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Is my child underweight? When you should (and shouldn't) worry

With childhood obesity rates soaring in the past four decades, increasing the cumulative incidence of Type 2 diabetes, hypertension and chronic kidney disease, it may seem frivolous to worry about a child who seems to be underweight.

»RELATED: 9 common infant illnesses (and when to worry)

But kids whose weight is below the range considered normal can incur health problems, too. Some underweight children have digestive conditions such as gastro-esophogeal reflux disease (GERD) or inflammatory bowel disease, for example. And while it's a lot more likely for a parent to worry for no reason about a seemingly underweight child, there are situations when it could be an issue.

"Being underweight is not the same as being thin or slender," noted Virginia Tech extension specialist and nutrition expert Elena Serrano. "Some children have a naturally slight build and maintain it with a well-balanced diet and physical activity. This is normal and healthy. However, true underweight may be a sign of dietary, health or emotional problems."

Some kids are thin and are just fine that way, noted Harvard Medical School assistant professor of pediatrics Claire McCarthy in Parenting.

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When should a parent worry about a kid being below the healthy weight range for her age? It's a good idea to do a little independent research and then consult your family's medical professional, according to the CDC.

You can start by getting an idea of your child's Body Mass Index and where it falls on the general scale.

BMI is a computation of a child's weight in kilograms divided by the square of his height in meters. For children and teens, BMI is both age- and gender-specific, earning it the tag "BMI-for-age."

According to Children International, it's super important not to use the adult BMI scale on kids: "They will frequently say a child is underweight when they're fine."

The CDC also warns parents not to consider its calculator as a replacement for a doctor's advice.

"The most important thing to know is why your child is underweight," McCarthy noted. "While some kids simply don't eat enough, there are many possible reasons, such as infections, food allergies, and intestinal, endocrine, heart, lung and liver problems. Your child should have a thorough checkup and may need a referral for testing with a specialist."

If you make it through that step with your child's doctor and everything is fine, there are a few strategies to potentially help your child gain weight without making a big deal about it, McCarthy said:

  • Let her eat fat, from putting butter on her bread to grating extra cheese over his pasta. "If he is 4 or older, try peanut butter," McCarthy said.
  • Limit juice, since kids who fill up on juice may lose their appetite. Too much juice can also cause diarrhea which will undermine weight gain.
  • Give your child high-calorie, protein-rich snacks like cheese, lunch meat, peanut butter crackers or full-fat yogurt.
  • Make your own supplementary shakes by mixing instant breakfast mixes, powdered milk or yogurt into whole milk and adding fruit for sweetness.

And don't worry overmuch, McCarthy advised. "Keeping in close touch with your doctor and playing with your child's diet will help him get on track."

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