Height and weight are genetically determined, so how much of BMI can be attributed to our genes? No one really knows, but if you are interested in numbers, it is estimated that 40-70% of one’s  BMI is genetically determined.

Beyond Size: Genetics and Body Shape

Genetic Predisposition to Fat Distribution

It is not just how much fat you have, but where your fat is located. All body fat is not equal. Body fat located in the abdomen, termed visceral fat, is strongly associated with negative health outcomes, such as increased risk of cardiovascular disease and type 2 diabetes. BMI does not distinguish between the location of body fat, which is most easily determined with a simple waist measurement, where the higher-risk threshold begins at a 40-inch+ waist for men and a 35-inch+ waist for women.

Genes play a significant role in determining the distribution of body fat. More than 1100 genetic loci have been identified that relate to obesity.

Influence on Muscle Mass and Metabolism

Muscle also weighs a significant amount, and high levels of muscle mass can easily skew BMI measures. I often say that BMI works well to estimate obesity in large populations, but on a case-by-case basis, it can be wildly inaccurate. It works for the population because most people don’t exercise, and consequently, their excess body weight is correctly assumed to be fat. Strength-trained individuals can significantly compromise the usefulness of the BMI. As one example, one of my students had a BMI of 35, categorizing him as grade II obesity, a criterion physicians could use to prescribe patients obesity-treating medications. He didn’t need them. At the time of this BMI measure, his percent body fat was consistently measured at <10%, which is very lean. He had significantly more muscle mass than the average person of his height (he was a two-time DII All-American football player, BTW).

So BMI ain’t all that. Body composition measures are much more accurate (though still not perfect) to assess an individual’s health status. Especially when combined with a measure of regional fat distribution. 

Is our DNA our Destiny?

You can’t change your inherited DNA. But lifestyle changes (namely diet and exercise) can make significant alterations in your BMI. At the same time, BMI is said to be 40-70% determined by genetics, which still leaves 30-60% to the environment. And 40-70% is a big range. The genetic contribution to BMI varies with age, with the strongest effect observed in childhood. This makes sense, as your behaviors throughout a lifetime take effect through adulthood and shape you into the person you are. You can’t change your genes, but you can change your behavior.  

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