Monday October 11 2010
Environmental factors can also determine people's body shape
“Curvy women can ditch the diet – after scientists found that a woman’s body shape is all down to her genes”, reported the Daily Express.
This large genetics study pooled data from 61 studies involving almost 200,000 people. It identified 14 areas of DNA likely to contain genes that affect waist-to-hip ratio, one of which was already known. A major strength is that it pooled the data from such a large number of people that it enabled the detection of areas that have only a small effect on waist-to-hip ratio.
These genetic areas have only a small effect on waist-to-hip ratio, accounting for just 1.03% of the variability seen between the participants in total. As other studies in twins suggest that genes may account for between 22% and 61% of variability in waist-to-hip ratio, there are likely to be many other genetic factors involved. More research will probably follow to identify the actual genes within these areas that are having an effect.
The newspaper’s report that body shape is governed entirely by genetics is incorrect. Environmental factors (such as diet and physical activity) also play a role.
Where did the story come from?
The study was carried out by a large international consortium of researchers, called the Genetic Investigation of Anthropometric Traits (GIANT) consortium. Funding was provided by a large number of governmental bodies, companies and charitable foundations. The study was published in the peer-reviewed journal Nature Genetics.
The BBC covers this study well, highlighting that the research has only identified some locations where genes involved in waist-to-hip ratio may lie, rather than identifying the genes themselves. It also notes that these locations probably only account for a small amount of the variation in waist-to-hip ratio.
What kind of research was this?
The aim of this research was to identify areas in the DNA in which genes that influence waist-to-hip ratio might lie. The researchers report that waist-to-hip ratio is affected by genetic factors that appear to be independent of body mass index (BMI) or the overall amount of body fat.
This was a statistical pooling (meta analysis) of genome-wide association studies. A genome wide association study is a type of case control study, comparing the DNA of cases and controls to identify variations that are more or less common in cases. Traits such as waist-to-hip ratio are likely to be affected by a large number of genes, each having a small effect (as well as environmental factors). Pooling a number of these studies improves the ability to detect genetic variations that are having a small effect.
What did the research involve?
The researchers pooled data from 61 genome-wide association studies looking at waist-to-hip ratio (WHR). Initially, they used data from 77,167 participants in 32 of these studies to look for variations that were more or less common in cases than controls. The genetic variations that were associated with WHR in this first pooling were then examined in the remaining 29 studies (113,636 participants) to confirm the initial findings. Finally, the two sets of studies were pooled to look at any variations that had shown associations in both sets of data. This provided an overall estimate of how strong the association was.
The study only included people of European descent, as including people of different ethnicities can influence the results. The analyses took into account the participants’ BMI and age. As women and men store fat differently, the researchers also investigated whether certain variations were associated with WHR in women but not men, and vice versa. The researchers then checked whether these areas overlapped with areas that had been associated with BMI in another analysis carried out by the GIANT consortium. They also looked at what genes were in the areas identified, how they might play a role in affecting WHR, and whether these genes were active in fat tissue.
What were the basic results?
In the first part of the study on data from 32 studies, the researchers identified 16 areas (loci) of the DNA that contained genetic variations associated with waist-to-hip ratio. These variations were then tested in the second pool of 29 studies. This confirmed that 14 areas of the DNA contained genetic variations associated with waist-to-hip ratio. Thirteen of these loci were new associations with waist-to-hip ratio, and one had been identified in a previous study. These variations also showed a strong association with WHR when all 61 studies were pooled. These loci included, or were near to, genes with a variety of roles in the body, including insulin signalling, activity of an enzyme that breaks down fats, and the making of fats.
Overall, the loci identified accounted for 1.03% of the variability seen in WHR. Each individual locus accounted for between 0.02% and 0.14% of the variability. Seven loci showed a greater association with WHR in women than in men.
Only four of the areas identified also showed an association with BMI.
How did the researchers interpret the results?
The researchers concluded that their findings provide evidence that multiple genes have an effect on body fat distribution. They say that this effect is independent of overall body fat, and it occurs differently in men and women.
This research has identified a number of areas in the DNA that are likely to contain genes that affect waist-to-hip ratio. The strengths of this study include the large number of people whose data has been pooled, allowing the detection of areas that appear to have only a small affect on waist-to-hip ratio. The study also benefitted from its use of an independent set of data to confirm the findings. It is likely that more research will be aimed at identifying the exact genes that are having an effect within these areas.
Overall, these identified loci have only a small effect on waist-to-hip ratio, accounting for 1.03% of the variability in waist-to-hip ratio seen in the participants. Studies in twins have suggested that between 22% and 61% of variability in waist-to-hip ratio may be accounted for by genetic factors. This means that it is likely that there are other genetic factors that have not yet been identified. Despite the Daily Express’s report that body shape is governed entirely by genetics and that women can “ditch the diet”, this is not the case. Environmental factors also play a role.
As different body fat distribution patterns have been found to be related to cardiovascular risk, further studies are likely to assess the link between the areas identified in this study and cardiovascular risk. Hopefully, an improved understanding of the genetics of fat distribution and obesity will lead to better ways of preventing obesity and reducing cardiovascular risk. However, much more work is required before this can become reality.