Are your genes making you fat?
It is certainly true that the genes a person is born with have an impact on body weight. Research suggests that the impact is the result of ongoing interactions between the person's environment and individucal genetic makeup. Studies indicate that 24-40% of the difference between individuals with respect to body fatness can be attributed to genetics.
However, it is important to recognise that, except in very rare cases, the genes that impact body weight do not directly cause obesity. Rather, genetic makeup influences the susceptibility to weight gain when the person lives in an environment that supports eating kilojoules in excess and/or limiting physical activity.
Over the last decade, our understanding of the role of genetics in determining body weight has significantly advanced. As the story has unfolded, the complexities have increased exponentially. Current findings suggest that, with only rare exceptions of a single gene mutation that results in severe morbid obesity, there are numerous genes (i.e. possibly several hundred) that affect one's susceptibility to overweight. Moreover, the various genes interact with one another to further influence vulnerability.
Birth weight links to obesity
Susceptibility to obesity does not appear to be limited to genes alone. There is increasing research that suggests that the environment to which a foetus is exposed during pregnancy impacts weight later in life. Indeed, many studies have found direct links between both high and low birth weights and a greater likelihood for overweight and obesity years later.
Genetics and behaviours
To add yet more complexity, newer research suggests that the role of genetics expands beyond the ultimate weight that shows on the bathroom scale. It is now widely recognised that the influence of genes extends to weight-related behaviours as well. Food preferences, dietary patterns and exercise behaviours are all likely to be determined, in part, by genes.
While genes play a part in determining susceptibility to overweight, they do not appear to play a role in the weight fluctuations or the typical weight gain experienced during the middle adult years.
The bottom line is that biology is not destiny. While genes influence susceptibility to overweight and have an influence on weight-related behaviours, the genes cannot be expressed unless the environment supports them.
In the past decade, it's become obvious that the role of genes in affecting weight, appetite and even our predisposition to exercise is a complex one. "Currently there are about 450 genes identified as having an influence on weight - and there will be more," says Karen Miller-Kovach, Weight Watchers Chief Scientific Officer. "There are undoubtedly a large number of genes involved in the regulation of body weight, energy expenditure, food preferences, eating, hunger and the way fat operates in the body," agrees Associate Professor Boyd Strauss of the Nutrition and Dietetics Department at Monash University.
In fact, some studies suggest genes may be responsible for between 25 and 40 per cent of the difference between individuals with respect to body weight. So - are we condemned by our genes or can this information help us fight weight gain head-on?
The obesity gene
Research into the impact of genes on weight accelerated in the early 1990s when a quest began for an obesity gene that could be modified to help control weight. "It was believed - and hoped - that there would be two or three genes in the human genetic pool linked with obesity," explains Karen Miller-Kovach. "There was a quest for a 'thrifty gene' - a gene that helped people store fat to help them survive when food was scarce. There were theories that if science could identify and modify that gene, it would solve the world's obesity problem. Then, in the mid 1990s, the 'leptin gene' was discovered. People thought 'this must be the gene' - but it wasn't."
In laboratory studies, leptin, a protein, decreased the body weight of mice by reducing their food intake and increasing their energy expenditure. After two weeks of treatment with leptin, mice lost 30 per cent of their weight. Genes carry the instructions to make leptin, but in obese mice, those genes were defective, so the obese mice ate more and exercised less. Leptin also plays an important fat-burning role in humans - but until recently, nobody knew how. Researchers at Brown Medical School and Harvard Medical School have found leptin triggers production of a chemical called aMSH, which sends a message to the brain to burn kilojoules. "If somehow, through a drug, you could increase activity of aMSH, you'd force the body to burn more kilojoules and lose weight," says Eduardo Nillni of Brown Medical School.
From foetus to fat?
Another more recent field of research in this area is epigenetics. "Epigenetics is the recognition that environment, particularly when a foetus is in the womb, also affects vulnerability to weight gain," says Karen Miller-Kovach. "For example, one study followed babies born during the German occupation of various European countries in the First World War, when there was famine. Children born during the famine were substantially heavier and more likely to become obese as adults than children born before or after the occupation. Why? While the foetus had restricted food supply, a biological programming occurred along the lines of 'when food becomes available you'd better keep hold of it'."
British researcher David Barker, who conducted this study, suggested chronic health issues such as obesity and heart disease could be an outcome of physiological adaptations or 'programming' an unborn baby forms when it is undernourished. Barker believes maternal nutrition is a major influence on the programming that affects childhood growth and obesity in later life. However, this is just part of the story. It's now widely recognised that the influence of genes extends beyond biological factors to weight-related behaviours such as food preferences, dietary patterns and exercise behaviours.
Karen Miller-Kovach believes research into how genes affect our predisposition to doing physical activity is significant. "You can take two groups of people and put one group on an exercise program, but at the end of the program, the weight change between the two groups is minimal," she says. "That may be because the group who exercise then unconsciously make up for that physical activity through the week. They spend a little more time on the couch, and they do that without realising it, because that's the way they are genetically programmed."
"If that sounds like you, a good strategy is to focus on incidental activity, rather than structured activity," says Karen Miller-Kovach. Park the car further away, use the stairs not the lift and walk more, because you won't compensate for that kind of physical activity by relaxing on the couch.
The bottom line
While researchers are studying the role of genetics in weight, it's important to realise that except in very rare cases, genes don't directly cause obesity. How and what we eat and the amount of exercise we do has a far greater impact on our weight. "The majority of obesity today is probably environmental rather than genetic," says Strauss. "It's to do with your level of physical activity and food intake."
Karen Miller-Kovach stresses that a healthy lifestyle is what makes the difference. "You may have a deck of cards stacked towards weight gain or you may be dealt cards where the genes are protective against weight gain," she says. "But don't think of these as genetic edicts, think of them as tendencies. Focus on behaviour to change those tendencies - for example, think carefully when choosing a drink if you have a tendency towards sweet things. "If everyone in your family has had a heart attack, you probably have a fair amount of genetic material that makes you vulnerable to a heart attack, too," says Karen Miller-Kovach. "But that doesn't mean you'll have a heart attack; your lifestyle can modify the risk. Similarly, just because you have a genetic predisposition to weight gain, it doesn't have to be your destiny."
- This content was written by Weight Watchers for Kidspot.
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Last revised: Friday, 15 May 2015
This article contains general information only and is not intended to replace advice from a qualified health professional.