Physical activity, healthy eating and BMI not linked in older teens: study

April 30, 2008

Contrary to what many researchers expect, physically active older teens don’t necessarily eat a healthier diet than their less-active contemporaries. And there appeared to be no link between body mass index (BMI) values and levels of physical activity, the research showed.

The study of 900 Vancouver-area teenagers in Grades 10 through 12 was conducted by Dr. Catherine Sabiston, of McGill University, and P.R.E. Crocker, of the University of British Columbia (UBC). The results of their research – conducted in Vancouver while Dr. Sabiston was still a PhD student at UBC – were published in the Journal of Adolescent Health earlier this year.

Overall, said Sabiston, now an assistant professor in McGill’s Department of Kinesiology and Physical Education, boys reported participating in more physical activities but ate a less-healthy diet than did girls. Moreover – and contrary to established wisdom in the field –researchers found that people with “healthier” BMI values were no more likely to be physically active than those with higher, “unhealthier” values. Unexpectedly, it was the latter who were more likely to eat a healthier diet.

“A lot of people are surprised,” Dr. Sabiston said, “but when you think about it, BMI doesn’t have a huge impact on physical activity. And in terms of diet, it actually makes sense that someone who is not happy with their body might try to eat more healthily.”

According to Sabiston, who is also director of McGill’s Health Behaviour and Emotion Lab, the results showed only a very weak correlation between physical activity and healthy eating, and virtually no correlation between an individual’s BMI and his or her level of physical activity. The study was undertaken to test a comprehensive model of physical activity and healthy eating behaviour in teens aged 15 to 18, partially in response to two perceived problems with existing research in the field.

“First of all, older adolescents are an unrepresented sample in research studies,” Sabiston said. “Researchers have generally looked at youths or at university populations and have completely missed this unique, intermediate age group.” Second, Sabiston said, many researchers have traditionally treated physical activity and healthy eating as separate phenomena, and have only rarely explored their similarities and differences simultaneously.

The study also found a significant difference in the way boys and girls approach physical activity and healthy diet. Boys, Sabiston said, need to attach value to a healthy diet and feel confident in their ability to follow a healthy diet before they’ll actually do it. Girls, she said, regardless of how they feel about their ability to eat a healthy diet, only need to feel it is important to do so before they’ll eat properly.

What this study really says, Sabiston explained, is that one cannot assume that someone who is physically active necessarily eats a healthy diet – or the reverse, that someone who is more sedentary or has a high BMI by definition eats a diet of junk food.

“This study drives home the point that as a society, we’re primarily focused on extrinsic things like appearance and weight versus the betterment of health,” Sabiston said. “From a public health perspective, this means we should probably focus on people who are at a healthy weight or even underweight, and emphasize that healthy eating is not just about weight-change.”

Source: McGill University