in what we call the aromatisation, or the conversion, of certain sex steroids, especially in young women," says Dr. Ross. "We see disturbances in their normal menstrual cycle due to these fluctuations of hormones as a consequence of excessive running, poor nutrition and very, very low body fat." And the effects are not limited to women. One former Olympic marathon champion was unable to impregnate his wife until he reduced his training volume and his body fat percentage increased slightly.

Recent research from Laval University's School of Nutrition has also discovered significant negative side effects from sudden weight loss. Dr. Angelo Tremblay, who has been studying the results of body-fat reduction for eight years, has observed a variety of complications following dramatic weight loss. These include a decrease in hormone production, a lower caloric expenditure during rest and a marked decrease in skeletal muscle enzymes leading to lower energy levels. "These are associated with a more pronounced weakness of our furnace," says Dr. Tremblay. "Basically this is not what we dream about…after weight loss."

Research has also shown that shedding most of one's body fat can release toxins into the system. The toxins, known as organochlorines, are stored in the body's fatty tissue, and as more body fat is burned, they become hyperconcentrated and enter the bloodstream.

So how do you know how fat you should be? The Canadian Society of Exercise Physiologists still recommends using the Body Mass Index in conjunction with a waist measurement (taken midway between the hips and the rib cage). The BMI is simply a ratio of body weight to height. For most men with a waist measurement under 40 inches, a BMI of 18.9 to 24.9 is considered normal. If you're above that, it's probably time to start cutting out the Pringles and the Häagen Dazs. If you're below it, then you might want to head down to the fridge right now.
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