What’s the Ideal BMI?

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 grimmer: “Obesity is always tragic, and its hazards are terrifying.” But it’s not just obesity. Of the 4 million deaths every year attributed to excess body fat, nearly 40% 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 flipside, 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 five-foot-six. 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 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 five-foot-six 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 spheres, get used to it!” 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 1% 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 quote-unquote “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.

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.”