Why BMI is important?

Body Mass Index (BMI) Is a Good Gauge of Body Fat

The most basic definition of overweight and obesity is having too much body fat-so much so that it “presents a risk to health.” A reliable way to determine whether a person has too much body fat is to calculate the ratio of their weight to their height squared. This ratio, called the body mass index (BMI), accounts for the fact that taller people have more tissue than shorter people, and so they tend to weigh more.

BMI is not a perfect measure, because it does not directly assess body fat. Muscle and bone are denser than fat, so an athlete or muscular person may have a high BMI, yet not have too much fat. But most people are not athletes, and for most people, BMI is a very good gauge of their level of body fat.

Research has shown that BMI is strongly correlated with the gold-standard methods for measuring body fat. And it is an easy way for clinicians to screen who might be at greater risk of health problems due to their weight.

Weight Gain in Adulthood Increases Disease Risk

In adults, weight gain usually means adding more body fat, not more muscle. Weight gain in adulthood increases disease risk even for people whose BMI remains in the normal range.

– In the Nurses’ Health Study and the Health Professionals Follow-Up Study, for example, middle-aged women and men who gained 11 to 22 pounds after age 20 were up to three times more likely to develop heart disease, high blood pressure, type 2 diabetes, and gallstones than those who gained five pounds or fewer.
– Those who gained more than 22 pounds had an even larger risk of developing these diseases.
– A more recent analysis of Nurses’ Health Study data found that adult weight gain-even after menopause-can increase the risk of postmenopausal breast cancer.

Healthy BMI in Children and Adolescents
It is normal for children to have different amounts of body fat at different ages, and for girls and boys to have different amounts of body fat. So in children and teens, the healthy range for BMI varies based on age and gender.

In the U.S., the Centers for Disease Control and Prevention (CDC) has developed standard growth charts for boys and girls ages 2-20 that show the distribution of BMI values at each age. By the CDC’s definition, a child whose BMI falls between the 85th and 94th percentile for age and gender is considered overweight. A child whose BMI is at the 95th percentile or higher for age is considered obese.

In 2006, the WHO developed international growth standards for children from birth to age 5, using healthy breast fed infants as the norm; in 2007, the WHO extended those standards to develop growth charts for children ages 5 to 19. Breastfed infants tend to gain weight more slowly than formula fed infants after 3 months of age, so the WHO growth standards have lower cut points for underweight and overweight to reflect this difference. The CDC now recommends using modified versions of the WHO growth standards for all children from birth to age 2. The International Obesity Task Force has also developed its own cut points for childhood overweight and obesity. At different ages, these criteria give somewhat different estimates of overweight and obesity prevalence.

BMI vs. Waist Circumference: Which Is Better at Predicting Disease Risk?

Body fat location is also important-and could be a better indicator of disease risk than the amount body fat.

Fat that accumulates around the waist and chest (what’s called abdominal adiposity) may be more dangerous for long-term health than fat that accumulates around the hips and thighs.

Some researchers have argued that BMI should be discarded in favor of measures such as waist circumference. However, this is unlikely to happen given that BMI is easier to measure, has a long history of use-and most important, does an excellent job of predicting disease risk.

In adults, measuring both BMI and waist circumference may actually be a better way to predict someone’s weight-related risk. In children, however, we don’t yet have good reference data for waist circumference, so BMI-for-age is probably the best measure to use.

As obesity rates have soared, people’s perceptions of what constitutes a healthy weight appear to have shifted: A recent U.S. study comparing weight perception surveys from the late 1980s to the early 2000s found that in the early 2000s, people were more likely to consider their own weight “about right” instead of “overweight.” Some of these people were truly at a healthy weight, but many of them were not.
Measuring BMI (and in children, BMI percentile-for-age) and tracking it over time offers a simple and reliable way for people to tell whether they are indeed at a healthy weight.

Sources

Download our FREE report on why sitting down too much is bad for you!