Researchers have shown that the influence of genes changes during the first years of life. In childhood, there are other factors in particular that affect weight.
From hi-tech countries to Burkina Faso, almost one-third of the world’s population is overweight. The Institute for Health Metrics and Evaluation in Seattle (IHME) estimates that 2.2 billion people are now overweight (1). Researchers are already talking about a disturbing global health crisis.
But what is the cause of the steady increase in overweight in the population? Basically, there are two major influencing factors: the environment and genes. But their influence also fluctuates, as we can see. What has an effect on weight during childhood can later lose its influence and vice versa. A large group of scientists led by Karri Silventoinen from the University of Helsinki (2) has now found evidence that environmental factors that are shared by siblings only play a greater role in early childhood. Their influence diminishes in adolescence, but the importance of genes increases.
In order to better understand the huge obesity problem, scientists are no longer looking for individual genes that fuel obesity or experiences that contribute to eating the wrong food. They also want to find out when their influence is greatest.
In The American Journal of Clinical Nutrition, more than a hundred researchers have tried to show how the influence of genes and the environment on people’s weight changes over the course of their lives. The researchers were particularly interested in the period from childhood to early adulthood, so they evaluated data from almost 88,000 twins between the ages of six months and 19 years. They come from dozens of research cohorts from a total of 20 countries.
In the end, the scientists saw a clear development: the older the subjects grew, the wider the range of Body Mass Index (BMI) measured. At the age of four, BMI scatters could be explained least by genetic effects. Here, the hereditability was only about 0.4 (for an explanation: 0 = the genes play no role, 1 = the environment has no influence). However, this changed with increasing age. When the test subjects were 19 years old, this value climbed to 0.75. It seems that environmental factors that are shared by twin siblings, such as eating in the family, still have quite a large influence on weight in childhood. From the age of 15, however, the scientists were no longer able to measure the effects of the common environment. During this time, however, the genetic influence increased.
The scientists are unable to say which genes are particularly important here, as such a study is only concerned with statistical relationships. However, dozens of genetic snippets associated with obesity are already known from genetics. One of the best known is the fat mass and obesity-associated protein (FTO). It was one of the first genes to be associated with obesity using genome-wide sequencing in 2007. Of all known candidate genes, this genome section probably has the greatest influence on the difference between human BMIs.
In their work, however, the researchers make it clear that health behavior is still important at an advanced age. Many of the genes in question would not have a direct effect on weight, but would, for example, influence food intake and other behavior patterns. And these can also be influenced by genetic predisposition. Variants of the FTO gene, for example, influence the ability of self-regulation and the way food is eaten. They often make it difficult but not impossible for the carrier to stay slim.
This is part 28 of a series covering twin health provided by Paul Enck from the Tübingen University Hospital and science writer Nicole Simon. Further studies in twin research can be found at TwinHealth website. Translation was done with the assistance of DeepL translator.
- Silventoinen K. et al. Genetic and environmental effects on body mass index from infancy to the onset of adulthood: an individual-based pooled analysis of 45 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins) study. Am J Clin Nutr. 2016;104(2):371-9.