Body mass index

Is your weight healthy? That depends on several factors, say the government's experts.

The first federal guidelines on obesity were released by the National Heart, Lung, and Blood Institute in June 1998. These guidelines provide definitions of being overweight and obesity, and evaluate the health risks associated with these conditions. Three criteria are used to assess those risks: how much of your weight is fat, where you carry your that fat on your body, and whether you have weight-related health problems.

In a strict medical sense, men with more than 25% body fat and women with more than 30% body fat are considered obese. But measuring body fat is not easy or cheap. The techniques used — including dual-energy X-ray absorptiometry scans and underwater weighing — are often costly, and usually they can be done only at special facilities.

So doctors and dietitians generally prefer a measurement called the body mass index. The BMI is derived from a simple mathematical calculation that takes into account both height and weight.

To determine your BMI, use our calculator or the following formula. W is your weight in pounds, and H is your height in inches: W divided by H², multiplied by 704.5, equals your BMI.

Then compare your result against the federal BMI guidelines below.

Most experts consider BMI a better guideline than the old "ideal body weight" tables and the Metropolitan Life Insurance Height and Weight Tables, which were primarily based on white populations and aren't representative of the general public. BMI also can be used regardless of gender, whereas men and women require separate ideal body weight tables.

Of course, that doesn't mean relying on BMI doesn't have its shortcomings. People under five feet tall may have high BMIs but not be truly overweight. The same goes for very muscular people. And when used with elderly patients, who have usually lost muscle with age, BMI may be inaccurately low. Nevertheless, BMI provides a reasonably good estimate of body composition and relative health risks.

Where your body fat is located, not just how much fat you have, is also an important consideration. You may have heard the terms apple and pear used to describe body types. Apples carry fat around their abdomen; pears around their hips and buttocks. Apples are at greater risk than pears for developing several serious conditions, including high cholesterol, heart disease, high blood pressure, and diabetes. Measuring your waist, which will be disproportionately large if you're an apple, is one way to assess your risk for these diseases.

According to federal guidelines, using waist size to predict the risk of weight-related health problems works best for people who are taller than five feet and have a BMI between 25 and 34.9. To measure your waist, wrap a flexible measuring tape around the narrowest point of your waist. To ensure accuracy, you may need someone else to read the tape for you.

This table shows you whether your BMI and your waist size indicate you are at increased risk for diseases and conditions associated with weight problems.

You can calculate your own BMI and measure your own waist. But if you're concerned about your weight you should see a doctor. He or she will consider your risk for weight-related health problems, and can help you determine whether you need to lose weight, and if so, how much. Health problems that can go along with being overweight and obesity include:

• High blood pressure
• Heart disease
• High total cholesterol
• Low HDL (the "good" cholesterol)
• High LDL (the "bad" cholesterol) and high triglycerides
• Type 2 diabetes
• Osteoarthritis
• Sleep apnea and other respiratory problems
• Cancer
• Menstrual irregularities and infertility

Risk factors that may contribute to being overweight include:

• Smoking
• Lack of physical activity
• Family history of early heart disease

If your BMI is between 25 and 29.9 and you also have two or more of the health problems or risk factors listed above, or if you have a BMI of 30 or higher whether or not you have any other risk factors, you need to lose weight. Rather than focusing on an ideal weight, your doctor will most likely suggest you begin by losing 10% of your current weight. Losing even a small amount of weight can significantly lower your risk of further health problems, and reduce or even eliminate current ones.

If you're overweight, but don't have a large waist and have fewer than two of the above risk factors, your doctor may advise you to focus just on not gaining any more weight, and coming in for regular health checkups.