There is a large body of evidence pointing to the extraordinary way in which our genetic make-up has been modified over thousands of years by adaptive responses to changes in our environment. Hitherto, much of this work has focused on infectious disease. For example, thousands of children are born with serious forms of anaemia-like sickle-cell disease and thalassaemia in tropical countries because carriers for these conditions - that is those who have only one defective gene rather than the two that are required to express the disease - are partly resistant to the ravages of malaria. Hence they tended to bear more children in malarious areas and the frequency of the gene rose until its advantage was balanced by the premature deaths of those carrying two copies.
There are many other examples of the advantages and downsides of adaptation to malaria. Millions of people who live in regions where malaria is endemic, or has been in the past, have a deficiency of an enzyme in their red blood cells that, although it makes them more resistant to malaria, also renders them prone to severe anaemia if they ingest fava (broad) beans or certain drugs. Favism, or Baghdad spring fever - names given to the malaise and anaemia that follows the ingestion of beans in particular seasons - has been recognised for a thousand years or more. Many other changes in our genomes have resulted from intense selection by infection and must be still occurring as Aids sweeps through Asia and sub-Saharan Africa. Less attention has been paid to the way in which our genetic make-up reflects differences in our diet. While it is known that our reactions to spicy or bitter foods, or our ability to tolerate lactose, are genetically determined, only recently has attention been paid to the complex interplay between cultural changes in our diet and our genes that have made us what we are today. This rapidly evolving story is outlined for the general reader in Gary Nabhan's Why Some Like it Hot .
Nabham's message is that these interactions are far more complex than geneticists would have us believe. His view is exemplified by his discussion of obesity and insulin-resistant diabetes, a condition that, it has been predicted, will affect some 300 million people by 2025. One explanation for this devastating epidemic is based on the "thrifty genotype" hypothesis, first proposed by the geneticist James Neel. The theory is that, because of intermittent periods of severe dietary privation, our hunter-gatherer forebears evolved genetic make-ups that are totally inappropriate for today's high-energy diets and fast-food habits.
However, it has not yet been possible to define the genes that might underlie the thrifty genotype. There is an extraordinary diversity in the frequency of this form of diabetes among the world populations. While it affects up to 80 per cent of Pima Indians and some populations of isolated Pacific islands, and is extremely common among Aborigines and the populations of the Indian subcontinent, it affects only 4 to 6 per cent of Europeans. Nabhan suggests that it is not so much the exposure to junk food and a thrifty genome that is responsible for this epidemic but rather the loss of traditional diets, which may even have protected against disease, a change that Europeans have had much longer to adapt to. He cites pilot studies in Hawaii that suggest that, while diet and exercise may help to improve the situation, the most important factor is a return to a traditional diet.
A short coverage of such a complex and rapidly developing field for general readers is bound to lead to some errors and speculation. For example, work on the effect of foetal development on the likelihood of developing obesity and diabetes in middle life, which is not mentioned, together with evidence that this form of diabetes reflects more than one genetic defect, have to be considered in any discussion of the role of the cultural effects of diet. And Nabhan's lurid description of gene therapy may well persuade the faint of heart never to visit their doctor again.
Nevertheless, this book offers a readable and thought-provoking account of the complexity of the interactions between nature and nurture that may have made today's populations what they are. Anybody who thinks science is dull should read Nabhan's account of the disastrous outcome of trying to woo a young woman who, because of her genetic make-up, was unable to tolerate his spicy and exquisitely prepared Mexican meal, not to mention his description of attempts to link the analgesic effects of genital stimulation with a genetic ability to tolerate spicy food - surely one of the most bizarre experiments ever performed in the name of science.
Sir David Weatherall is emeritus regius professor of medicine, Oxford University, and chancellor, Keele University.
Why Some Like it Hot: Food: Genes and Cultural Diversity
Author - Gary Paul Nabhan
Publisher - Island Press
Pages - 233
Price - £17.95
ISBN - 1 55963 466 9