Too good to be true?

Remember how we always say that diet and exercise aka lifestyle stuff, are abysmally poor explanations or treatments for weight problems. The message is starting to sink in but there is still way too much barking up that wrong tree. A report in The Journal of the American Medical Association Pediatrics seems to show that lifestyle, aka obesogenic environments, can make a big difference.

Investigators in England did a project using 925 twin pairs of 4 year olds and studied a different and important way of looking at the environmental impact on obesity. (1) We already know that genes are mostly to blame for the problem but the environment probably has a different influence on obesity depending on those genes. They observed that “the hypothesis that obesity-related genes are more strongly associated with BMI in more obesogenic home environments,” makes sense because, “The heritability of BMI is higher in populations with higher average BMIs, in countries with higher gross domestic product, in populations born later, and in families of lower socioeconomic status.”

These guys showed that a genetic predisposition to obesity estimate for child BMI was 86% for children raised in homes with higher obesogenic risk, but only 39% for counterparts raised in homes with lower obesogenic risk. An almost 50% effect of environment on obesity. That’s an amazing lot of effect.

This led to headlines like “A Healthy Home with More Fruits and Vegetables and Less Time Watching TV Can Cut Children’s Genetic Obesity Risks By HALF, Study Finds.” (2)

The way they measured the heritability was by looking at their twins. Genetic and environmental contributions to variation in a trait can be estimated by comparing similarity between identical twins (who share 100% of their genes) with that between fraternal twins (who share approximately 50% of their genes). Although finding twins raised by different families as has been done in the past would have been a better way to tease family genes and environmental effects apart, those creatures are harder to find.

The way they measured the “obesogenic home environment” was a complicated sum of scores created to capture food, physical activity, and TV watching in the home.

But this big effect of the obesogenic environment seems to contradict abundant other evidence in general populations at every age and the fact that taken individually diet, physical activity and time watching TV don’t seem to make a meaningful difference. These authors themselves concede that, “home-based childhood obesity prevention and treatment efforts, .. have been ineffective.” Maybe looking at gene-environment interactions and lumping lots of stuff simultaneously into the environment can show a bigger difference.

In an accompanying editorial (3) it is pointed out that the environment is very complicated. Other aspects of the environment might be the real factors. “One such example is the degree of cognitive enrichment or stimulation of the home environment. Indeed, children living in home environments that are less cognitively enriched (eg, fewer games, books, and activities) are more likely to become obese.”

But perhaps what really nails the problem with this study was simply put by my friend David Allison, Dean of The Indiana University School of Public Health-Bloomington. “Gene-environment correlation is not necessarily gene-environment interaction.”

Still, if the environment makes that much difference why can’t it be detected, even if at a lower effect size, in a mixed population? I don’t know. There’s something else wrong here that I can’t figure out. Maybe it’s somewhere in the math.

1. Schrempft S et al. Variation in the Heritability of Child Body Mass Index by Obesogenic Home Environment. JAMA Pediatr. Published online October 1, 2018. doi:10.1001/jamapediatrics.2018.1508


3. Faith MS, Epstein LH. Healthy Homes and Obesogenic Genes in Young Children. JAMA Pediatr. Published online October 1, 2018. doi:10.1001/jamapediatrics.2018.1945