How much should teens weigh to prevent heart disease as adults?

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We’ve long known that being overweight can lead to cardiovascular disease and early death. We also know that being overweight as a teen makes it more likely that someone will be overweight as an adult, which is why we pediatricians talk so much to teens and their parents about getting to — and staying at — a healthy weight.

But we may have set the bar too low. A study just published in The New England Journal of Medicine (NEJM) says that if we really want to prevent heart disease in adulthood, our teens should be much thinner than we currently tell them to be.

The number doctors use to figure out if someone’s weight is healthy is the body mass index, or BMI. The BMI is a person’s weight in kilograms divided by the square of their height in meters, and it is a very useful number when it comes to describing how much weight a person is carrying on their body frame. It’s kind of a thinness/fatness index, if you will. For adults, we say that the BMI should be between 18.5 and 24.9 (25 is the cutoff for overweight). For youth 2-20 years old, we use BMI percentiles: we say that a BMI percentile between 5 and 85 is fine, between 85 and 95 is overweight, and 95 or greater is obese.

In Israel, 17-year-olds have a medical exam as a screening for military service. As part of this medical exam, they get a height and weight measurement. For the NEJM study, researchers looked at the height and weight data for 2.3 million Israeli adolescents (mostly boys) taken between 1967 and 2010. They calculated the teens’ BMIs and then followed them out until 2011 to see who died from cardiovascular problems like heart attacks, strokes, or sudden death (which is usually from a cardiovascular cause).

They found that teens whose BMI was in the overweight or obese range, were much more likely to die of cardiovascular causes as adults. Compared with those whose BMI was in the 5^th to 24^th percentile, teens whose BMI was in the 95^th percentile or greater were 4.9 times more likely to die of cardiovascular disease, 2.6 times more likely to die of stroke, and 2.1 times more likely to die of sudden death. Those whose BMI was in the 85^th to 94^th percentile also had higher risks: 3 times for cardiovascular disease, 1.8 times for stroke, 1.5 times for sudden death. No surprise.

But here’s the surprise: that risk of cardiovascular disease didn’t start at the 85^th percentile. The risk started at the 50^th percentile. The teens whose BMI was in the 25^th to 49^th percentile had the lowest risk of cardiovascular disease.

This is a big deal. It goes against what we have been telling people for years.

Let’s use some weights to illustrate. For a 17-year-old boy who is 5 foot 9 inches (average height for 17), the 25^th to 49^th percentile would be a weight between 132 and 143 pounds. But currently, we tell that boy that it is fine for him to weigh up to 170 lbs. A 17-year-old girl who is an average height of 5 foot 4 inches should weigh between 112 and 122 pounds to prevent heart disease; currently, we tell her that it is fine to weigh up to 148 pounds.

Bottom line: if we want to our children to grow into healthy adults, we need them to be thinner than we currently tell them to be.

Now, there are limitations to this study. The researchers didn’t have a lot of data on the BMIs or lifestyle habits of the teens as they grew, there weren’t as many women as men in the study, and we don’t know if its findings are true for everyone worldwide. But its findings can’t be ignored.

The Centers for Disease Control and Prevention (CDC) has a calculator you can use to find out a child or teen’s BMI percentile. It also has calculators for adults.

If your child or teen’s BMI is in the 50^th percentile or higher, don’t panic. Talk to your doctor — and rather than concentrating on a particular weight, concentrate on lifestyle habits. In particular, strive to be sure your child

• is active for an hour a day
• eats a diet that includes five servings of fruits and vegetables daily, is low in refined carbohydrates and sugar (including sweetened beverages like soda), low in saturated fat, and high in whole grains
• gets 8-10 hours of sleep a night
• spends less than 2 hours a day on entertainment media.

If your child is far off from these, at least set him in the right direction — like by having one extra serving of fruit or vegetables a day, going for a 20-minute walk together, or turning off the TV a half hour earlier at night. Small steps can be the best way to begin — and if we care about the health of our children and our society, we need to begin.