Based on the available information outlined above, it seems very likely that biome depletion is by far the most significant and most profoundly influential consequence of genetic/cultural mismatches in postindustrial society. However, other consequences which apparently exacerbate the effects of biome depletion are evident. For example, vitamin D deficiency has reached epidemic proportions in postindustrial society, is known to impact inflammation during early development [52
], and, like biome depletion, has been linked to a spectrum of allergic, autoimmune, and inflammatory diseases [53
]. Further, epidemiology and other circumstantial evidence link vitamin D deficiency to autism [56
]. Interestingly, the widely publicized association between rainfall and autism [58
] can be accounted for, perhaps more than in part, by decreased exposure to sunshine in areas with high rainfall, and the subsequent impact of lower vitamin D levels.
Vitamin D deficiency is another consequence of postindustrial culture's mismatches with human biology (). Vitamin D production requires exposure of oils on the body's surface to sunlight. The resulting photochemical reaction and production of vitamin D is greatly reduced by components of postindustrial culture that reduce exposure to sunlight (e.g., indoor work environments, sunscreen).
The association of vitamin D deficiency with a spectrum of hyperimmune-associated diseases places this consequence of biology/culture mismatch alongside biome depletion as an underlying agent destabilizing immune system function in postindustrial populations. However, vitamin D deficiency is an ancient problem, as indicated by the ancient occurrence of rickets, whereas epidemics of hyperimmune-associated diseases are more recent in nature. Thus, it seems likely that vitamin D deficiency is a contributor to hyperimmune-associated disease, but not sufficient by itself to lead to disease. Further, the prevalence of vitamin D deficiency in postindustrial society is somewhat less than the prevalence of biome depletion: although the prevalence of vitamin D deficiency is substantial, with one study finding 24% of individuals deficient (≤20
ng/mL) with an additional 34% having levels that were insufficient (≤29
], the prevalence of biome depletion in postindustrial populations is essentially 100%. Thus, it is expected that the impact of vitamin D deficiency on immunity in postmodern culture may be a matter of exacerbating the problems associated with biome depletion more so than a significant problem when considered in isolation.
Some mechanisms by which vitamin D might stabilize the immune system have been elucidated. Vitamin D acts as a signaling molecule to enhance immune cell function in the presence of infection and is important for maintaining the normal interface between the microbiome and the immune system [60
]. However, the mechanisms by which vitamin D interacts within the body are complex, with the molecule being intertwined either directly or indirectly in virtually all aspects of human biology. The active form of vitamin D binds and activates the Vitamin D receptor, altering the expression of a variety of genes involved in such diverse processes as cell growth and proliferation, bone remodeling, and calcium homeostasis [61
]. It has been estimated that vitamin D binds human DNA in more than 2,500 places and changes the expression of more than 200 genes [62
]. Thus, while it is known that vitamin D is required for normal immune function, it is likely that vitamin D deficiency could cause a general destabilization of the human biome independent of the immune system, particularly during early development. Fortunately, regardless of the extent to which vitamin D deficiency destabilizes the human biome, dietary supplements are readily available, and thus this consequence of culture/biology mismatch is easily avoided.