Synergistic effect of vitamins A and D

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Very important article by Chris Masterjohn.   Based on the recent Chinese paper using human stem cells in-vitro.  The paper demonstrated that vitamin A acts synergistically in triggering expression of neurogenin-3.  Neurogenin-3 is necessary for activation of stem cells development, in that case - for replacing the damaged insulin-producing islet cells such as in diabetes type 1.

I think that study is important for several reasons:

- Adequate intake of both A and D3 is one of the critcial factors in tissue regeneration, propably not only limited to pancreatic islets but generally to all other organs,

- Supplementation of only one of the vitamins is inadequate because both act synergistically.

- I suspect (speculating) that beta carotene might not be adequate if vitamin D3 level is low, and vice versa - supplementation of D3 might not be effective in the absence of A.  We shall keep in mind that beta carotene (pro-vitamin A)  from plants is NOT the true vitamin A!   This may explain a curious fact of difficult to treat vitamin A or vitamin D3 depletions frequently reported by vegans.

Vitamin A,D3,K2 - toxicity levels

This is a short comment to summarize the situation, based on information gathered from the most recent nutrition studies as well as some of the most useful nutrition blogs (Stephan's, Peter's and Chris Masterjohn's - see the links on the left margin).

The issue of supposed toxicity level of D3 is being slowly unraveled in recent research. It turns out that A,D3 and (probably) K2 must come together in balanced proportions, thus some studies that were increasing D3 alone without A reported toxicity of D3 at a lower level than other studies where D3 may have come from natural produce (like fish oil) thus came together with vitamin A! A and D3 always seem to come together in animal food (any exceptions?). The same story repeats with studies analyzing toxicity of vitamin A. Again, the toxic A levels come out lower when only A is being increased on it's own, versus if it comes with natural fats like fish oil, butter, liver, yolks etc. Also, it appears that the body's synthesis of A out of carrotenoids from vegetables (or absorption) is impaired without sufficient level of fat in food. Subsequently, the sunlight-induced synthesis of D3 out of cholesterol, or the utilization of D3 seem to be deficient when not enough vitamin A is present in the system.

All of the above is absolutely consistent and may explain why do low fat vegetarians, even in sunny places like Arizona or Florida may and often do get deficient in both A and D3.