How much body fat do you have? The number on a bathroom scale usually offers a clue, but it doesn't always tell the complete story.
For decades, researchers have used body mass index (BMI) to determine whether a person's weight is optimal and as an estimate of a person's proportion of body fat. The BMI formula uses body weight in kilograms divided by the square of height in meters — or, in American measurements, body weight in pounds divided by the square of height in inches and then multiplied by 703. (If you want to avoid doing math, use an online BMI calculator like the one at www.health.harvard.edu/bmi.)
There are four categories of BMI:
underweight: less than 18.5
normal weight: 18.5 to 24.9
overweight: 25 to 29.9
obesity: 30 or higher.
"BMI provides a good estimation of a person's amount of body fat, and the number can offer guidance on whether people should make lifestyle changes, like modifying their diet and adopting more cardio and strength training," says Dr. Frank Hu, a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health.
Still, BMI is far from an ideal measuring system, and for many older adults, it may not be the best way to gauge body fat.
It's essential to keep tabs on body fat as you age. Our bodies need some fat to perform everyday functions. But carrying too much can raise your risk of heart disease, diabetes and certain cancers. In general, the higher the BMI number, the greater the risks.
While research has found BMI a strong predictor of health risks, it has its faults. First, while it has some correlation to overall fat mass, BMI can't distinguish between subcutaneous fat (the kind that lies just below the skin) and the more dangerous visceral fat, which lies deep in the body and surrounds organs.
Studies also have suggested the BMI scale may be less accurate for certain ethnic groups, like Asians, who tend to naturally carry more body fat than people of European descent, and Black men, who tend to have lower body fat percentages than white men.
BMI also can't be adjusted to account for normal physical changes. "As men age, they naturally lose muscle mass, which can get replaced with fat," says Dr. Hu. "So, even though their weight may be about the same, they may have added extra fat that BMI won't recognize."
Measure of success
Are there any alternatives to BMI? The most accurate way to measure fat mass is with computed tomography (CT) and magnetic resonance imaging (MRI) scans. These imaging techniques also can identify amounts of subcutaneous and visceral fat.
Another high-tech approach is dual energy x-ray absorptiometry (DEXA), which measures fat via X-ray beams that pass through body tissues at different rates. However, DEXA cannot distinguish between subcutaneous and visceral fat. "The major downside to these is that they require specialized machines, which makes the tests expensive," says Dr. Hu. "They also are not widely available to most people."
The solution? Grab a measuring tape.
"It's not highly technical, but measuring your waist circumference is an easy way to identify increased fat mass, since most men accumulate extra fat around their midsection," says Dr. Hu.
This doesn't mean BMI is useless. In fact, Dr. Hu suggests older men use both BMI and waist circumference to measure and monitor their fat mass.
"Neither one is perfect, but taken together, they can help create a more accurate reading of your accumulated fat," he says. "Perhaps best of all, they help you stay mindful about your weight so you can note any increases before they escalate."
The line on waist size
Waist circumference is the distance in inches around the abdomen. Here's how to accurately measure it:
1. Stand and place a tape measure around your bare waist, at or just below your belly button.
2. Keep the tape snug but not so tight that it compresses the skin.
3. Relax and let your breath out before measuring.
4. Do it at least twice to get an accurate reading.
Ideally, your waist size should be no more than one-half of your height in inches. For example, a man who is 6 feet tall could feel good about a waist size of 36 inches or less. Even if you're above your ideal goal, track your measurement monthly to see if you're headed in the right direction.
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