GeneWatch UK today responded to a new twin study claiming that differences in body mass index and waist size were 77% governed by genes (1).
"Making claims of strong genetic influence based on twin study results is highly irresponsible," said Dr Helen Wallace, Director of GeneWatch UK. "Twin study findings depend on the assumptions that are made about how genes and lifestyle interact. It is impossible to draw reliable conclusions about the importance of genetic effects from twin data alone".
Twin studies are well known to exaggerate the importance of genetic effects in common conditions and diseases because the way the data is analysed is based on assumptions originally made by the eugenicist Ronald Fisher in 1918. Complicated effects - such as the interactions between multiple genes and different foods - are assumed to be less important than simple ones, but this now seems unlikely to be true. Fisher's assumptions always give the highest possible estimate of 'heritability' and heritability can be high even if there is no genetic effect at all (2). The recently discovered 'obesity gene' (called the FTO gene) has only a small effect, accounting for about 1% of the differences in body mass index in the UK population.
"We are facing an epidemic of obesity and it is critically important that people are not misled about their health," said Dr Wallace. "Although genetic differences do play a role, this study is likely to exaggerate their importance. Overweight people have overweight pets and they don't share any genes with their pets".
For further information contact:
Dr Helen Wallace: 01298-24300 (office), 07903-311584 (mobile).
Notes for editors
(1) Wardle et al. (2008) Evidence for a strong genetic influence on childhood adiposity despite the force of the obesogenic environment. American Journal of Clinical Nutrition, 87, 398-404.
(2) Wallace HM (2005) A model of gene-gene and gene-environment interactions and its implications for targeting environmental interventions by genotype. Theoretical Biology and Medical Modelling, 3:25. http://www.tbiomed.com/content/3/1/35