Are Normal-Weight Americans Over-Fat?

Readers of these pages will recall that I am not a fan of defining obesity based on BMI because this measure of “fatness” does not perform well when applied to individuals. Thus, people with identical BMI levels can have remarkably different levels of body fat (not to mention remarkable differences in their actual health status).

This, incidentally, is also true for people with BMI levels in the so-called “normal” range, a finding which prompted Marie-Pierre St-Onge from Columbia University, New York, to ask whether normal-weight Americans are perhaps “over-fat” (rather than overweight). Her commentary was just published online in OBESITY.

it has previously been suggested that a normal body fat level should be around 21–32% for 21- to 39-year-old, normal-weight women and 8–20% for men of the same age and BMI category.

However, a recent analyses of data from National Health and Nutrition Examination Surveys (NHANES) 1999–2004, reported that the 5th percentile for body fat, which should represent the leanest of the US population, corresponds to 28 and 17% body fat for women and men, respectively.

Most disturbingly, the 50th percentile of body fat in the US population was as high as 41 and 28%, for women and men, respectively.

When categorized by BMI and age, the data also show high percent body fat values, particularly in lower BMI categories.

Thus, clearly, even normal-weight Americans appear to have more body fat than is considered “normal”.

This is an important finding, because “normal” BMI levels are generally interpreted as being “healthy”, meaning that no “weight-loss” intervention is recommended.

As I have pointed out before, BMI levels are limited in defining body-fat-related health, because an excess BMI alone does not necessarily reflect ill health.

On the other hand, it now seems that a normal BMI level does not necessarily reflect good health either, as a substantial number of people with normal BMI levels may well have excess body fat, which can potentially contribute to cardiometabolic and other “body-fat-related” health problems.

Perhaps, it is time for clinicians to turn to more accurate measures of body-composition when defining obesity rather than simply using the rather crude surrogate of BMI.

Edmonton, Alberta

St-Onge MP (2010). Are normal-weight americans over-fat? Obesity (Silver Spring, Md.), 18 (11), 2067-8 PMID: 20978478

Li C, Ford ES, Zhao G, Balluz LS, & Giles WH (2009). Estimates of body composition with dual-energy X-ray absorptiometry in adults. The American journal of clinical nutrition, 90 (6), 1457-65 PMID: 19812179