Assessment of obesity

Editor – In their article¹ the authors do not address one of the two fundamental flaws in BMI, that it is proportional to height, the other being it takes no account of composition and little of distribution.

It assumes the body is a cylinder of constant height and so works better than ponderal index for those around median height because girth increases with obesity.

However, the error caused is not small. It invalidates any comparison between populations of differing heights separated in time or space. Consider the height range 1.50 to 2.075 m. The BMI of a tall person of the same body mass as a small person BMI 25 would be 34.5 and compared to one of average height, 1.75 m, 30.5. When fit tall people have a BMI in the obese range this is frequently attributed to muscle mass rather than simply to height. The consequences of over-recognition of overweight in the tall and healthy likely to be less relevant than the under-recognition of undernourishment in the tall and elderly. For a BMI of 19 the same range would be 19 to 26.25, with the majority appearing to be adequately nourished. The slight exaggeration of the growth of population obesity is of little practical consequence as perhaps is the overestimate of obesity in the tall, but the underestimate of undernutrition in the tall and elderly is potentially dangerous.

Unlike BMI the preferable alternative, waist to height or span, does not need a scales, takes into account fat distribution, is independent of height, and particularly if span is used does not underestimate undernutrition in the elderly.

• © Royal College of Physicians 2023. All rights reserved.