Friday Favorites: What’s the Ideal BMI and Waist Size?

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We seem to have become inured to the mortal threat of obesity. If you go back in the medical literature a half century or so, when obesity wasn’t just run-of-the-mill, the descriptions are much more grim: “Obesity is always tragic, and its hazards are terrifying.” But it’s not just obesity. Of the four million deaths every year attributed to excess body fat, nearly 40 percent of the victims are just overweight, not obese. According to two famous Harvard studies, weight gain of as little as 11 pounds from early adulthood through middle age increases the risk of major chronic diseases, such as diabetes, cardiovascular disease, and cancer. The flip side, though, is that even modest weight loss can have major health benefits.

What’s the optimal BMI? The largest studies in the United States and around the world found that having a normal body mass index, a BMI from 20 to 25, is associated with the longest lifespan. Put all the best available studies with the longest follow-up together, and that can be narrowed down even further to a BMI of 20 to 22. That would be about between 124 to 136 pounds for someone who stands 5’6″. You can pause the video here to use this unisex chart to see what your optimal weight might be based on your height.

But, even within a normal BMI, the risk of developing chronic diseases, such as type 2 diabetes, heart disease, and several types of cancer starts to rise towards the upper end, even starting as low as a BMI of 21. A BMI of 18.5 and 24.5 are both considered within the normal range, but a BMI of 24.5 may be associated with twice the heart disease risk compared to 18.5. Look at this diabetes graph among women: a five-fold difference in diabetes rates, all within the so-called ideal range under 25.

Just as there are gradations of risk within a normal BMI range, there is a spectrum within obesity. Class III obesity, a BMI over 40, can be associated with the loss of a decade of life or more. At a BMI greater than 45, such as a 5’6″ person at 280 pounds, life expectancy may shrink to that of a cigarette smoker.

There are, however, so-called “obesity skeptics” that argue that the health consequences of obesity are unclear, or even greatly exaggerated. They are a motley bunch, ranging from feminists, queer theorists, and new ageists to “far right wing, pro-gun, pro-America websites where the idea [is] that obesity alarmists are nanny-state communists who simply want to stop us from having fun….”

Unlike activists who, for example, organized to raise consciousness and stamp out the AIDS epidemic, the size acceptance movement appears to have the opposite goal, rallying for less public awareness and treatment of the problem. (They do have good slogans though: “We’re here, we’re sphere, get used to it!”). I’m all for fighting size stigma and discrimination—I have a whole section on weight stigma in my new book—but the adverse health consequences of obesity are an established scientific fact.

Can’t you be fat but fit? In a study of more than 600 centenarians (those living over 100), only about one percent of the women, and not a single one of the men, were obese. But there does appear to be a rare subgroup of obese individuals who don’t suffer the typical metabolic costs, such as high blood pressure and cholesterol. This raises the possibility that there may be such thing as “benign obesity” or “healthy obesity.” It may just be a matter of time, though, before the risk factors develop. And even if they don’t, followed long enough, even “metabolically healthy” obese individuals are at increased risk of diabetes, and fatty liver disease, and cardiovascular events, such as heart attacks, and/or premature death. Bottom line: there is strong evidence that so-called “healthy obesity” is a myth.

Many “fat-activists” try to downplay the risks of obesity, even as they may be among the epidemic’s greatest victims. Lynn McAfee is the director of medical advocacy for the Council on Size and Weight Discrimination, and routinely takes part in obesity conferences and government panels on obesity. “I’m not actually particularly that interested in [health],” she is quoted as saying, “and God I hate science.”

There was a book originally published in the 80s, and then repeatedly republished, entitled Dieting Makes You Fat. Since most people who lose weight go on to regain it, the concern is that there may be adverse health consequences of so-called yo-yo dieting. This idea emerged from animal studies that showed, for example, detrimental effects of starving and refeeding obese rats. This captured the media’s attention, leading to a pervasive common belief about the “dangers” of weight cycling, discouraging people from even trying.

