Your body mass is quite simply, how much you weigh. It's used to calculate your body mass index, your body weight divided by the square of your height. Body mass alone doesn't tell you much, except for the obvious fact of how much your body is affected by the Earth's gravity because of how large or small it is. But with body mass index, you are able to compare your height and weight -- or mass -- with people of a similar height, in this way understanding better where you fall in a comparison of the heaviest and lightest people who have a skeleton roughly the same size as your own.
History
Body mass index as it is known today emerged in 1972, when a researcher, Ancel Keys, asserted that the ratio was a great proxy for understanding body fat percentage among ratios of weight and height. Though in his paper he stressed the importance of understanding that BMI was inappropriate for individual diagnosis but useful for population studies that increasingly studied weight given the rise of obesity in Western societies. But because it was such an easy number to calculate, BMI came to be used often for individual diagnosis, in spite of its limitations.
Advantages
Knowing how your body mass compares with the body mass of others of your gender and age can help you gain context for how much you weigh compared to the rest of the population across the country. This can help you get perspective. A 16-year-old girl who notices the popular girls at school are thinner than she is -- lots thinner. She could feel like like she was abnormal, believing the skinny girls were normal and she was overweight. However, using her body mass to calculate her body mass index, her doctor could help her compare how much she weighed with how much it's healthy for girls her age to weigh, and understand that maybe the skinny popular girls were the ones with a problem.
Disadvantages
The downside of using your body mass to calculate your body mass index is that there's temptation to use it as an authoritative statistic that tells you you're definitely obese, or definitely underweight. This is because your body mass -- how much you weigh -- is determined by things like how muscular you are. Since muscle weighs more than fat, two people of the same weight would have the same BMI, even though one person could be just a muscle man and the other one could be carrying a bit too much fat.
Implications
According to the CDC, adults 20 and older have the same weight status categories, regardless of differences in gender or age; kids and teens, though, have age- and sex- specific interpretations. For adults, the standard weight status of someone with a BMI below 18.5 is underweight; BMI range of 18.5 and 24.9 is labeled "normal"; a BMI between 25 and 29.9 is considered overweight, and those with a BMI range of 30 and more is considered obese.
Variance
Though these figures are generally strong indicators of body fatness, the CDC does acknowledge that even for adults the correlation can vary by sex, race, and age, because again, your body mass is just a number and doesn't tell you anything about where the weight comes from -- muscle or fat. The CDC says women generally have more body fat than men, older people on average tend to have more body fat than younger adults, and "highly trained athletes" could have a high BMI because of their increased muscularity rather than increased body fatness. If you have questions or concerns about the appropriateness of your body mass, or weight, for how tall you are, bring them to your family doctor. There are serious consequences of overweight and obesity for adults, including being at higher risk for hypertension, high cholesterol, type 2 diabetes, coronary heart disease, stroke and some cancers.
Neck Circumference Measurement as Alternative to Body Mass Index
An article in "Pediatrics" journal published July 2010 announced that for children, there seems to be a better way to measure obesity than calculating body mass index. Researchers at University of Michigan's Mott Children's Hospital found measuring neck circumference was a reliable way for telling whether kids were overweight or obese. They noted that body mass index doesn't tell you what's responsible -- fat or muscle -- for someone's weight. By comparison, they noted that the chances of a child having an excessively muscular neck, skewing results, were low. They also said that while your body mass index can't tell you where in the body fat is accumulated -- fat around the middle of a person's body is associated with raised risk for high blood pressure, diabetes and high cholesterol -- but there's a strong correlation between high neck circumference measurements and "fat around the middle."
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