To me, I am less convinced that the elevated levels in the Masaai represent an ideal. Do all hunter gatherers have such high levels? Unclear. Perhaps the Masaai have such high levels as a quirk of their genetics, and their particular lifestyle. Does that necessarily translate into an ideal level for all humans? Possible, but there just isn't enough data. On the other hand, there are many studies across populations that find levels plateauing around 20 ng/mL for healthy rural cohorts, with no negative outcomes.
Why does the body stop producing Vitamin D around 50 ng/mL, if it wasn't the ideal? Vitamin D toxicity only kicks in at extremely high levels, several orders of magnitude higher than are naturally produced. So why would nature even bother to set an upper limit at 50 ng/mL? It doesn't, though. Vitamin D3 is produced by a reversible photosynthetic reaction, and is constantly being degraded as well under sun exposure. 50 ng/mL possibly represents the upper limit at the speed with which D3 can be transported away from the dermis, and the limits of natural exposure levels. My feeling is that rather than an evolutionarily fine tuned level, this is a "good enough" local maxima as an upper limit for D3 production under natural conditions. If the Masaai were somehow exposed to 20 hours of full sun, their levels might very well be much higher.
As a thought experiment, if humans were exposed to 20 hours of sunlight, would we evolve fewer keratinocytes to keep vitamin D levels at the 50 ng/mL level? Or perhaps evolve less efficient enzymes to produce lower levels of the precursor? My impression is that there isn't a strong enough disadvantage to higher levels of Vitamin D for evolution to push in that direction. So rather than an ideal level, 50 ng/mL might simply represent the upper limit of what humans can produce under natural conditions.
Why does the body stop producing Vitamin D around 50 ng/mL, if it wasn't the ideal? Vitamin D toxicity only kicks in at extremely high levels, several orders of magnitude higher than are naturally produced. So why would nature even bother to set an upper limit at 50 ng/mL? It doesn't, though. Vitamin D3 is produced by a reversible photosynthetic reaction, and is constantly being degraded as well under sun exposure. 50 ng/mL possibly represents the upper limit at the speed with which D3 can be transported away from the dermis, and the limits of natural exposure levels. My feeling is that rather than an evolutionarily fine tuned level, this is a "good enough" local maxima as an upper limit for D3 production under natural conditions. If the Masaai were somehow exposed to 20 hours of full sun, their levels might very well be much higher.
As a thought experiment, if humans were exposed to 20 hours of sunlight, would we evolve fewer keratinocytes to keep vitamin D levels at the 50 ng/mL level? Or perhaps evolve less efficient enzymes to produce lower levels of the precursor? My impression is that there isn't a strong enough disadvantage to higher levels of Vitamin D for evolution to push in that direction. So rather than an ideal level, 50 ng/mL might simply represent the upper limit of what humans can produce under natural conditions.