Even the animal data is inconclusive, though. For example, weight cycling mice makes them live longer. Most importantly, though, a review of the human data concluded that “evidence for an adverse effect of weight cycling appears sparse, if it exists at all.” Bottom line: “Yo-Yo Dieting is Better Than None.”

Ideally, we’d get down to a BMI of 20 to 22, but body mass index doesn’t take the composition of the weight into account. For example, bodybuilders are heavy for their height, but can be extremely lean. The gold standard measure of obesity is percentage body fat, but an accurate calculation can be complicated and expensive. All you need to measure BMI is height and weight, but it may underestimate the true prevalence of obesity.

The World Health Organization defines obesity as a body fat percentage over 25 percent in men or 35 percent in women. At a BMI of 25, which is considered just barely overweight, body fat percentages in a representative US sample of adults varied between 14 percent and 35 percent in men, and 26 percent and 43 percent in women. So, you could be normal weight but actually obese. Using the BMI cutoff for obesity, only about 1 in 5 Americans were obese back in the 90s. But based on their body fat, the true proportion even back then was closer to 50 percent. Half of America is not just overweight, but obese.

So, just using BMI, doctors may misclassify more than half of obese individuals as being just overweight or even normal weight, and miss an opportunity to intervene. The important thing is not the label, though, but the health consequences. Ironically, BMI appears to be an even better predictor of cardiovascular disease death than percentage body fat. That suggests that excess weight from any source—fat or lean—may not be healthy in the long run. The lifespan of bodybuilders does seem to be cut short. They have about a third higher mortality rate than the general population. The average age of death was around 48 years old––but this may well be due in part to the toxic effects of anabolic steroids on the heart.

Preeminent nutritional physiologist Ancel Keys (after which “K-rations” were named) suggested the mirror method: “If you really want to know whether you are obese, just undress and look at yourself in the mirror. Don’t worry about our fancy laboratory measurements; you’ll know!” All fat is not the same, though. There is the pinchable superficial flab that you may see jiggling about your body; but then there’s the riskier, deeper visceral fat which coils around and infiltrates your internal organs. Measuring BMI is simple, cheap, and effective, but does not take into account the distribution of fat on the body, whereas waist circumference can provide a measure of the deep underlying belly fat.

Both BMI and waist circumference can be used to predict the risk of death due to excess body fat, but even at the same BMI, there appears to be nearly a straight-line increase in mortality risk with widening waistlines. Someone with “normal-weight central obesity,” meaning someone not even overweight according to BMI, but fat around the middle, may have up to twice the risk of dying compared to even someone who’s obese according to their height and weight. This is why the current recommendations recommend measuring both BMI and waist circumference. This may be especially important for older women, who lose approximately 13 pounds of bone and muscle as they age from 25 to 65, while quadrupling their visceral fat stores (men tend to only double). So, even if a woman doesn’t gain any weight based on the bathroom scale, she may be gaining fat.

What’s the waistline cut-off? Increased risk of metabolic complications starts at an abdominal circumference of 31.5 inches in women, and 37 inches in most men, though closer to 35.5 inches for South Asian, Chinese, and Japanese men. The benchmark for substantially increased risk starts at about 34.5 inches for women, and 40 inches for men. Once you get over an abdominal circumference of about 43 inches in men, mortality rates shoot up about 50 percent compared to men with 8-inch-smaller stomachs, and women suffer 80 percent greater mortality risk at 37.5 inches compared to 27.5 inches. The reading of a measuring tape may translate into years off one’s lifespan.

The good news is the riskiest fat is the easiest to lose. Your body appears smart enough to preferentially shed the villainous visceral fat first. Although it may take losing as much as 20 percent of your weight to realize significant improvements in quality of life for most individuals with severe obesity, your disease risk drops almost immediately. At 3 percent weight loss (just 6 pounds for someone weighing 200 pounds), your blood sugar control and triglycerides start to get better. At 5 percent, your blood pressure and cholesterol improve. Just a 5 percent weight loss (about 10 pounds for someone starting at 200) may cut your risk of developing diabetes in half.

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