Humans, over many generations, have evolved specific diets following environmental and cultural restraints. For example, vegetarianism, which is nowadays more and more common in Western cultures, has been traditionally the standard diet in India and in other cultures (Jainism, Hinduism, Buddhism…). Considering the tight relationship between nutrition and DNA, it’s not hard to imagine that different diets have affected the human DNA along the history. Can we spot such influences even now? This topic was investigated by a team of scientists from Cornell University (USA) who studied the genetic makeup (genotype) in a traditional vegetarian population in India, comparing it to other non-vegetarian populations. Their results point towards a genetic predisposition that vegetarian populations developed through the evolution to favor their diet.
Vegetarians have indeed access to a different range of nutrients compared to carnivores. This is particularly evident in the metabolism of fatty acids. Two of them (linoleic acid LA and alpha-linolenic acid ALA) are considered essential because the body can’t produce them; the only access to them comes from the food, mainly from plants. LA and ALA are necessary for human health because they are the starting point for other specialized fatty acids (long chain polyunsaturated fatty acids – lcPUFA), known also as omega-3 or omega-6 if they derived from ALA or LA respectively (See figure for a review).
The enzymes FADS1 and FADS2 are responsible for the conversion of LA and ALA into different lcPUFAs; however, the conversion rate is usually quite inefficient. Meat and seafood eaters can overcome such inefficiency since such foods contain the lcPUFAs; vegetarians instead have only access to the two essential fatty acids LA and ALA and therefore they must rely on the enzymatic conversion that happens inside the cells of their body.
For vegetarians, an enhanced activity of those enzymes represents an advantage to get the proper amount of lcPUFA. And indeed the research has identified a version of the FADS2 gene that is particularly frequent in traditionally vegetarian populations. Such version of the gene, the allele I, has an insertion of a region (22 base pairs) that enhance the expression of the FADS1 enzyme and consequently has a higher conversion rate of the essential fatty acid LA, derived from plants, into the omega-6 AA, found in meat. The second version of the gene, without the inserted region (deletion, allele D,) leads to a lower conversion rate. People who possess two copies of the I allele (I/I genotype) had an adaptive advantage in South Asians because of their traditional plant-based diet, which explains its high frequency. However an excess of omega-6 fatty acids is also harmful, therefore for an individual with an I/I genotype is suggested to stick to a vegetarian diet to avoid an excess of omega-6 sources.
This study reinforces the idea that a diet depends on the genetic makeup of each person and it also suggests that, as a general guideline, it might be safer to stick to what our ancestors ate because that has influenced the evolution of our genes.