Fit or Fiction: Can BMI Determine Your Ideal Weight?

I've heard that body mass index is the best way to determine my ideal weight. True or false? --Josephine Marino, N.Y.

What's your dream weight? Chances are, you have some number tucked away in your brain, and maybe you've even started a program to achieve your goal. But how did you arrive at that magic number? Is it what you weighed in high school? Is it what Jennifer Aniston says she weighs? Is it what you think you should weigh?

Actually, there is a whole range of numbers that tell you about your level of "fatness" or "slimness." They each reveal something slightly different about your health filtered through the lens of how much body fat you carry on your body and where it's stored. Body mass index, or BMI, is just one of those numbers. To help you understand how all these measurements fit together, let me list and describe them in order, from least helpful to most helpful in terms of helping you figure out what the heck you should weigh.

Scale Weight: Your weight doesn't reveal how much of your body consists of fat (as opposed to, say, muscle, bones, blood, and organs). This is why two people of approximately the same height and weight can be vastly different dress sizes. Your scale weight may give you a rough sense of whether you're heading in good direction, but it isn't a complete picture.

Body Fat Percentage: Also known as a body-composition, this measure tells you how much of your weight is composed of fat and how much is composed of lean body mass -- including muscle, bones, blood, water, organs and connective tissue. There are more than a half dozen testing methods and most of them require help from a doctor or personal trainer. The general consensus is that men should not exceed 20 percent body fat and women should not exceed 30 percent. However, because your disease risk is influenced by the location of your fat, it's impossible to equate a body-fat score with increased risk of disease. A woman who is 30 percent fat but carries the bulk of her fat in her hips or thighs is likely to be at lower risk for diabetes and heart disease than a woman with the same body fat percentage who carries her weight in the middle.

Body Mass Index: This measurement is a way of relating your height and weight to determine how "fat" you are. Click here to calculate your BMI. If your BMI is 25 or over, you are considered overweight. BMI is a good, though not perfect guide, for determining whether or not you should lose or gain weight. For example, BMI measurements for extremely muscular athletes, very thin and very obese people are not very accurate. It doesn't distinguish between sexes or ethnicity, either.

Waist to hip ratio: This is a measurement that uses the circumference of your waist and girth of your hips to determine whether or not you should consider yourself overweight. Here's a link to a waist-to-hip ratio formula. To be considered healthy, women should have a waist-to-hip ratio of less than 0.8 and men should have a ratio of less than 0.9. This means that "pear shaped" people who carry their weight further south on their bodies tend to be healthier than "apple shaped" people who hold their weight in the middle. I like this measurement because it's easy for someone to figure out themselves without any help. It's also considered to be a more accurate measurement of health risk than BMI.

Waist measurement: Lately, a lot of medical experts say you can tell a lot by simply keeping track of your waist circumference. There is mounting evidence that health risk increases with a waist measurement of over 40 inches in men and over 35 inches in women.

Fat Mass Index: This is a brand new measurement that was recently proposed by the investigators who run the National Health and Nutrition Survey (NHANES), an ongoing, nationally representative health survey. They have proposed that Fat Mass Index, or FMI, which could replace BMI as a more accurate way of assessing obesity. Like BMI, FMI is a ratio between height and weight except that it only factors the part of your body weight accounted for by fat.

If you're head is spinning, I get it. That's a lot of numbers to choose from when you're trying to decide whether or not you need to lose weight. I always tell clients to look at a few different numbers so they can see where they are trending. If all your numbers come up high, it's time to seriously consider salads. If one pops out as an anomaly (and BMI is usually the one that will), it may say more about your body type than your health.

I'd be interested to know what you think of this number soup. Confusing? Make sense? Have a better way? Post here or tweet me http://twitter.com/Lizzyfit. I also invite you to link with me on linkedIn.