Body Mass Index, or BMI, has been used for many years as a way to estimate a person’s body composition. You may have stumbled upon BMI calculators on the web and entered in your height and weight. Calculating your BMI is a helpful way to learn where you stand in terms of disease risk. However, the BMI also has several limitations.
What Is BMI?
BMI is a calculated number based only on your height and weight. There are certain BMI classifications for adults and different classifications used for children. The main purpose of calculating your BMI is to determine your body’s level of adiposity, or body fat content. Also, research studies use the BMI of participants as data to correlate with disease risk and health. To figure out your BMI:
Step 1 – Calculate Your BMI
Step 2 – Determine Your Classification
- Underweight = < 18.5
- Normal weight = 18.5–24.9
- Overweight = 25–29.9
- Obese = > 30
Benefits of BMI
The main benefit of the BMI is that it gives a general estimate of where you stand in relation to weight and disease risk. Doctors and dietitians commonly use the BMI of patients to determine how likely they are to get certain diseases (such as heart disease). However, the BMI should not be the only tool you use to measure your body composition. Other measurements such as waist circumference, waist-to-hip ratio, and bioelectrical impedance can give you a better understanding of your body composition. In addition, biochemical data such as blood cholesterol, blood glucose and blood pressure should also be used to determine your current health and disease risk.
Limitations of the BMI
The BMI has many flaws. Considering it only takes into account a person’s height and weight, it doesn’t account for how much of your body is actually fat, bones, and muscle. For example, a professional football player may have a BMI of 30, which would indicate he is obese. However, he may be perfectly healthy and the extra muscle mass inaccurately inflates his BMI. The BMI is also not specific to different ethnicities and body types. For example, an Asian American’s BMI may indicate she is “underweight,” when she really just has a comparatively smaller frame.
What to Use
The BMI is a fast way to calculate where you might stand in terms of your weight status and health risk. However, take your BMI with a grain of salt, as it may not give you the full picture. The BMI may not be accurate if you:
- Do frequent strength training and have a high amount of lean muscle. This may inaccurately skew your BMI towards being overweight.
- Have a naturally smaller or larger frame.
- Carry most of your body weight in your midsection and belly.
- Are female and have a larger hip or breast size. This may also skew your BMI towards being overweight.
- Are elderly, as different weight guidelines may apply to those of an older age.
Here are some other methods you can use in conjunction with the BMI to determine your health risk:
- Waist to Hip Ratio: Measure your waist and hip using a tape measure. Divide your waist measurement by your hip measurement. A ratio of greater than 0.85 for women and 0.9 for men indicates a higher risk for chronic disease.
- Skin Fold Caliper: This measurement “pinches” your body at various places (like your arms and belly). It determines a body fat percentage, which can tell you where you stand in terms of disease risk. Many gyms have personal trainers who are trained in administering a skin fold caliper.
- Bioelectrical Impedance: This sends an electrical signal through your upper or lower body to estimate a fat percentage. This measurement is usually very quick and easy. Some doctors and outpatient dietitians may have access to bioelectrical impedance.
- Use Your Mirror: You know your body better than anyone else. Weight gain in the midsection and belly is associated with a higher risk for developing chronic disease. On the other hand, gaining weight in the hips and legs is associated with a lower risk.
- More comprehensive measurements include hydrostatic weighing, which consists of being weighed underwater, and DEXA scan, which measures your bone density and calculates your body fat content. These measurements are very accurate, but also much more expensive and may not be practical for the average person.
Bobby is the community and social media manager at Zipongo. He has a degree in nutrition and dietetics and previously worked as a health educator.