Measuring Fitness: Understanding Weight, BMI and Body Composition

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Credit: Ben Holland

 

Correction: An earlier version of this article incorrectly stated that you need a weekly deficit of 500 calories to lose a pound of fat in a week. The correct figure is 3,500 calories a week or 500 calories a day.

By Sarah McGowan

When you think about athletes and weight maintenance, you usually think about wrestlers trying to make their weight classes not quidditch players training during their summer season. Yet weight is an important element to any sport that can impact your speed, power and overall athleticism. Just remember: less isn’t always more. 

BMI and Body Composition

When you go for a yearly physical, the doctor will figure out your Body Mass Index (BMI) to determine if you are at a healthy weight for your height. BMI is essentially a ratio of your weight and height. As children, this is a perfectly acceptable way to judge health, but as we get older it becomes a little problematic. That’s because BMI just accounts for your weight, without any consideration of the variations in body composition.

Your body is composed of different tissues, which all vary in density, volume and ultimately weight. You have your skin, eyes, brain, bones, internal organs and water all contributing to that number you see on the scale. But the two most important tissues that contribute to your weight are muscle and the ever-dreaded fat. Muscle is more dense than fat, so a pound of muscle takes up a smaller volume than a pound of fat. That means you can actually lose fat, gain muscle, and get more fit—all while staying the exact same weight. Similarly, you can gain weight and not go up in pant-size because it’s just new heavier muscle showing up on the scale. The different density between muscle and fat make BMI an incomplete picture of health; you can actually gain muscle and lose fat, but ultimately gain weight which will make your BMI higher.

Scientists have tried to figure out ways to measure our body composition. The gold standard is underwater weighing. Muscle is denser than fat, so a person with more muscle will weigh more submerged under water than someone with more fat. That means scientists can compare your underwater weight with your land weight to calculate fat percentage. Of course, not everyone has access to a giant tank or an underwater scale. But most gyms will have other devices to calculate body fat percentage. There are skin-fold calipers which measure fat just underneath the skin’s surface, as well as bioelectrical impedance which use the conductivity of muscle fibers to measure how quickly a small electric current travels through your body. Both are harmless but have their limitations.  A skin-fold measurement has to be taken by the same individual every time, while bioelectrical impedance can be affected by your hydration levels and any food content in your system.

Because of the limitations of BMI, you should try to focus less on weight and more on your body fat percentage. Biological athletic males are usually between 6-13%, while females hover around 16-20% body fat. The average, acceptable percentages for biological males are 18-25% and 25-31% for females.

How Do I Change my Body Fat Percentage??

The best method to lose fat is similar to the methods most people take to lose weight. To lose one pound of fat a week, you want your daily calorie intake to be about 500 calories less than your calories burned. You can accomplish this by either decreasing the amount of calories you eat by 500, or burning 500 more calories a day. This pace—losing one to two pounds every seven days—is a healthy goal for weight loss. To gain weight, you essentially take the opposite approach.     

Don’t try to expedite the process through extreme restriction. Not only is this unhealthy, but it is ineffective. Extreme diets can deplete your body of essential minerals and vitamins, and make your body believe you are starving. When in starvation mode, your body will drop your metabolic rate and store any excess energy as fat for survival. If the calorie deficit is too great, it will also begin to break down muscle and use that as energy. Muscle is the most expensive tissue to maintain in your body, so in a time where calorie input is too low, the body will break down muscle for energy and to decrease metabolism. And you definitely want to keep that muscle; not only does it facilitate better athletic performance, but your muscle composition will actually affect your metabolism. More muscle means a higher resting metabolism, which means you will burn more calories throughout the day. In other words, extreme dieting may make you lose weight, but you will also lose muscle, making your journey towards fitness a whole lot harder.

To gain muscle, you should explore specific weight lifting programs and eat more to sustain that new level of activity. But eating more doesn’t mean eating everything and anything. To make gains, try to focus on consuming more protein and carbohydrates. Your body will use the protein to build muscle and the carbohydrates as fuel.

Another thing to keep in mind during your journey towards a healthier body: everybody’s process is different and there is no set of streamlined steps towards success. Some people are genetically predisposed towards fast metabolisms or better muscle composition. For instance, most biological females will convert energy to fat easier than males, while most males will build muscle easier than females. There are a lot of factors that go into fitness, and a lot of methods to track your progress, but with the right tools you will find your way to a happier, healthier weight.