Exercise, genetics and obesity

Behind the Headlines

Wednesday September 1 2010

Lifestyle factors are more likely to influence weight than genetics

A “genetic excuse for obesity ‘is a myth’”, reported The Daily Telegraph. It said, “people could work off around 40 per cent of the extra weight that “fat genes” laid on them by exercising.”

This news report is based on a study that looked at how much physical activity over 20,000 people in Norfolk did and whether they were genetically more likely to be overweight. The researchers found that, although some genes increased the likelihood of having a higher BMI (body mass index), being active meant that these “genetically predisposed” individuals were less likely to be overweight. At the same time, being inactive increased the amount of weight they were likely to gain.

Current recommendations are that everyone should do at least five 30-minute sessions of moderate exercise a week as part of a healthy lifestyle. The results of this research suggest that this is beneficial for maintaining a healthy BMI, even in people who may be genetically prone to being overweight.

Where did the story come from?

The study was carried out by researchers from the University of Cambridge and was funded by Cancer Research UK, the Medical Research Council, British Heart Foundation, Food Standards Agency, Department of Health and Academy of Medical Sciences. It was published in the peer-reviewed medical journal PLoS Medicine.

The Telegraph, Sun and Daily Express all accurately reported the results of this study. The newspapers quoted the study’s author, Dr Ruth Loos, who stated: “It goes to show we’re not complete slaves to our genetic make-up.”

What kind of research was this?

This was a cohort study that investigated the extent to which people with a genetic susceptibility to becoming obese can change their weight with exercise. The research was based on previous genetic studies, which had identified 12 possible positions on 11 genes where differences between people in their DNA sequence could influence BMI. However, although the studies showed an association between variations in the genetic sequence at these positions and BMI, they only seemed to have a very small effect on a person’s risk of obesity. This suggested that lifestyle played a greater role, and the new study aimed to investigate this in more detail.

What did the research involve?

The participants were part of a larger cohort study, called the EPIC-Norfolk study, which involved 25,631 people living in Norwich. The participants were aged 39–79 years old during a health check that took place between 1993 and 1997. They had a second health check between 1998 and 2000. During the health checks, the participants’ weight and height were measured and their BMI calculated. In a questionnaire, the participants were asked about the amount of physical activity they usually did each week, at work and during their free time. Based on this questionnaire, they were classified as:

  • inactive (sedentary job with no recreational activity)
  • moderately inactive (sedentary job with less than half an hour a day recreational activity, or a standing job with no recreational activity)
  • moderately active (sedentary job with half an hour to one hour of recreational activity a day, or a standing job with less than half an hour of exercise a day, or a physical job with no recreational activity)
  • active (sedentary or standing job with more than one hours’ recreational activity a day, or a physical job with some recreational activity, or a heavy manual job)

The researchers had DNA from 21,631 participants of the larger cohort. These participants were all of white European descent. The researchers looked at the genetic sequence at the 12 positions on the 11 genes to see if the genetic variations associated with susceptibility to obesity were present. At each of the 12 positions, the participants were given a score, which indicated whether their DNA sequence gave them an increased genetic predisposition to becoming obese. The scores were then added together to give an overall score.

The researchers used a standard statistical technique, called logistic regression, to assess the strength of the association between an increased genetic predisposition for obesity and high BMI at the first health check. They then determined if they could still predict whether an individual would be obese, based on their genetic predisposition, if the analysis was repeated with people grouped according to their activity levels.

The researchers then looked at the interaction between genetic predisposition and physical activity, and the likelihood that a participant would put on weight in each year between the first and second health checks (a period of one to seven years).

What were the basic results?

The researchers found that for each of the 12 genetic variations that increased the predisposition for obesity, there was a 0.154kg/m2 increase in BMI. This corresponded to a 1,445g increase in body weight for each variation in an individual who was 1.70m tall.

Each increase in physical activity level was associated with a 0.313kg/m2 reduction in BMI. This corresponded to a decrease of 904g in body weight for a person who was 1.70m tall.

When the participants were grouped according to the four physical activity levels and the association between genetic predisposition and BMI was assessed, the researchers found that physical activity modified the effect on BMI of the genetic predisposition score. An increase in genetic predisposition score was associated with a 0.205kg/m2 increase in BMI in the inactive individuals (an extra 592g for a person 1.70m tall), but only a 0.126kg/m2 increase in active individuals (an extra 364g for a person 1.70m tall).

The researchers found that physical activity modified the association between genetic predisposition to obesity and BMI at the first health check and over follow-up.

How did the researchers interpret the results?

According to the researchers, their study shows that “a physically active lifestyle can modify the genetic predisposition to obesity”. They say that “living a physically active lifestyle is associated with a 40% reduction in the genetic predisposition to common obesity” and “promoting physical activity, particularly in those who are genetically predisposed, may be an important approach to controlling the current obesity epidemic.”

Conclusion

This large cohort study found that physical activity decreased the likelihood of having a higher BMI in people with a genetic predisposition to being overweight. One strength of this study is that it looked at a large population, which is important for assessing gene-environment interactions. However, the study has some limitations which the researchers highlight:

  • The amount of physical activity was assessed with a self-administered questionnaire. Reporting physical activity in this subjective way may have led participants to over- or under-estimate the amount of physical activity they did.
  • The participants included in the study were all white and of European descent. This population may not reflect the UK population as a whole.

This study shows that, although some people may have a genetic predisposition to being overweight, physical activity can prevent weight gain in these individuals. Current recommendations are that people should do at least five 30-minute sessions of moderate activity a week as part of a healthy lifestyle.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Your genes made you fat? Rubbish… Just get some exercise. The Sun, September 1 2010

Weight: why having fat parents is no bar to being trim. Daily Express, September 1 2010

Genetic excuse for obesity 'is a myth'. The Daily Telegraph, September 2010

Links to the science

Li S, Zhao JH, Luan J et al. Physical Activity Attenuates the Genetic Predisposition to Obesity in 20,000 Men and Women from EPIC-Norfolk Prospective Population Study. PLoS Med 2010; 7(8)

Shaw KA, Gennat HC, O'Rourke P, Del Mar C. Exercise for overweight or obesity. Cochrane Database Syst Rev 2006, Issue 4

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