In clinical medical practice, obesity assessments are typically based upon the Body Mass Index (BMI) a measure of weight for height. Similarly, most obesity related population statistics are reported based upon the BMI. The BMI is commonly utilized because of its ease of use but fails to capture the spectrum of obesity related cardiometabolic risks. Studies have indicated that a high body-fat percentage, even with a normal body weight, results in an elevated cardiometabolic risk.
An article published in the August, 2013, issue of Obesity, studied the effect of a high body-fat percentage in 12,386 middle aged adults (30 to 49 years) with a normal BMI (18.5 to25). Subjects were excluded if they had been treated for hypertension, diabetes, or hyperlipidemia, or diagnosed with a chronic disease or pregnant. They were also excluded if any of the blood tests were grossly abnormal (greater than three standard deviations from the norm) or failed to respond to all questions on a lifestyle questionnaire.
Based upon the questionnaire, subjects were stratified as to smoking history (nonsmoker, smoker, exsmoker), alcohol consumption (nondrinker, moderate drinker, heavy drinker), and exercise (nonexercise, irregular exercise, regular exercise). Coincidental with measuring height, weight and waist circumference, the body fat and body muscle percentages (BF% and BM%) were measured using a bioelectric impedance device. The device was calibrated to a DEXA scan. Blood tests included total cholesterol and triglycerides, HDL and LDL cholesterol fractions, fasting glucose and insulin. Insulin resistance was calculated from this data.
Within the normal weight range (BMI: 18.5 to 25), obesity was defined as BF% greater than 25% in men and 30% in women. Cardiovascular disease risk factors were defined as: (1) high blood pressure greater than 130/85 independently, (2) hyperglycemia with fasting glucose greater than 100, and (3) dyslipidemia with triglycerides above 150, or HDL cholesterol less than 40 for men and 50 for women, or LDL cholesterol more than 160.
Based on BF% and BMI, as defined above, 4.5% of men and 21.9% of women were classified as normal weight obese (NWO). Those classified as NWO exercised less frequently and had a higher abdominal fat distribution. Those with NWO had higher blood pressure, total cholesterol, LDL cholesterol, triglycerides and insulin resistance than the normal BF% group. Adjusted for age, all of these cardiometabolic risk parameters were elevated in the NWO compared to those with normal BF%. After adjusting for age, fasting glucose and HDL cholesterol were not significantly different between groups. After adjusting for the lifestyle factors, fasting glucose and blood pressure did not significantly differ in the male subjects.
This study supports the concept that the BMI is a crude indicator of the relationship between weight and health. This also reveals why basing an insurance surcharge based upon BMI is inappropriate.
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