Thursday February 7 2008
Lifestyle factors are the main cause of obesity
Childhood obesity is down to nature not nurture, reports The Times and other news sources. Genes account for “more than three quarters of the difference between children’s waistlines, with lifestyle factors such as diet and exercise playing a much smaller role”, the newspaper adds. All the news stories focus on a message that it is wrong to blame parents for their child’s weight as much of the variation is due to genetics.
The research behind these stories has looked at the “heritability” – an estimate of the extent to which characteristics (e.g. physical, behavioural, personality) are determined by genetic make up – of body mass index and waist circumference using a UK twin study which compared identical and non-identical twins. A limitation with these studies is that they cannot identify which genes are responsible.
The genetic component of risk for obesity is likely to be complex, including genes that affect appetite, personality, as well as how fat is deposited. However, a predisposition to obesity does not mean a child will definitely be overweight and parents should not abandon a healthy lifestyle, as there is good evidence of the benefits of weight reduction on health.
Where did the story come from?
Dr Jane Wardle and colleagues from University College London carried out this research. The study was supported by a grant from the Biological and Biotechnology Research Council. It was published in the (peer-reviewed): American Journal of Clinical Nutrition.
What kind of scientific study was this?
This study was a twin study carried out on a subset of identical and non-identical twins who were enrolled in a larger study – the Twins’ Early Development Study (TEDS). TEDS is a cohort study of twins born between 1994 and 1996 in the UK. For this particular study, the researchers were interested in quantifying the genetic and environmental influences on body mass index (BMI) and waist circumference (WC).
In 2005, parents were sent a questionnaire and a tape measure and asked to measure the waist circumference and the height of their child. Of the 8,978 families they contacted, 62 per cent returned the questionnaire, and after excluding families where a twin had a specific medical condition and for other reasons, 5,092 families (twin pairs) remained in the study. Within a year of the parents returning the questionnaire, the researchers visited the homes of 228 families to measure height, weight and waist circumference themselves. This allowed them to assess how similar the parent and researchers’ measurements were.
Using a complex modelling technique, the researchers compared the physical (BMI, WC) similarities between identical twins with the physical similarities between non-identical twins to determine what contribution “genetics” had on these characteristics. They also compared the average height, weight, BMI and WC of the twins with population averages in 1990.
What were the results of the study?
The researchers found that, overall, the twins’ heights and weights were greater than the1990 averages, though BMI was similar. Waist circumferences were substantially higher than in populations in 1990, particularly in girls. They also found that identical twins were more likely than non-identical twins to have similar BMI and waist circumference measurements, suggesting a genetic component to these characteristics.
Using the modelling method, the researchers conclude that variation in BMI scores is 77 per cent heritable, while variation in waist circumference is 76 per cent heritable. They also found that the “shared-environment” had little effect on BMI and waist circumference (10 per cent each).
What interpretations did the researchers draw from these results?
The researchers say that their modelling shows a substantial genetic influence on BMI scores and waist circumference and that their study is the first to have quantified the heritability of waist circumference. They have found that waist circumference is as heritable as BMI (though 40 per cent of this was due to different genetic factors). Their findings, say the researchers, mean that “blaming” parents for their child’s obesity is wrong.
What does the NHS Knowledge Service make of this study?
The study has demonstrated that BMI and waist circumference are heritable traits and that the genetic component has a greater influence than the environmental component.
The researchers discuss important criticisms of twin studies, which hold for this study:
- Firstly, the common finding that the shared environment has little effect. In studies of obesity, this is surprising considering the fact that many models suggest that the environment is “the root cause of obesity”. They say that this finding suggests caution when assuming that if all parents followed “current child-feeding recommendations, the obesity problem would be solved”.
- Secondly, twin studies assume that identical and non-identical twins share the same environment (in the uterus and in the family). There is discussion in the scientific literature about whether this is an accurate assumption, however the researchers here say that the effect is small and “it would not materially change the conclusion”.
- Thirdly, such studies do not identify genes responsible for traits or behaviours. No major genes that cause obesity have been identified and obesity is likely to be due to the influences of many different genes, affecting appetite as well as how fat is stored.
Importantly, parents should not give up on healthy lifestyles. Having a gene that predisposes to obesity does not mean a child will become obese. As Jane Wardle, the lead author of the study is quoted on ITN as saying, “children born with "Billy Bunter" genes are not inevitably overweight but have to work extra hard to stay slim”. The researchers give the example of phenylketonuria, a strongly inherited condition that can be entirely treated by environmental interventions. This is still a complex and controversial area; there is a great deal of research into strategies to prevent or treat obesity, and exercise and diet have been shown to result in weight loss in and/or improved cardiovascular risk factors in overweight or obese individuals.
Of all the interventions which could tackle the “obesity epidemic” addressing eating and physical activity habits in childhood is a more practical and realistic intervention than gene therapy.