What is your BMI? Should you even care?

 

Although it was not called Body Mass Index (BMI) at the time, the concept has been in use since about 1850. The name BMI was coined in 1970 and is a number derived from an individuals height and weight. A convenient and simple way of classifying a person from underweight to obese, it has come under fire recently for its shortcomings as a predictor of a person's present and future health. BMI provides a simple (many now say simplistic) numeric measure of a person's thickness or thinness, allowing health professionals to discuss weight problems more objectively with their patients. BMI was designed to be used as a simple means of classifying average sedentary (physically inactive) populations, with an average body composition (muscle to fat ratio).

A study published by The Lancet in 2009 involving 900,000 adults showed that overweight and underweight people both had a mortality rate higher than normal weight people as defined by BMI. But in an analysis of 40 studies involving 250,000 people, patients with coronary artery disease with normal BMIs were at higher risk of death from cardiovascular disease than people whose BMIs put them in the overweight range (BMI 25–29.9). One study found that BMI had a good general correlation with body fat percentage, and noted that obesity has overtaken smoking as the world's number one cause of death. But it also notes that in the study 50% of men and 62% of women were obese according to body fat defined obesity, while only 21% of men and 31% of women were obese according to BMI, meaning that BMI was found to underestimate the number of obese subjects. A 2010 study that followed 11,000 subjects for up to eight years concluded that BMI is not a good measure for the risk of heart attack, stroke or death. A better measure was found to be the waist-to-height ratio. A 2011 study that followed 60,000 participants for up to 13 years found that waist-to-hip ratio was a better predictor of heart disease mortality.

Recently both the medical establishment and statistical community have highlighted the limitations of BMI. The BMI was always designed as a metric for European men. For women, and people of non-European origin, the scale is often biased. As noted by sociologist Sabrina Strings, the BMI is largely inaccurate for black people especially, disproportionately labeling them as overweight even for healthy individuals.

It is very simple to compute your BMI if you know your height and weight. There are easy to use BMI Calculators. But should you bother?

It is worth knowing your BMI as it gives you a quick "drive by" snapshot of your weight/health status. But you should not rely on it overmuch, or be too concerned if you are in an other than "normal" category. It is a good starting point for a conversation with your primary healthcare provider but not a very accurate forecaster of your health.

To read more about BMI, its limitations, and the alternatives you might consider, click here.