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Body fat may matter more than Body Mass Index (BMI)

A few years ago I shared with readers a study that found that even when people's Body Mass Index was within normal range, if their Waist to Hip ratio was over .90 (the World Health Organization defines this as centrally obese) they were more likely to die of any cause than those whose Body Mass Index and Waist to Hip Ratio were within clinically normal parameters.

That study is just one indication that how much body fat someone is carrying, as well as where that fat is located on their body, may be a greater indicator of metabolic risk than Body Mass Index alone.

Today's research, published in the journal Nutrition (2021;85:111134) looks more closely at both body fat and specific health risks.

This study was carried out in Brazil among nearly 800 men and women who were born in Sao Paolo, Brazil in 1978 and 1979. At birth their birth weight was recorded as well as information regarding their parents. Over 20 years later the participants were between 23 and 25 years of age and underwent height, weight, and waist measurements as well as skinfold measurements to assess body fat. Blood was drawn for insulin and cholesterol tests and the participants responded to dietary and lifestyle questionnaires.

Over ten years later the participants were between 37 and 39, and while they again were weighed and measured and surveyed and provided blood for testing, this time they underwent a body scan known as air pletysmography that allows much greater accuracy in the measurement of body fat percentage. Another test, dual-energy x-ray absorptiometry, provided measurements of muscle mass and total body weight.

The authors classified the participants into one of three groups:

HWI (Healthy Weight): those with a BMI in the clinically normal range with a body fat measurement (via skinfold) below the 90th percentile;
NWO (Normal Weight Obesity): those with a BMI in the clinically normal range with a body fat measurement (via skinfold) above the 90th percentile; and
OWO (Overweight/Obesity): Body Mass Index over 25 as well as a body fat measurement above the 90th percentile

For those 23-to-25-year-olds the authors also analyzed their clinical scores to determine if they had Metabolic Syndrome, a collection of metabolic abnormalities that indicates a greater risk of cardiovascular and other metabolic diseases. These abnormalities include:

Waist Circumference 90 or more centimeters for men and 80 or more centimeters for women;
Triglyceride scores over 150;
HDL cholesterol scores over 40 for men or 50 for women;
High blood pressures of either >130 systolic or >85 diastolic;
Fasting blood glucose equal to or greater than 100.

In all cases, if the participants were being treated with medications for cholesterol, high blood pressure, or insulin-controlling medications, the authors took that as proof that the participant met the criteria for the abnormality.

The same information from the participants at ages 37-39 was gathered and the same analysis was performed - and those who were considered to have Metabolic Syndrome at in their 20's were excluded from the final analysis.

The researchers compared the Metabolic Syndrome status of the participants in their 30's with their classification by body composition groups (HWI, NWO, and OWO).

The authors state that after taking into account demographic, dietary, and lifestyle variables, "We did not find differences in the [relative risk] for [Metabolic Syndrome] between [Normal Weight Obesity] and [Overweight/Obesity]."

What I found interesting about their results is that while those with Overweight/Obesity were 161% more likely to develop Metabolic Syndrome than those with clinically normal weight status, those with Normal Weight Obesity - excessive body fat percentage - were 187% more likely to develop Metabolic Syndrome.

This pattern of slightly (but perhaps not statistically significant) greater risk for those with NWO than OWO holds true for the risk of excessive waist circumference, cholesterol scores, and blood glucose levels. Those with Overweight/Obesity were 172% more likely to have high blood pressure compared to those of clinically normal weight, while those with Normal Weight Obesity were 144% more likely to have high blood pressure.

What this means for you

This suggests to me that excess body fat - and not just total weight - could be more dangerous to your health than a number on a scale. Certainly this aligns with previous research showing that BMI is inaccurate for those with low body fat, as well as research showing the risks of a higher Waist to Hip ratio. Here's how to calculate your Waist to Hip ratio.

First posted: June 16, 2021