Stephanie wrote in about a very common problem:

My daughter is 4 years old. She isn’t the tallest cat in town (she is about the 15th-25th percentile for height), and her BMI always ends up being in the high range (like over 85%). I worry about it. I am very health conscious for myself and my family. We live by all of the “rules.” And yet.

The family doctor doesn’t worry — been shrugging it off since day one. Maybe because both Dad and I are very lean. Maybe because, as patients of hers, she knows we are a very healthy family (regular exercise, healthy diet, no smoking, healthy pregnancy with aforementioned child). Family doc knows we have never fed our kid a drop of juice, no fast food, homemade meals, limiting screen time, healthy choices …

So I’m stumped. Why the high BMI for my daughter? I would love to hear some solid, scientific data about why this could be, as opposed to: “Meh, she’ll be fine.”

We know that obesity, in the long run, isn’t good — but we can’t even agree on what “obesity” is. BMI, or body mass index, is a single number that basically reflects weight-for-height. We figure that the more someone weighs for their height, the more likely they are to weigh “too much.” What we really need is a measure that tells us when someone’s weight is unhealthy, or likely to lead to ill health. Instead, we use that BMI number, a very poor predictor of individual health outcomes.

There are several reasons why BMI is not a great way to discriminate between healthy and unhealthy weights.

A BMI doesn’t reflect the difference between lean muscle mass and fat mass. What’s unhealthy is excess body fat, not excess body muscle. A muscular, lean individual with little body fat may have a “high” measured BMI because muscle has weight.

BMI doesn’t distinguish between kinds of body fat. We know that visceral fat — the kind in your belly, or the kind that contributes to an “apple” shape — has far more long term negative consequences for health than fat distributed in the lower body.

Criteria for “healthy” versus “unhealthy” BMI are based only on statistics, not on individual health outcomes. We’ve decided that anyone above the 85 percentile for BMI (down to age 2) is overweight, and anyone above the 95 percentile for BMI is obese. This compares a child or adult’s BMI against historical data, which assumes that people thirty years ago had a BMI distribution healthier than today. While that’s generally true for the population (obesity-related health problems are genuinely much more common now), that doesn’t mean it’s specifically true for each individual or child. In other words, relying on statistics forces us to oversimplify and generalize instead of focusing on ways to individualize our approach to maximize health.

Finally, improved diet and exercise habits improve health outcomes, even if the BMI doesn’t change. Over-focusing on BMI can lead to discouragement, preventing steps that can really improve well-being in children and adults.

So what should Stephanie’s mom do? Forget the BMI and keep up those good healthy life habits. Stay active. Turn off the TV. Eat moderate-sized portions, slowly, eating mostly plants and whole-grains. Eat as a family, and share cooking and cleaning chores together. Avoid eating out or doing take-out too often, and stay away from sweet drinks (soda and juice are equally unhealthy). Enjoy eating and playing, together as a family, and don’t worry about the numbers on the scale. The BMI is one thing, maybe a starting point to remind us to keep up healthy habits. But it’s a terrible target to use as a goal for your child’s body.

Roy Benaroch is a pediatrician who blogs at The Pediatric Insider. He is also the author of Solving Health and Behavioral Problems from Birth through Preschool: A Parent’s Guide and A Guide to Getting the Best Health Care for Your Child.