Scientists are changing their minds about the best way to monitor body fat. Body mass index, or BMI — long considered the gold standard for evaluating an increased risk of health problems due to weight — is far from a perfect measure, says Dr. Arya M. Sharma, an obesity researcher at McMaster University in Hamilton, Ontario.

The BMI doesn't take into account the amount of muscle a person has, and is less accurate in older people, who lose muscle and bone and gain fat with age. It is also less accurate in certain Asian ethnic groups because their body fat percentage at various BMIs differs from that of Caucasians.

It also doesn't measure visceral fat, which may be more significant to overall health than knowing one's BMI.

The best way to measure visceral fat is by a CT scan, which shows the amount and location of body fat. It's not possible to do the expensive scans on everyone, however. Luckily, using a tape measure will work fine, too.

Don't assume you're too fat around the middle — or just fine — without measuring your waist. While most overweight people have excessive visceral fat, even people whose BMI is considered normal can have an unhealthy waist circumference.

In a study published last year by University of Pittsburgh researchers, abdominal fat was linked to metabolic syndrome in older people regardless of whether they were normal weight, overweight or obese. Having more subcutaneous fat in the thighs, however, seemed to protect individuals from metabolic syndrome. To assess abdominal fat, measure either your waist or your hip-to-waist ratio.