Rising trend may be due to “pathoenvironment”

EDITOR,--The prevalence of obesity has reached epidemic proportions in the United States and many European countries. Andrew M Prentice and Susan A Jebb provide convincing evidence that “simple gluttony” cannot account for this increased prevalence since over the past 20 years food intake has decreased whereas body mass index has continued to increase in Britain.1 The authors conclude that physical activity has decreased even faster over the same period and so obesity can be blamed on “sloth.” Many readers may incorrectly conclude from this study that obesity is simply the result of a lack of the willpower to control eating and exercise.

Epidemiological and genetic studies suggest, however, that obesity in a given population is largely determined by the environment, whereas the variability among individuals within a given environment is largely determined by the individual genetic responses to that environment.2 3 I therefore propose that the high prevalence of obesity in some environments is the consequence of normal, genetically determined physiology in a “pathoenvironment.” Throughout most of its history, humankind has evolved in a restricted environment characterised by scarcity of food and a need for high levels of physical activity. Survivors of those times were probably people with a “thrifty genotype,” which made them fatter during times of plenty so that during lean times they could survive on their own energy stores.4 More than 30 years after the proposal of the thrifty genotype hypothesis,5 however, we are still waiting for the identification of specific “survival” genes, which are likely to be common among many native populations with a high prevalence of obesity, such as Pima Indians, Australian Aborigines, and Pacific Islanders. Some people in Western societies may also have inherited stronger metabolic drives to eat more or exercise less than others and will, therefore, become obese in the present pathoenvironment.

To combat obesity on societal levels, public health strategies should be designed to make the environment less pathogenic by reducing the energy density of readily available food and increasing physical activity. These programmes should be targeted at children as early as the primary grades in school. Intervention programmes will not, however, be accepted until it is recognised that obesity is not simply due to the gluttony and sloth of undisciplined people but often results from genetically determined metabolic drives to eat more and exercise less in a pathoenvironment.