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The controversy about the influences of nature versus nurture has been going on for centuries. The last 20 years of the 20th century have seen tremendous advances in molecular sciences, which have enabled a greater appreciation of the importance of genetics in many areas of human biology. The human-genome project, which commenced in the 1980s, plans to map all human genes (a directory that will provide genetic ‘addresses’ equivalent to about eight major city phone books) by the year 2010. In the end, it is likely that what will be proven is what we have always known, that parents are responsible for a lot, including the degree of body fatness in their children. On the other hand, genetic influences do not necessarily imply predetermination. The influence of a genotype (the genetic ‘blueprint’) which favours obesity will only be translated into the phenotype (the manifest characteristics of the genotype of an obese person) under certain conditions. Unlike having blue eyes or fair hair, the genetic expression of fatness is only manifest given the right conditions. In other words, an environment which favours energy surplus is virtually a prerequisite for the obesity genes to show themselves.
The studies of genotype and body fat are usually based on comparisons of body size in families and some of the most informative studies involve twins. Identical twins share 100 per cent of their genes, whereas non identical twins only share 50 per cent of their genes. By comparing the similarities in body size between the two types of twins, an estimate of the genetic contribution can be made. Three major twin studies carried out in the late 1980s showed some startling findings, in particular, amazing similarities in body fatness and body shape in identical twins who had been reared apart since birth, some never even having met! In general, genes explain about 25-40 per cent of the variation in body fatness, although it has to be remembered that these estimates are based on people living in fairly similar environments.
Some of the most significant research in this area has come from Professor Claude Bouchard and his group at Laval University in Quebec. Their experiments are carried out with pairs of twins who are kept in comfortable holiday-type accommodation for months at a time, and compare responses to various eating and exercise regimes. Their findings confirm that there is a wide range of responses to identical environmental influences and that the degree of response (such as the degree of weight gained for a given calorie excess) is largely genetically determined.
Genetic influences are unlikely to be the result of a single ‘fat gene’. Genes probably influence all aspects of energy balance including food preferences, nutrient digestion and processing, fat burning and storage and physical activity levels. In fact, to the mid-1990s, a total of 24 genes had been specifically identified as related to some aspect of obesity, but scientists believe several hundreds more are likely to be involved.
One key factor which appears to be at least partly inherited, for example, is food preference. Researchers at the University of Cincinnati examined preferences for 17 different types of foods ranging from fruit to snacks, chips and hamburgers. Comparisons were made between young (9-18-year-old) identical and non-identical twins living together. Frequency of eating and preferences for different foods were rated on a series of scales which indicated that genetic factors (e.g. in identical twins) do, indeed, appear to account for certain food preferences. The main heritable factor in preference appears to be sensitivity to, and preference for, bitter compounds in foods. Preferences for orange juice, broccoli, cottage cheese, chicken, sweetened cereal and hamburgers, for example, all appear to have a hereditary component.
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