Here's some bad news for older women: They're likely fatter than young women of the same height and weight. That means, new research has concluded, that the body mass index cutoff for when people are considered obese (30) should be lower for postmenopausal women.

"Your body changes as you age, but BMI cut points don't," said Hailey Banack, a postdoctoral fellow who works with the Women's Health Initiative at the State University of New York at  Buffalo. "That number means something different in an older lady than in a younger lady."

This has implications for research involving groups of women, she said, because definitions may affect study results.  The health impact of obesity has been cloudier in older people, Banack said. Results could be skewed by the fact that many older women with non-obese BMIs actually have lots of body fat. Her study on body fat and BMI was published last month in the journal Menopause.

What the results mean for individual women is less clear.  Doctors who work with overweight women said they look at multiple indicators of health risk, including blood pressure and cholesterol levels, rather than focusing on BMI. Still, if you're taking comfort in having a BMI under 30, you might want to rethink that.

An imperfect measure

BMI is often used as a measure of obesity because it's easy.  All you need to calculate it is your height and weight.  Currently, doctors consider people with a BMI of 25 to 30 to be overweight.  Those with higher BMIs are considered obese.  A 5-foot-4-inch woman has a BMI of 30 at 174 pounds.

But it is well-known that BMI is an imperfect measure.  Athletes who are in prime physical condition, for example,  may have high BMIs because they are so heavily muscled.

Unfortunately, most older women don't have that problem.

Gary Hunter, an exercise physiologist at the University of Alabama at Birmingham, has long studied how body composition changes with age.  An average woman loses 13 pounds of muscle and bone mass from age 25 to 65, he said.  He hasn't studied men, but thinks they would lose at least as much muscle because they start out with more.  Typically, we replace that muscle with fat.  "You probably haven't increased brain tissue," joked Hunter, who is 74.

Meanwhile, women "have an incredible shift in where they store their fat," Hunter said.  Younger women tend to store fat between muscle and skin — what's known as subcutaneous fat.  This is the kind of fat you can pinch at your waist.  In older people, more of the fat is stored deeper in the belly, below the muscle.  This is what's called visceral fat, and it's more dangerous from a health standpoint.

Hunter measured visceral fat with cross-sectional scans of women's stomachs taken at the level of the belly button.  At 25, a woman with a normal BMI has about 40 square centimeters of fat in such a scan.  By 65, most women have gained 10 to 15 pounds and have about 130 square centimeters of visceral fat.  The risk for cardiovascular problems and cancer goes up at around 80 square centimeters and possibly above 40, Hunter said.

A rough substitute:  If your waist circumference is more than 35 inches for a woman or 38 for a man, you've probably got too much visceral fat, he said.

Hailey Banack is a postdoctoral fellow at at the State University of New York at Buffalo.
Courtesy of Hailey Banack
Hailey Banack is a postdoctoral fellow at at the State University of New York at Buffalo.

Banack's study analyzed how the percent of total body fat compared to BMI in 1,329 postmenopausal women ages 53 to 85.  Her team used a full-body DXA scan, the kind of imaging often used to measure bone density.

Using BMI as a measure, 21 percent of the women in the study were obese. When Banack used percentage of body fat to define obesity, many more women fell into that category, a sign that they had been misclassified with BMI.  When 35 percent body fat was the cutoff — a frequent standard, Banack said — 66 percent were considered obese.

Banack concluded that BMI cutoffs of 25 to 27 would more accurately reflect obesity in postmenopausal women.

A 5-foot-4-inch woman with a BMI of 25 weighs 145 pounds.

"The evidence is showing the cut point needs to be lowered in older women," Banack said.

Slowing fat gain, muscle loss

The best ways to combat muscle loss and visceral weight gain are the usual recommendations:  Exercise and eat healthy food.  Some muscle loss is inevitable, Hunter said, but we can slow the decline or regain some strength at any age.  People who stay thinner and exercise have less visceral fat.

Janine Kyrillos, director of the Jefferson Comprehensive Weight Management Program at Bala, said the new study comes as no surprise.

"We've known that BMI is a very inconsistent, or unreliable, predictor of obesity and metabolic syndrome," she said.  Doctors even have an acronym for the kind of unhealthy patient that BMI misses:  TOFI or Thin on the Outside, Fat on the Inside. These are patients with normal BMIs whose blood sugar is rising because of their unhealthy abdominal fat.  Kyrillos suggests that they avoid refined, processed foods, and take up exercises such as walking, running and weight lifting.

Anastasia Amaro, medical director of Penn Metabolic Clinic, which focuses on nonsurgical weight management, said many programs for overweight patients are starting to use waist circumference more.  Another measurement that's gaining fans is waist-to-hip ratio or waist divided by hip, which should be less than 0.8 for women and less than 0.9 or 1.0 for men.

She doesn't set BMI goals for patients. Though there's no strong scientific research showing how much body fat is too much, there's good evidence, she said, that even a 10 percent weight loss is "clinically meaningful."

Anne Cappola, an endocrinologist at Penn Medicine who has studied frailty among older people, agrees.  "The message should be about fitness rather than a particular BMI number," she said.