Hypothesis: The ideal weight range, adjusted for height, in adults that is associated with the lowest mortality risk is not constant and varies with age, ethnicity and state of health.
Discussion: When Body Mass Index (BMI) was introduced to replace ideal body weight in the late 1970s, Andres concluded that the relationship between BMI and mortality was U-shaped AND the BMI nadir for mortality increased with age. But such findings challenged the beautiful simple hypothesis that lower BMI is better, and so Andres’ caution was unheard or dismissed.
Over the last decade however, there has been an exponential rise in the reporting of apparently paradoxical findings in the relationship between BMI and mortality. The obesity paradox refers to observations that being overweight or even mildly obese may provide mortality advantage when compared with ‘normal’ weight individuals under a range of human conditions. Yet until recently, the majority of obesity researchers have been reluctant to support any paradox, believing that the observations arose from epidemiological biases. So where are we today?
There are now abundant data indicating that ethnicity, age and chronic disease alter the BMI-mortality relationship and there are many biologically plausible reasons for the observations. Further driving the need to rethink our definitions of ‘ideal BMI’ is the paucity of any clear evidence that weight loss reduces mortality in people in the overweight and class I obese ranges, as the only convincing evidence comes from bariatric surgery populations (BMI>35).
There is no “obesity paradox” to explain if we can let go of the biologically implausible concept that a single ideal weight range fits all. Perhaps lifestyle advice should focus less on intentional weight loss, which is difficult to achieve, and more on quality nutrition, physical activity, fitness and maintaining function in chronic disease states and with aging.