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> Rather importantly, they note that while parathyroid hormone levels are inversely proportional to Vit D serum levels, they start to go up with high levels of

> Rather importantly, they note that while parathyroid hormone levels are inversely proportional to Vit D serum levels, they start to go up with high levels of Vit D. ...in that particular polynomial fit. > Keeping PTH levels low is likely beneficial. The Nairobi study found PTH levels were lowest in people with vit D levels around 30 ng/mL. The minimum of the polynomial fit is around 35. If you look at the scatter plot, there are only six samples near 50 ng/ml and they're all over the place. You can not draw your conclusion from that. There's no significant deviation between 40-50 ng/ml. My question is, why would the body stop producing Vitamin D around 40-50 ng/ml if 30 was optimal? Why not stop earlier? > Keeping PTH levels low is likely beneficial. Not sure where you get that.

valarauko 6y ago
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.

valarauko 6y ago

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.