Body mass index, or BMI, is an easy-to-calculate measure of obesity based on the ratio of your weight and your height. It's convenient to use because it doesn't require expensive equipment or a clinical setting to measure -- just knowledge of your height and weight -- and it's useful for most people to get a general idea of your disease risk. BMI isn't perfect, however, and while it's beneficial in some circumstances, it has some major limitations if you're trying to use it to gauge your individual disease risk.
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Advantages: Accurate Measurements Across a Group
Body mass index works well for what it was intended to do; measure rates of obesity in a population. Because it's a general measure of obesity that will work for most people, looking at changes in BMI level allow researchers to get a good idea of how rates of overweight and obesity differ over time, or between populations, explains the Centers for Disease Control and Prevention. Because it's a relatively simple way to measure obesity in a population, this allows health researchers to more easily gather data they can use to investigate the obesity epidemic or, for example, look at how dietary patterns affect the risk of obesity in large groups of people. It's not prohibitively expensive to measure -- unlike, for example, body fat measurements -- so researchers can afford to can look at larger groups of study subjects to pick out trends in larger segments of the population.
BMI can also help your physician gauge your general risk of obesity-related diseases, though BMI is best used in combination with other measurements to get a more complete look at your health.
To calculate your BMI, use this equation: BMI = weight / (height X height) X 703. Plug in your weight in pounds and your height in inches. Alternatively, use an online BMI calculator that does the math for you. A BMI between 18.5 and 24.9 is considered a healthy weight.
Disadvantage: BMI Misses Normal Weight Obesity
Because BMI is simply a measure of your weight versus your height, it doesn't take into account where that weight comes from -- lean tissue or fat. For this reason, you might have a normal "healthy" weight, according to your BMI, but still face health risks due to excess body fat. For example, excess abdominal fat that pushes your waistline to larger than 35 inches for women and 40 inches for men ups your risk of obesity-related diseases, according to the National Institutes of Health, regardless of your BMI. And normal weight obesity -- which happens when you're overfat, but not overweight, according to BMI -- increases your blood lipid and blood pressure levels, which increases your risk of heart disease.
Disadvantages: BMI Overestimates Risk for Some
While BMI might underestimate the risk for people with normal weight but high body fat, it can overestimate the risk for muscular, healthy people. Someone putting in serious time at the weight room might have a body weight that pushes their BMI into the "overweight" or "obese" category, even if they carry very little body fat -- and therefore have a lower risk of obesity-related diseases than someone at the same weight with more fat tissue.
BMI also doesn't distinguish between the type of fat you carry -- subcutaneous or visceral fat. While subcutaneous fat -- the fat you see under your skin, which you can pinch -- affects the way you look, it's visceral fat -- "hidden" fat located deep in your abdomen that surrounds your internal organs -- that poses the highest health risk. If you're slightly overweight, but most of your fat is from subcutaneous fat -- for example, in your hips and thighs -- you'll face a lower health risk than someone who has the same weight and fat level, but stores most of his fat as harmful visceral fat.
Take a Holistic Approach to Measurements
Take a more holistic approach and use several measurements to asses your weight and health. In addition to BMI, consider your waist size -- to ensure you're under the recommended waist size for your gender. Instead of measuring weight loss in terms of where you fall on the BMI scale, look at a combination of pounds and inches lost and consider a professional body fat measurement to get an accurate look at whether you have a healthy body fat level.
Stay motivated and happy with your health routine by focusing on performance-based goals instead of simply aiming to fall into a certain BMI range. Reward yourself for making healthful meals at home for the week, eating your recommended intake of veggies for the day, running faster without losing your breath, or lifting a heavier weight during your workouts, for example, rather than being able to check the "normal weight" box when it comes to BMI.