Vitamin A inhibits/aids in the absorption of Vitamin D
Q: I know that some people think vitamin A inhibits the absorption of vitamin D (but not carotene) – I think I learned this from the Vitamin D Council website. Anyway, it is interesting that DFH says in their info on the D Complex that Vitamin A is needed to absorb Vitamin D. I will have to hope that DFH is right and not the other researcher! What is your take on this ‘argument’?
A: Vitamin A & D both need fat from the diet to be absorbed, which is why cod liver oil is such a good medium for the absorption of these 2 vitamins.
Vitamin D is absorbed in the small intestine by nonsaturable passive diffusion that is dependent on micellar solubilzation and, hence, the presence of bile salts. The fastest absorption appears to be in the upper portions of the small intestine, but owing to the longer transit time of food in the distal portion of the small intestine, the greatest amount of vitamin D absorption probably occurs there. Vitamin D enters the lymphatic circulation predominantly (about 90% of the total amount absorbed) in association with chylomicra, with most of the balance being associated with the alpha-globulin fraction. The efficiency of this absorption process for vitamin D appears to be about 50%.
Most of the preformed vitamin A in the diet is in the form of retinyl esters. Retinyl esters are hydrolyzed in the lumen of the small intestine to yield retinol. The overall absorption of retinol from retinyl esters appear to be high (about 75%) and is also dependent on micellar solubilzation; this process appears to be minimally affected by the level and type of dietary fat, although the absorption is appreciably less efficient at very high vitamin A doses.
Vitamin D & A can possibly compete for absorption because they both need micelles to pass through the intestinal wall and excessive vitamin A levels may antagonize some of the vitamin D being able to pass through. It has been noted that vitamin D increases the need for vitamin A, and these two vitamins have a collaborative relationship. Research in osteoporosis shows that in the absence of adequate levels of vitamin D, vitamin A supplementation can contribute to adverse effects such as bone loss (because it affects the osteoclasts, (which are cells that break down bone). Yet, when vitamin D is present at sufficient levels, this does not seem to occur.
There are also roles of vitamins A and K2 in protecting against vitamin D toxicity.
The roles of all the fat soluble vitamins are complex and many, and often still being discovered and better understood, but there remains a bit of controversy in the understanding of these nutrients in the body, but science is moving forward all the time in helping us have a better understanding of the importance of having the fat soluble vitamins in optimum amounts in our diets and supplements.