The muddy waters of body mass index (BMI)

"The road is long, With many a winding turn, That leads us to who knows where"

The lyrics of the Hollies hit He Ain't Heavy, He's My Brother fit nicely along the convoluted road that is the relationship between body mass index (BMI) and health.

From being an accepted arbiter of whether you were overweight, obese or a member of that elusive category, normal, the emperor’s clothes have become somewhat tattered of late. Add conflicting advice on healthy eating, and the world of fitness and health has become most uncertain.

BMI, which is calculated by dividing your weight (in kg) by the square of your height (in metres), gained currency as a more accurate measure of "healthy" weight following the publication in 1972 of a paper in the Journal of Chronic Diseases by Ancel Keys.

He argued that BMI was, “if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity”.

Keys was prescient in describing BMI as “not fully satisfactory”. Using the ultimate outcome of mortality, the optimal BMI associated with lowest risk of all causes of mortality is no longer certain.

Given the relationship between increased weight and a greater incidence of diabetes and heart disease, you would expect that a rising BMI would be associated with increasing mortality. However, compared with normal weight, being underweight is associated with increased mortality, and a moderately elevated BMI is associated with lower mortality. This unexpected relationship is called the obesity paradox.

In a paper published earlier this month in the Journal of the American Medical Association, Danish researchers found that the optimal BMI associated with lowest mortality had increased from 23.7 to 27 over three decades. In addition, they reported the risk of all-cause mortality linked to a BMI of 30 (traditionally the cut-off point between being overweight and obese) now equates to the risk associated with having a BMI of 18.5-to 25 (underweight/ normal range).

Their finding calls into question the validity of the World Health Organisation (WHO) overweight categories, which define a BMI of 20-25 as normal, with 25-30 classified as being overweight.

“If this finding is confirmed in other studies, it would indicate a need to revise the WHO categories presently used to define overweight, which are based on data from before the 1990s,” the authors say.

Why the increase in BMI associated with lowest all-cause mortality has occurred over time is a mystery that needs further study.

Is the improved treatment of cardiovascular disease in people who remain overweight conferring a survival advantage that is independent of the person’s weight?

How is the known link between obesity and higher rates of cancer feeding into this mortality decline? Is weight gain in later life more or less life-limiting than being overweight from childhood?

It may be time to move away from a BMI-focused approach at the level of the individual patient. For example, obesity staging systems focus on overall cardiometabolic health, rather than BMI.

Better measurements of body fat, such as waist circumference, may also help. And some mechanism for incorporating a person’s exercise levels into the obesity “equation” is worth exploring also.

The publication in Britain last week of a controversial report, advocating that we eat more fat, muddies the waters even more.

The National Obesity Forum and the Public Health Collaboration called for a diet low in refined carbohydrates but high in healthy fats, saying it offers “an effective and safe approach for preventing weight gain and aiding weight loss”.

There is no doubt that, from the frontline of clinical practice, guidelines suggesting high-carbohydrate, low-fat diets were a universal panacea, did not reduce obesity levels. Looking back, dietary guidelines demonising fat were an open invitation to increase sugar and carbohydrate consumption.

Between measuring and dieting, overweight/ obesity is truly in a “terrible state o’ chassis”.

mhouston@irishtimes.com

@muirishouston