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JOHN LEHMANN/The Globe and Mail

Does child care make kids fat? That's the question answered by a study by Chris Herbst and Erdal Tekin, forthcoming in the Review of Economics of the Household.

Herbst and Tekin use the U.S. Early Childhood Longitudinal Study to study the connection between child care arrangements and children's body mass index (BMI) scores for 3,113 kindergarten-aged children living with single mothers.

Of children who had been cared for by a parent in the year before going to kindergarten, about one in four – 24 per cent – had a BMI score in the overweight range in the fall of their kindergarten year, and one in ten was obese. Obesity rates were higher for children who had any kind of child care arrangement before kindergarten, but the children with the greatest obesity risk were those in centre-based care before starting kindergarten: one in three were overweight in the fall of their kindergarten year, and 14 per cent were obese.

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Yet these numbers, in isolation, prove nothing. For example, a number of studies (for example, here and here) have found that maternal employment increases the risk of childhood obesity, and children in child care generally have employed mothers. That, or something else, could explain the correlation between child care and BMI.

So Herbst and Tekin did a regression analysis that included controls for a wide range of factors that might be expected to influence a child's BMI: race, gender, the number of hours/day spent watching TV, the number of days/week the child gets vigorous exercise, the family's income, the child's health, and so on.

Even with these other controls, children in centre-based care had a five percentage point higher chance of being obese in the fall of their kindergarten year, as compared to children cared for by a parent.

So if your children are in centre-based child care, should you be worried?

Not necessarily. First, Herbst and Tekin considered U.S. child care centres, a number of which are run for profit, and serve a different market niche from Canadian centre-based care. ( Here is a typical U.S. menu.)

Second, thinner isn't necessarily better. Herbst and Tekin's study included many low income children, and found that centre-based care allowed children at the bottom of the weight distribution to gain weight, as well as children at the top. More generally, BMI-based measures do not distinguish between healthy and unhealthy weight gain.

Third, other research, has found that children similar to the ones Herbst and Tekin studied, particularly disadvantaged girls, benefit in other ways from early interventions such as child care.

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Finally, the experience of single-parent families studied by Herbst and Tekin might not generalize to the U.S. population as a whole, let alone Canada.

But a simple input-output analysis suggests that child care will lead to weight gain if children have lots to eat and a relatively small play area.

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