Wednesday, November 2, 2016

Study: South Indians but not Europeans have a specific genetic adaptation to vegetarian diet

This study by Kothapalli et al. got a lot of press some months ago when it was first released. However I am not aware of any reports that discussed its importance for understanding genetic differences between ethnic groups that influence how well an individual may adapt to a vegan or vegetarian diet. 

Kothapalli et al. studied the SNPs that determine an individual's ability to convert plant-source linoleic acid (LA) and alpha-linolenic acid (ALA) into, respectively, arachidonic acid (AA) and docosahexaenoic acid (DHA).

Unsupplemented vegans depend on this pathway to get adequate AA and DHA.  This study showed that a majority of people of European descent have a quite limited ability to perform this conversion, whereas people of South Indian descent have a specific genetic adaptation that facilitates this conversion.


Production of the enzyme FADS1 necessary for endogenous synthesis of these fatty acids is controlled by the insertion/deletion (indel) genetic variant rs66698963, of which the deletion is the minor allele.  Thus, individuals who carry D/D (deletion/deletion) genotype should have a reduced ability to produce AA and DHA from precursors than I/I (insertion/insertion) genotype individuals.

Kothapalli et al. found that the D/D genotype predominates in people of European descent, while the I/I genotype predominates in people of East Indian descent.  D/D is 14 times more common among Europeans compared to Indians, while I/I is 3.75 times more common among Indians as among Europeans.

In graphic form:



The long practice of vegetarian diet among Indians would exert a positive selection for I/I high-converter genotype among them.


Kothapalli et al. found higher frequencies of I/I in not only Indian but also African populations.


Notice that D/D frequency is highest to lowest thus: Europeans (25%), East Asians (16%), Africans (5%), South Asians 1.8%.  D/D plus I/D frequency from highest to lowest:  Europeans 83%, East Asians 70%, Africans 47%, and South Asians 30%.

Experiment showed that people with D/D and I/D genotypes are much less efficient converters of LA to AA compared to I/I genotypes:


  
Kothapalli et al. found evidence that the higher frequency of I/I was positively selected by generations of widespread practice of vegetarian diet in India. 

 

This data suggests that individuals with African and South Asian ancestry are more likely than individuals of European or East Asian ancestry to be able to adapt to a vegan diet without supplementation of long-chain polyunsaturated fats such as DHA. 

Individuals with I/I genotype will be prone to accumulate an excess of pro-inflammatory AA when eating diets rich in LA from vegetable oils, which may increase their risks for chronic diseases involving inflammation, such as heart disease.  

Individuals with D/D and I/D genotype will, as Kothapalli et al. point out, be protected from excess conversion of LA to AA, but, on the other hand, D/D and I/D individuals – that's about ~83% of Europeans and 70% of East Asians – probably have a dietary requirement for DHA and perhaps also AA, particularly during pregnancy.  

 

Europeans do not have a tradition of generations of widespread practice of vegetarian diet.  In addition, unlike Africans, Europeans and East Asians have 1-4% uniquely Neanderthal genes, and Neanderthals lived for ~300,000 years in Europe independently of Africans, at a time when vegetarian diets were impossible for Europeans.   A diet providing AA, EPA and DHA from fish and meat selects for D/D genotype, to prevent the accumulation of excesses of these long-chain fatty acids, which can have toxic effects.

This provides evidence that in respect of endogenous production of AA and DHA most Europeans and likely most East Asians are genetically poorly adapted to an unsupplemented meatless diet whereas majorities of South Asians or Africans are better suited to meatless diets.

In Powered By Plants I cited Carlson and Kingston who argued against a human dietary requirement for DHA on the basis that there have existed millions of individuals around the world, including adherents of several religious doctrines that emphasize abstention from animal consumption (Jainism, Buddhism, Taoism), which form a significant proportion of modern populations, who have produced children with normal brains:
“In the case of these vegetarians, many have maintained such a restricted diet for generations. Neurological impairment under generational deficiency of DHA should result if dietary DHA is essential for neural function. Given that these populations experience normal brain growth and development in the absence of dietary DHA, it seems reasonable to question the nature of our dietary requirement for n-3 fatty acids. If preformed DHA is essential...the expected outcome of a vegetarian lifestyle is the failure of neural growth and development. On the other hand, there is no evidence to suggest that the capacity for DHA synthesis in vegetarians is limited...A logical explanation involves the sufficiency of LNA from the dietary intake of plants to provide sufficient DHA for the neural development of these populations.”
What Carlson, Kingston, and I neglected to consider was human biodiversity.  We were operating under the assumption that no significant genetic differences exist between different ethnic groups exposed for thousands or even hundreds of thousands of years to very different environments and diets.  This assumption is incompatible with the theory of evolution which predicts the evolution of genetic differences between ethnic groups produced by many generations of exposure to different environments placing very different demands on the inhabitants.

In other words, the fact that some isolated ethnic groups of humans appear to have the ability to produce healthy children 'despite' generations of subsistence on vegetarian diets doesn't serve as evidence that all humans of all ethnicities have the same ability.  On the contrary, from an evolutionary perspective these groups have this ability not despite their having lived for generations on restricted diets, but because they have lived for generations on specific diets. 

This research shows us that we can no longer make general statements about the nutritional requirements or abilities of the human genus or species.  On the contrary we should assume not that all ethnic groups are metabolically alike, but that each ethnic group has distinct genetic, physical and metabolic characteristics due to having been geographically isolated for sufficiently long periods of time. 




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