At one time, anyone who weighed 20 percent more than the ideal weight for his or her age and height was defined as clinically obese. But this is not as accurate an indicator as the body mass index (BMI), which is now the best measurement of obesity. The BMI is calculated by dividing your weight (in kilograms) by your height (in meters) squared. (The formula used is BMI = kg/m2 if you're doing this on your calculator.) BMI charts abound on the Internet, on the backs of cereal boxes, and in numerous health magazines. Most people can now easily find BMI converters on the Internet, where you simply type in your weight in pounds and your height to arrive at your BMI. A good chart will calculate BMI by gender and sometimes age ranges.
Currently, a BMI of 18.5 or less indicates that you are underweight. A BMI between 18.5 and 24.9 is normal. The most recent clinical guidelines define people with a BMI between 25 and 29.9 as overweight, and those with a BMI between 30 and 34.9 as obese (mild to moderate). A BMI between 35.0 and 39.9 would indicate severe obesity; people with a BMI of 40 or greater are considered morbidly obese. These are the clinical guidelines for defining obesity as of 1998. Changes in the average weight of people in North America since then means that at least 75 million more North Americans are now considered obese who were not considered obese in 1998.
Obesity rates in children and teens are calculated through BMIs that are at or above sex- and age-specific weights within the 95th percentiles. This is a more conservative approach to account for growth spurts.
Obesity experts consider the North American lifestyle to be the single largest contributing factor to obesity. Although social, behavioral, metabolic, cellular, and genetic factors all contribute to obesity, some obesity genes are "turned on" only when they are exposed to an environment conducive to weight gain, such as a sedentary and food-rich environment.
North Americans have the highest obesity rates in the world. More than half of North American adults and about 25 percent of North American children are now obese. These figures reflect a doubling of adult obesity rates since the 1960s and a doubling of the childhood obesity rate since the late 1970s—a staggering increase when you think about it in raw numbers. Obese children will most likely grow up to become obese adults, according to the most recent research.
Waist circumference is another factor in calculating obesity—particularly abdominal obesity. Men with a waist circumference of 40 inches or more are at increased risk of obesity-related health problems, while women with a waist circumference of 35 inches or more are at an increased risk of obesity-related health problems.
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