Heredity
Seen from a Darwinian point of view, efficient "food-metabolisers", or individuals with low basal metabolic rates played an important role with regard to food scarceness. Thus, gene constellations ensuring hereditary transmission of these characteristics were preferred in the selection process. However, the initial advantages of the "frugal" energy balance are no longer valid under the current economical and socio-cultural circumstances of industrial nations. At changed living conditions the "provident" genes promoted by selection are now causing a negative effect, so that adiposity will be fostered now and in the future. Genetic characteristics that were once favourable are now becoming an unfavourable selection of great disease potential (see also secondary diseases).
According to and depending on opinions of individual scientists and physicians, the hereditary rate is stated as 40 to over 70%, though no one doubts its existence as such anymore. The extent to which the environment plays a role and also the frequency with which certain predispositions dependent on environmental changes occur cannot yet be definitively concluded because the phenomenon is multi-factorial and involves a great variety of individual components.
In mice, monogenic models have already been examined (Ob-gen), in which – following mutation of this particular gene – the mice showed an unchecked increase in weight (see also Energy Balance). It is also known that point mutations of single components can severely impact the control circuits of these control mechanisms.
Especially studies on the Pacific Islands (Ross, 1994) showed the severe influence of hereditary and environmental factors. Due to industrialisation the average body weight within the population increased dramatically during the course of only a few decades, where the degree of obesity fluctuated strongly in direct relation to the extent to which western lifestyles (BMI>26) were adopted: males /females 1.9 / 14.2% in the highlands of Papua New Guinea, as opposed to 84.7 / 92.8% on the island Nauru. This difference was not evident just 30 years earlier. Further epidemiological studies confirmed the influence of civilisation on a given population's body weight.
Obesity increasingly occurs within families. Large-scale twin studies and examinations were conducted with adopted persons to investigate whether a distinction can be made between genetic and environmental influences. The most important twin study of all (Twin Register of the National Academy of Sciences / National Research Council, 1978) analysed data of 1974 monozygotic (mono-ovular) and 2097 dizygotic (hetero-ovular) twins pairs, regarding height and weight between the ages of 20 and 45 years. The results showed heredity of 77% at the age of 20 years, and 84% at the age of 45 years. Taking into account all possible objections, these values are considered very high, and correspond to the extent of hereditary transmission of body height. Further studies allowing more efficient discrimination of environmental factors confirm these data and supplement them with common and individual environmental factors. Correlation of these factors reveals that monozygotic twins raised separately show a hereditary transmission rate of 74% in males, and 69% in females (Pedersen 1984).