Vitamin D is a steroid hormone that can be produced endogenously from cholesterol by the action of ultraviolet light on the skin. The vitamin D is then hydroxylated to 25-hydroxyvitamin D in the liver, and subsequently further hydroxylated to 1, 25-dihydroxyvitamin D in the kidney. Alternatively vitamin D in the form of ergocalciferol (vitamin D2) or cholecalciferol (vitamin D3) can be obtained from plant sources or animal sources in the diet, respectively. These dietary sources of the vitamin then undergo the same hydroxylation reactions as those experienced from endogenously synthesised vitamin D. Calculations as to the intakes of vitamin D from various sources have been made, and in a recent paper (here) a discrepancy was found between the production and intake of vitamin D with the recorded plasma levels of 25-hydroxyvitamin D (the biologically accepted biomarker for vitamin D status). In other words, the plasma levels of the subjects were higher than could be accounted for by known dietary sources and endogenous skin production.
In response to this discrepancy the authors suggested that some food may contain preformed 25-hydroxyvitamin D and that this may contribute to the higher than expected plasma levels of the vitamin. For example, some evidence suggests that beef may contain between 100 and 400 IU of vitamin D equivalents (in the 25-hydroxyvitamin D form). The estimate that 25-hydroxyvitamin D is more potent biologically by a factor of about five times suggests that a 170 gram serving of beef may provide enough vitamin D to explain the discrepancy in the plasma levels of some subjects (which was around 1600 IU) if the upper figure is accepted. However, in a recent letter to the editor1, the vitamin D researcher Dr Cannell pointed out that this might not be the case, and that the preformed 25-hydroxyvitamin D in meat might be much lower than could account for the missing 1600 IU. Dr Cannell suggests that other sources of vitamin D may exist in the human diet and that vitamin D synthesis might occur through the presence of radiation other than ultraviolet B.
For example, some foods of plant origin may contain the cholecalciferol version of the vitamin. Generally it is accepted that the vitamin D form found in plants is the ergocalciferol form of the vitamin. However, some research has shown that vitamin D3 glycosides are detectable in tomato and pepper leaves so there is a realistic possibility that other vegetables may contain the vitamin D3 form of the vitamin. The fruits of tomatoes and peppers have not been tested so it is unclear if they contain vitamin B3 glycosides. In addition, Dr Cannell suggested that radiation other than ultraviolet B may account for vitamin D production in the skin. This is because the type of radiation is not important, only that the wavelength has sufficient energy. For example, ultraviolet C light can trigger the isomerisation reaction that causes the formation of vitamin D. It is therefore possible that X-rays may do the same as this is the next shorter wavelength. As humans are exposed to x-rays from space, this may explain the discrepancy in vitamin D levels.
Dr Robert Barrington’s Nutritional Recommendation: The complexity of the vitamin D requirements for humans is only just being realised. Generally it is acknowledged that those living at high latitudes must supplement with vitamin D during the winter months. Until the discrepancies over possible food intakes, endogenous production and plasma levels are elucidated, it is strongly recommended that vitamin D supplements are taken during times of low solar radiation exposure.
RdB
