Overweight and obesity are at epidemic proportions, and excess body fat is associated with coronary artery disease, hypertension, diabetes, pulmonary disease and arthritis. Body composition, or the relative percentage of body weight that is fat or fat-free tissue, can be estimated with both laboratory and field techniques that vary in complexity, cost and accuracy. Two useful measures that are easy to administer are the Body Mass Index, or BMI, and a comparison of hip and waist circumference, which is called the waist-to-hip ratio, or WHR.
Calculating BMI
Calculate your BMI by dividing your weight in kilograms by your height in meters squared. Divide your weight in pounds by 2.2 to derive your weight in kilograms. Multiply your height in inches by 0.0254 to determine your height in meters, and multiply this value by itself to calculate your height in meters squared. For example, a person who is 5 feet 10 inches tall is 1.78 m, which equates to 3.16 m-squared. If this person weighed 185 lbs., his weight would be 84.1 kg and his BMI would be 26.6.
Calculating WHR
Determine your WHR by using a measuring tape to measure the circumference of your waist and your hips. Make these measurements in either inches or metric units. Measure your waist at the navel while standing relaxed without pulling in your stomach. Measure your hips over the buttocks where girth is largest. Divide waist circumference by hip circumference to determine your WHR. For example, a person with a waist circumference of 38 inches and hip circumference of 45 inches would have a WHR of 0.84.
Healthy BMI
Health problems related to storage of excess fat increase with a BMI above 25 for most people. The Expert Panel on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults lists a BMI of 25.0 to 29.9 as the criterion for being overweight and a BMI greater than or equal to 30 as the criterion for obese. An unusually low BMI can also signal increased health risk. In healthy people, the low point of the U-shaped curve depicting the relationship between BMI and mortality occurs between a BMI of 23.5 and 24.9 for men and 22.0 and 23.4 for women.
Healthy WHR
Health risk increases with WHR, and standards for risk vary with age and gender. For example, risk for young men is very high when WHR exceeds 0.94, while for young women, a WHR greater than 0.82 indicates very high risk. These thresholds increase with age. For example, for those aged 60 to 69, values greater than 1.03 and 0.90 suggest very high risk for men and women, respectively. Generally speaking, WHR greater than 0.80 for women and 0.95 for men increases the risk of death even after adjusting for differences in BMI.
BMI Limitations
BMI is determined exclusively from measurements of weight and height, which means fat cannot be distinguished from muscle, bone and blood plasma. These can increase from exercise training without increasing health risk. For example, a person who trains regularly with weights might have what appears to be an unhealthy BMI even though his body-fat percentage is not excessive. This is particularly problematic for large-size, track-and-field athletes, bodybuilders, weightlifters, heavier wrestlers and American football players. Age, gender, ethnicity and race also dissociate BMI from body fat. For example, a higher body-fat percentage at a given BMI is characteristic for Asians compared to Caucasians.
WHR Advantage
WHR provides a strong indicator of disease risk because where you store your fat is very important. There are two types of fat-distribution patterns that humans display. The android pattern is characterized by fat in the abdominal region and when this storage is excessive, it is called central or android-type obesity. Men are generally more likely to display this pattern. Peripheral, or gynoid, obesity is characterized by fat in the buttocks and thighs. Android fat poses a greater health risk because it is more apt to be mobilized. Mobilization supports processes that cause heart disease. Android fat is also associated with endometrial cancers.
References
- "ACSM's Guidelines for Exercise Testing and Prescription, 6th Edition"; American College of Sports Medicine; 2000.
- "Exercise Physiology: Nutrition, Energy, and Human Performance, 7th Edition"; W.D. McArdle, et al.; 2010.
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