Childhood Obesity: The Death of a Paradigm

Author: Timothy Olds

Date: 03 May 2010

Myth and legend may constitute a significant proportion of what passes for rational discussion of public issues. It’s always hard to tell since it takes a lot of work to uncover and then question the shared assumptions of debate. In this article, Timothy Olds pulls up his sleeves and takes a closer look at tales about childhood obesity.

There is probably no public health issue as divisive, as controversial, as contested as childhood obesity. In 2002, the relentlessly pessimistic Sydney-based paediatric researcher Kate Steinbeck asserted that “childhood obesity is at epidemic proportions, and Australia needs to respond, just as it did to the AIDS threat.” Just a few years later, David Kirk could write that “there is no foundation to justify the confident and loud assertions that we are in the midst of an obesity crisis.”

Amid all this crossfire, the “logic diagram” of childhood obesity is familiar to us all: kids are eating more than they used to, and more of the bad stuff (soft drinks, fast foods, fat, trans fats). They are also less active than they used to be, and the imbalance between energy in and energy out means that childhood obesity is not only increasing, but increasing exponentially.

One consequence is that today’s kids are less fit than we were when we were young. This isn’t so bad in itself — after all, fat kids aren’t keeling over dead in the streets — but fat kids become fat adults, and fat adults do die younger from cardiovascular disease and diabetes, so childhood obesity is a public health “time-bomb”.

How good is the evidence for this familiar narrative? Are kids really eating more than we did when we were young? A systematic review by Stevenson of 2547 reports of self- and parent-reported energy intake in developed countries dating back to 1854 found no evidence of an increase in children’s energy intake. In fact, the review found that since 1955, kids’ energy intake had been falling at the rate of 3% per decade. Fat intake peaked at 40% of total calories in 1960, and has fallen steadily to 30% since then. Intake of saturated fat has also fallen. This is sometimes dubbed “the American paradox”: the parallel rise in obesity rates and the popularity of healthier food.

Are kids less active today than we were as youngsters? There are simply not enough data to decide. Harten and Olds have shown that the percentage of kids walking to school has declined at the rate of about 2% per year since the 1970s, participation in PE appears to be down, and a UK study’s meticulous vignette from 1919 showed kids who were almost unbelievably active by today’s standards. But high quality longitudinal data are simply not available.

There is, on the other hand, no doubt that kids are less fit today. Grant Tomkinson’s landmark study of over 50,000,000 children from 50 countries showed that aerobic fitness has been declining at the rate of 3-4% per decade since 1970: our kids are likely to be 10-15% less fit than we were when we were their age. Jumping and sprinting performance is also declining.

But surely kids really are getting fatter? Yes and no. There have been rapid increases in fatness, measured either by weight for height or skinfold thickness, since 1960, at the rate of about 7% per decade. But consistent and strong Australian and overseas data show a stabilisation since the late 1990s. There has been no increase in children’s weight status over the last decade.

Even so, levels of childhood overweight are high — about 5-6% of Australian kids are obese and another 15-20% overweight. If fat kids become fat adults, then there is a looming public health crisis. But do they? Statistically, yes, but the relationship is perhaps not as strong as many people think. A 20-year Australian follow-up study by Alison Venn found that 57% of obese kids become obese adults, and are six to seven times more likely to become obese adults than normal weight children. Nevertheless, the overwhelming majority of obese adults (perhaps 90%) were not obese as children. And about 70% of overweight adults were not overweight or obese as children.