How much weight is too much?

From a health standpoint, what's a healthy weight, and at what weight is a person at risk for developing health problems? Several studies suggest that many teens don't know when they've reached an unhealthy weight - and neither do their parents. In fact, the considerations are different for adults and teens. For adults, the gold standard for defining healthy weight is something called body mass index, or BMI, a number that measures weight in relation to height. When an adult's BMI is 25-plus, health risks increase compared with someone who has a BMI of less than 25; 25 to 29.9 is considered overweight, and 30 or above is obese. (BMI is calculated by dividing weight in pounds by the square of height in inches, then multiplying that number by 703. BMI charts are available in books, at doctors' offices, and on many health-related Web sites.)

For children and teens, the picture is more complicated, because what's considered a healthy weight and normal amount of body fat changes with age and differs in boys and girls. The best way to determine a teen's weight status is to consult a physician and to use special pediatric growth charts developed by the Centers for Disease Control and Prevention for children ages two through nineteen. These charts plot weight according to percentiles for kids of the same age, gender, and height. (The Centers for Disease Control and Prevention provides an interactive computerzied BMI calculator that does the work for you. Visit this Web site: http://apps.nccd.cdc.gov/dnpabmi/calculator.aspx.)

BMI and the BMI growth charts are only rough guides for a healthy weight. For instance, some teens fall into the "at risk" category for being overweight becuase they're muscular, not because they hae excessive body fat levels.

It's also important not to go by any single measure of BMI percentile for a child or teen. Rather, BMI percentile should be watched over time to see if a child is staying on track. According to Roman Shypailo, a body composition expert at the Baylor College of Medicine, "The important thing is to watch for significant 'drifting' in a teen's BMI percentile, either up or down." In other words, if there's a sudden large increase or if a teen creeps up by three or four units every year, that's probably cause for concern. But if he or she has been on the same track for years - even if it's somewhat on the high side - there's less reason to worry.

Although it's unwise for kids and their parents to be obsessively calculating BMI and BMI percentile, it's probably a good idea to calculate them periodically, with a physician's help.