Conventional wisdom, along with boatloads of scientific evidence, point to obesity being universally unhealthy, leading to diabetes, cancer, heart disease, and many more problems. But in recent years, that conventional wisdom has been challenged by a “U.”
That “U” appeared on graphs charting the link between body-mass index — a common but imperfect gauge of whether or not someone’s weight is healthy, calculated simply by dividing their mass by the square of their body height in meters — and their risk of death. Numerous epidemiological studies have found that people in the “overweight” category (BMI 25-30) surprisingly have the lowest mortality risk, while those categorized as “obese” (30-35) have little or no increased risk over the “healthy” (18.5-25). At the extreme ends of the BMI spectrum, both the “underweight” (less than 18.5) and the extremely obese (35+) have a greatly increased risk of death. Furthermore, numerous studies also have suggested that obesity might lower the risk of death for older people and patients with various chronic diseases.
Considering what we know about the health pitfalls of increased body fat, one would expect a mostly straight line of rising mortality risk as one goes from a BMI of healthy to obese. That’s why the “U-shaped” mortality curve has been dubbed the “obesity paradox.”
But in recent years, that paradox, and the studies that created it, have come under fire. Critics chiefly contend that BMI is a flawed way to determine whether someone has obesity. That’s because it does not measure the composition of one’s body mass — that is, how much is fat and how much is muscle. Nor does BMI measure where fat is located, which can make a big difference. Visceral fat jammed among internal organs is much worse than subcutaneous fat stored just beneath the skin. For example, an extremely fit and muscular individual could easily make it into the obese BMI category. At the same time, a “skinny” individual with a lot of body fat nestled dangerously around their mid-section could be categorized as “healthy.”
Why has BMI been so frequently used in epidemiological studies? Because it’s convenient, and readily calculated based on self-report. On the other hand, measuring body fat requires subjects to take a trip to a lab or to conduct the measurement on themselves, which can be quite difficult for a layperson to do accurately.
