Thursday, September 6, 2007

Some Fitness Math

Over the next few posts I want to shift from motivation and examples for losing weight to some more quantitative information that's helpful in evaluating your needs and in setting appropriate goals. In particular, there are a few numbers that anyone concerned about their weight should know. First, what do you weigh? Simple enough, just get on a scale. Second, how much should you weigh? Third, how many calories per day do I need to maintain my current weight (which will help you know how much you you should be targeting if you want to lose weight.)

Am I a Healthy Weight?

One common and useful measure of the body's relative weight is the Body Mass Index (BMI). Online BMI calculators may be found at Free Dieting and HealthLink. This measures a the persons weight relative to their height. It is an important number to know because life expenctancy decreases for those with sustained BMI of 26 or more. (People who are overweight or obese are at higher risk for chronic conditions such as high blood pressure, diabetes, and high cholesterol, but you probably already know that.) Waist size above 40 inches (for men) or above 35 inches (for women) is also associated with increased health risk. The CDC has a BMI calculator for children and teens as well as adults.

Calculation of your percent body fat is another good indicator of whether your current weight is healthy. A useful body fat percentage calculator may be found at the freedieting website.

Several sites put together this information to give an ideal weight. In reality, any weight where you feel good about yourself and are in the healthy range for body fat percentage is a great weight.

How do I Determine My Body's Daily Calorie Needs?

An easy way to learn how many calories you need each day is to use this online calculator which accounts for age, gender, weight, height and activity. Another good one is the Mayo Clinic calorie needs calculator. The Healthy Body Calculator gives both caloric needs and body mass index. There are many other calculators or simple formula, but if they don't include factors like weight or activity level, they're over-simplified.

For you engineers out there, the Health Recipes website has a very good article on "How to Determine Your Body's Daily Calorie Needs." It covers this topic in considerable detail. The question is tougher than it looks because the body's needs depend on its basal metabolic rate (BMR), energy expended during physical activity, and the thermic effect of food (energy required to digest and metabolize food, about 10% of your caloric intake). A calorie needs calculator that lets you pick the formula to use may be found at the freedieting website. This website also has an ideal weight calculator and BMI calculator as well. There is also a weight loss calculator that will give the date you should expect to hit a target weight for a variety of levels of daily caloric deficit. (Hmm, it seems that I like the Harris-Benedict formula, which overestimates my caloric needs by about 200 calories compared to what is probably more accurate, the Mifflin-St Jeor formula) What I like about this calculator is that it gives a rock-bottom value for how many calories per day I should not go under to avoid fat starvation effects.

Next time we'll look at how this information is useful for weight loss, followed by discussion of the impact of exercise on weight control.

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