Scientists at the University of Chicago claim to have found genetic evidence to support the sodium-retention hypothesis, revealing that the frequency of one version of a gene that plays a crucial role in salt retention correlates with distance from the equator.
"The surprise was finding that as populations moved away from the tropics the original or normal version of the gene became less and less common and the 'broken' version more frequent, which suggests it is protective," said study author Anna Di Rienzo, associate professor of human genetics at the University of Chicago.
High salt consumption is a significant risk factor in developing high blood pressure, a cause or contributing factor in 170,000 deaths a year in England alone. Last month the UK's food agency backed a £4 million campaign to slice into salt consumption, concurrently encouraging the food industry to continue with salt reduction programmes.
Chair of the Food Standards Agency, Sir John Krebs said at the time: "The food industry is about two-thirds of the way to reaching our target of a 1g reduction in processed foods by the end of 2005."
The ultimate ambition is to reach a target of 6 grammes a day by 2010, "that will require further action by consumers and industry".
Humans need salt, sodium chloride, to transport nutrients, transmit nerve impulses or contract muscles, such as the beating heart. The average adult contains about 250 grams of salt, enough to fill three small saltshakers. This salt is constantly lost through sweat and urine and replaced through the diet.
In the sub-Saharan African regions where humans first appeared, available salt must have been limited and quickly lost through sweat. People who were better at retaining salt may have had a significant survival advantage. This advantage decreased as humans spread to cooler climates.
"There seems to be a strong selective advantage conferred by the non-functioning protein, and that advantage increases with latitude," says Di Rienzo.
Di Rienzo's team of evolutionary biologists looked at the genetics of salt processing. They focused on a gene called CYP3A5, part of a family known as cytochrome P450 genes, which help the body break down and eliminate a wide range of compounds, including many drugs and salt. In the kidney, CYP3A5 acts to retain salt. One version of this gene, however, a mutation known as 'CYP3A5 *3', produces a truncated, non-functional protein.
The researchers looked at variations of this gene in 1,064 individuals drawn from 52 populations scattered around the world. The mutation was least common in some natives of sub-Saharan Africa, ranging from a low of only 6 per cent of Yorubans in Nigeria (Latitude 8°N) to 31 per cent among the Mandenka of Senegal (12°N).
Rates were higher among populations in East Asia, ranging from 55 per cent among the Dai of China (21°N) to 75 per cent among Han Chinese (32°N) to 77 per cent among Japanese (38°N) and 95 per cent among the Uygur of China (44°N).
Rates in Europe are uniformly high, ranging from 80 to 95 per cent in Italy, France and Russia. The highest rate, 96 per cent, was found among the Basque, an ethnic group of uncertain origins now concentrated in the Pyrenees mountains (43°N).
Full findings for the study are published in the December issue of the American Journal of Human Genetics, now available online.