Why is the BMI not useful for postmenopausal women and the elderly?

Previously In Defense of Processed Food, I pointed out limitations in the BMI as an index of body weight. Unfortunately, the BMI has now become the primary health index. The BMI underestimates obesity in short people and overestimates it in tall people. Children and youth are special cases. Today’s young are evaluated against a reference standard collected over a period between 1963 and 1994. “Overweight is defined as a BMI at or above the 85th percentile and below the 95th percentile for children and teens of the same age and sex. Obesity is defined as a BMI at or above the 95th percentile for children and teens of the same age and sex” as compared to the reference standard.

BMI & postmenopausal women

I am afraid there is bad news for postmenopausal women. As they age, the amount of bone and lean-body mass tends to decrease relative to the amount of fat. Even if a woman maintains the same weight after menopause, the percentage of body fat will increase. Gaining weight during this period of life is generally more likely to lead  to increases in the percentage of body fat. A study published recently in Menopause (1) indicates that the cutoff for obesity in postmenopausal women be lowered from 30 to 27, which would also mean a lowering of the cutoff for overweight as well. The study has prompted an editorial in JAMA (2) to consider re-evaluating these cutoffs for this group. My precautions about making policy based on a single study still hold, but these results are another indication that BMI should not be used as a universal health index.

BMI and the elderly

But now it gets complicated. A comprehensive meta-analysis of 32 studies relating BMI and death rate found that overweight seniors were less likely to die at a given age than thinner ones. A BMI of 25-29 appears to be more protective than one of 23 or below! Death rate is not the only game in town as higher weights are associated with more chronic diseases like diabetes and more joint-replacement surgeries. Like women, men tend to lose lean-muscle mass at the expense of accumulating more fat as they reach their “golden” years. Since postmenopausal women are either old age or approaching it, should they be trying to lose weight or be satisfied with being slightly overweight? To further muddle the situation, fat around the waist is more dangerous than fat accumulating in the hips and buttocks.

Proposed alternatives to the BMI

Before widespread use of BMI to define overweight and obesity, obesity was considered to be over 25% body fat in adult males and over 35% body fat in adult females. The American Council on Exercise puts those cutoff points at 26% for males and 32 % for females. There are many ways to measure % body fat. Sophisticated methods like hydrostatic weighing and air-displacement plethysmography are accurate but costly. Use of calipers that can be purchased from amazon.com or available at most gyms are only as accurate as the skill of the person taking the measurements. A method that can also be done at home is waist-to-height-ratio, which may be more accurate than the BMI, particularly for the elderly. A quick calculator is available on the web if you can dodge the ads. There are many other suggested alternatives to the BMI, but most of them are too difficult to measure, calculate and understand.

A personal perspective

A few weeks ago, I started back on the Sonoma Diet as I was getting heavier than I would like to be. My BMI, using an online calculator, was 25.4 and thus overweight, In the interim I have lost 7 pounds to bring my BMI down to 24.1 or desirable* weight. During the progress of my diet I note that sometimes when I get up in the morning, void my bladder and weigh I am at a desirable weight. By the time I am ready for bed, however, I am back to being overweight. I realize that weighing more than once a week can be considered to be obsessive. In my defense, frequent weighing was merely to achieve a better scientific understanding of the ups and downs of weight loss.

Belly fat–now and then.  Can you tell a difference?

Since I started this diet, I have come upon the concept of waist-to-height ratio. The news here is not as good. I have no idea of what my waist was at the beginning of the diet, but using three online calculators, I find that I need to lose from three to seven inches around my waist to be a healthy, desirable number. It seems that determining a healthy waist-to-height ratio is not an exact science. I know that I am losing belly fat as my pants are getting looser. If I lose as much as five inches around my waist, I will greatly lower my BMI, but will that affect my ability to live through a serious illness? Will a little higher BMI help protect me? By looking at the information available, I have no clear guidelines. Bottom line, it is much easier to adopt the BMI as an overall health index, which is why it is used universally.

Where do I go from here?

For me the BMI and waist-to-height ratio are just two numbers that I need to look at. It is not a healthy practice to have major weight swings. It is also not advisable to look at either one of these measures as a definitive indicator of good health. There are many other numbers that should be monitored such as a blood lipid profile and A1c value to obtain a broader picture of health status. I intend to pursue my current weight loss to bring my BMI down to about 23.5 and attempt to maintain it below 24 in the future. I will also measure my waist circumference monthly to see if I can reduce it to a healthier range with increased exercise. Those values will not be only two numbers that my doctor and I will continue to look at as I monitor my health in the golden years.

Next week: Defending the American public from fake news on science and health

*I use “desirable weight” instead of “normal weight” as only a minority of Americans fit into the desirable weight category which makes it clearly un-normal

References:

(1) Banack, H., Wactawski-Wende, J., Hovey, K.M. and Stokes A., 2018. Is BMI a valid measure of obesity in postmenopausal women? Menopause 25:307-313.

(2) Rubin, R., 2018, Postmenopausal women with a “normal” BMI might be overweight or even obese. JAMA 319:1185-1187.

 

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