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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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