Vitamin A is largely ignored by the health-seeking crowd because many people are not certain what it does exactly, they’ve heard it’s toxic, and they assume there is already some vitamin A in their multivitamin so why the excitement?
The Wrong Form of Vitamin A is Found in Most Multi-Vitamin Formulas
It’s not so-called “Pro-Vitamin A” or “Beta-carotene” that you want–no. However, what does work is retinyl palmitate, otherwise known as vitamin A palmitate or retinol.
Vitamin A Produces Energy
Despite voluminous research on Vitamin A, it has only been recently discovered that a certain form of vitamin A may boost energy levels significantly.
In 2009, researchers at the Sloan-Kettering Institute for Cancer Research identified a previously unknown and vital role for retinol. It turns out that retinol is needed by mitochondria (the power plants of our cells) and without it, the enzyme kinase C delta cannot be activated. 
This can lead to a drop in oxygen consumption and ATP production – ATP being the fuel for our bodies at the cellular level.
The study’s authors also reported that only retinol qualifies as a mitochondrial response modifier. They went on to say that the connection to energy production in cells may explain the multiple effects and largely unexplained pathologies that result from a vitamin A deficiency, including immune deficiency and male sterility, and cannot be reconciled using retinoic acid (from pro-vitamin A).
Retinol can be found directly in animal sources of vitamin A or converted from retinal in the body, which is usually produced from ingesting pro-vitamin A (beta carotene).
While that may seem like both animal and plant sources provide the retinol our bodies need, it’s not exactly the case. It has long been assumed that pro-vitamin A automatically converts to retinal on demand but we now know that up to 50% of the population has a defect in the enzyme responsible for converting pro-vitamin A to retinal. If you’re only consuming pro-vitamin A, no retinal means no retinol.
This is important to note for people getting their vitamin A from multivitamins because the vast majority of them include provitamin A. Vegetarians should also take note because plants do not contain retinol so supplementing with retinyl palmitate, a plant-based precursor of retinol, may be desired.
If you’re low on energy or have undiagnosed symptoms, try supplementing with 10,000 IU of vitamin A daily from animal sources (usually liver), or with retinyl palmitate for vegetarians, and see how you feel.
Vitamin A + Vitamin D = Better Health
Vitamin D has received a lot of important attention over the last few years while vitamin A has been virtually ignored. What’s often forgotten is that rarely do nutrients work in isolation, and Vitamin D is no exception.
Vitamin A is an essential co-factor for vitamin D function. Vitamin A and D work in tandem through the same receptor sites across the entire body; in fact there are receptors for vitamin A and vitamin D in every cell.
Unfortunately, vitamin A deficiency is more prevalent than was once thought. One reason is that many popular foods are not particularly rich in vitamin A. Moreover, as previously stated beta-carotene and other sources of pro-vitamin A may not be converted to usable forms of vitamin A in up to 50% of the population.
High levels of vitamin D also have the effect of lowering vitamin A in the body, which is not uncommon with current widespread supplementation of vitamin D and disinterest in vitamin A. This was shown in 2010 when researchers at the University of Texas Southwestern Medical Center found that high levels of vitamin D lowered stored levels of vitamin A in the liver.
How They Work Together
Fat soluble vitamins, especially A and D, optimize immune response in infections, inflammatory disease and tissue damage.  Vitamins A and D synergistically guard against the invasion of pathogenic mycobacteria in a dose-dependent manner. 
While vitamin D provides immune defenses, vitamin A is essential for the immune function that eradicates harmful microbes. In autoimmune diseases vitamin D decreases pro-inflammatory T cell cytokines while vitamin A increases regulatory T-cells, which reduces hypersensitivity reactions. 
Vitamin A and D share a role in the prevention of diabetes, leukemia, prostate cancer, heart disease and osteoporosis. , , , , .
However, when these two vitamins are unbalanced problems begin to develop, such as in bone health. Studies involving low or high consumption of either vitamin A or D reveal problems in bone metabolism resulting in bones becoming more likely to fracture.
Perhaps one of the most important studies on the relationship of vitamin A and D was shown in developing brains. Vitamins A and D both create a lifelong “imprint” on tissues for chemical signaling in the brain via neurotransmitters such as dopamine and serotonin or 5-hydroxytryptamine. The study authors concluded that the use of vitamins A and D show a profound and lifelong effect of newborn hormonal imprinting on neurotransmitter production of the adult brain suggesting that the effect of vitamins A and D must be taken into consideration during gestation. They are also important for a healthy nervous system. 
With the increasing awareness of Vitamin D deficiency, there is a concern about an insufficiency of its vital co-factor vitamin A. High doses of vitamin D used alone may lead to potential adverse effects like increased fractures and poor bone health, more infections and cancer. 
Recommended and safe levels are between 5,000 IU and 10,000 IU of vitamin D and 10,000 IU and 15,000 IU of vitamin A daily.
Can Vitamin A Be Toxic?
While often thought of as potentially toxic, Vitamin A is very safe and only toxic when consumed in amounts of 100,000 IU (International units) or more per day over several months. A daily dose of 10,000 IU is sufficient to maintain health while larger doses may be taken to overcome a deficiency.
According to the Merck Manual, vitamin A toxicity is rare. One case of vitamin A toxicity in adults was reported in Arctic explorers who developed drowsiness, irritability, headaches and vomiting, followed by skin peeling. This was not long after consuming several million IUs of vitamin A derived from polar bear and/or seal liver. The explorers soon recovered and experienced no long-term side effects.
When larger doses of vitamin A are taken in the absence of Vitamin D there have been reports of lower bone density in some cases. Conversely, high doses of vitamin D in absence of Vitamin A can also create some problems, such as immune and bone problems. This is why it is important to consume levels of each vitamin for optimal health.
Retinol, which will convert into All-trans-retinoic acid (RA) plays a critical role in maintaining immune system balance. The relationship between T1 and T2-helper cells depends on both retinol and vitamin D. Both vitamins are greater than the sum of their parts. 
. J Biol Regul Homeost Agents. 2010 Oct-Dec;24(4):385-90.
. J Microbiol Immunol Infect. 2008 Feb;41(1):17-25
. J Leukoc Biol. 2009 Oct;86(4):959-69.
. Diabetes. 2009 January; 58(1): 24–25.
. Biochem Pharmacol. 2009 Jun 1;77(11):1694-701.
. Biochim Biophys Acta. 2011 Aug;1812(8):1023-31
. J Clin Invest. 1996 Apr 1;97(7):1577-88.
. Br Med J. 1975 Sep 13;3(5984):647-8.
. Horm Metab Res. 2009 Apr;41(4):277-80.
. Orv Hetil. 2011 Feb 27;152(9):323-30.
. J Immunol. 2013 Sep 15;191(6):3152-60.