A fat-soluble vitamin that most people think they're getting enough of, and most people are wrong. Vitamin A isn't a single molecule, it's a group of compounds (retinol, retinal, retinoic acid) that regulate everything from immune function and skin turnover to testosterone production and gene expression. It's involved in over 500 gene-signalling pathways and is essential for vision, reproduction, cellular differentiation, and keeping your epithelial tissues (skin, gut lining, respiratory tract) intact.
The reason most active people should care: vitamin A is directly required for testosterone synthesis, muscle stem cell maintenance, thyroid function, and immune regulation. If you train hard, eat high protein, run a caloric deficit, avoid liver and full-fat dairy, or follow a plant-heavy diet, you are almost certainly not getting optimal levels. Outright clinical deficiency is uncommon in developed countries, but subclinical insufficiency is far more common than bloodwork suggests, because serum retinol doesn't drop until liver stores are nearly depleted. Your blood can look fine while your tissues are running low.
Vitamin A exists in two dietary forms. Preformed vitamin A (retinol, retinyl esters) comes from animal sources, liver, eggs, full-fat dairy, fatty fish, and is absorbed at 70-90% efficiency. Provitamin A (beta-carotene, other carotenoids) comes from orange, yellow, and green vegetables and must be converted to retinol in the body. Here's the practical issue: beta-carotene conversion is wildly inefficient. The standard conversion ratio is 12:1 (12 mcg beta-carotene = 1 mcg retinol), but a study on BCMO1 gene variants found that common genetic polymorphisms can reduce conversion efficiency by up to 69%. If you carry these variants (roughly 40-45% of people of European descent do), you could be eating plenty of carrots and sweet potatoes and still be functionally deficient in active vitamin A. This is why relying solely on plant sources is a gamble. Liver is the single most concentrated food source of preformed vitamin A, and if you're not eating it, cod liver oil or a retinol supplement is the practical alternative.
Once absorbed, retinol is stored in the liver's stellate cells as retinyl esters. The liver holds about 80% of total body stores, which is why liver toxicity is the primary risk of overconsumption, and why the liver is also the best food source. From storage, retinol is released bound to retinol-binding protein (RBP) and transthyretin (TTR, which also transports thyroid hormones, more on that below). It's then converted in target tissues to retinoic acid, the biologically active form that drives most of vitamin A's effects through nuclear receptor signalling (RAR and RXR receptors).
Testosterone and male reproductive health: Vitamin A is essential for testosterone production. It crosses the blood-testis barrier as retinol, is stored in Sertoli cells, and is converted to retinoic acid locally. Retinoic acid directly stimulates Leydig cell steroidogenesis by upregulating StAR protein and P450c17 enzyme expression, two rate-limiting steps in testosterone synthesis. Animal studies consistently show that vitamin A-deficient males develop testicular atrophy, loss of germinal epithelium, and significantly reduced serum testosterone. One of the more striking human studies assigned 102 teenage boys with delayed puberty to receive either hormone therapy (testosterone or oxandrolone) or vitamin A plus iron supplementation. After six months, the vitamin A group showed growth acceleration equivalent to the hormone-treated groups, and was the only group to show increased testicular volume (indicating actual onset of puberty) at the six-month mark. Vitamin A also decreases estrogen production in the testes and supports spermatogenesis, retinoic acid is required for the initiation of meiosis in germ cells.
Immune function: Vitamin A is sometimes called the "anti-infection vitamin" and for good reason. Retinoic acid regulates both innate and adaptive immunity: it's required for the differentiation and function of T cells, B cells, natural killer cells, and macrophages. It maintains the integrity of mucosal barriers (gut, respiratory tract, skin) which are your first line of defense. Retinoic acid also drives B cell class-switching to IgA, the antibody that protects mucosal surfaces. Deficiency impairs all of these systems and is associated with increased susceptibility to respiratory infections, diarrheal diseases, and slower recovery. This matters practically: if you're getting sick often, especially respiratory infections, and your zinc and vitamin D are already covered, vitamin A status is the next thing to look at.
Skin and anti-aging: This is where most people encounter vitamin A, through topical retinoids (tretinoin, retinol serums). But oral vitamin A matters too. Retinoic acid regulates keratinocyte proliferation and differentiation, supports collagen synthesis, inhibits matrix metalloproteinases (the enzymes that break down collagen), and reduces sebum production. Serum retinol levels are significantly lower in acne patients compared to healthy controls. For severe acne, isotretinoin (Accutane) is a synthetic retinoid that works by massively reducing sebum output and normalising skin cell turnover, but it's essentially a high-dose, targeted vitamin A derivative. A review of studies found that oral vitamin A (retinol) at doses of 100,000+ IU was effective for acne, but at those doses you're approaching toxicity territory and it should only be done under medical supervision. For general skin quality and anti-aging, maintaining adequate vitamin A status through diet and moderate supplementation supports skin from the inside, while topical retinoids work from the outside.
Thyroid function: Vitamin A and thyroid hormones share transport proteins (transthyretin) and nuclear receptor pathways (RXR). Vitamin A deficiency impairs iodine uptake by the thyroid, reduces thyroglobulin synthesis, and disrupts the pituitary's ability to regulate TSH. A randomized controlled trial in premenopausal women found that 25,000 IU/day of retinyl palmitate for 4 months significantly reduced serum TSH, suggesting vitamin A supplementation may reduce the risk of subclinical hypothyroidism. This is particularly relevant for women, who are more prone to thyroid dysfunction. If you have hypothyroid symptoms and your iodine and selenium are already covered, checking vitamin A status is worthwhile.
Women's health beyond the basics: Vitamin A plays a role across the entire female reproductive axis, from hypothalamic-pituitary hormone secretion to ovarian steroidogenesis, oocyte maturation, and uterine implantation. It's required for proper functioning of the ovaries, fallopian tubes, uterus, and vaginal epithelium. In women undergoing IVF, higher follicular concentrations of retinoic acid correlated with better embryo quality. Women with poor BCMO1 conversion (the genetic variant mentioned above) may be especially vulnerable to functional deficiency, and this can present as unexplained fertility issues, vaginal dryness, or poor cervical mucus, symptoms that are rarely attributed to vitamin A but have been consistently observed in vitamin A-deficient animal models. Women on hormonal contraception should also be aware that some research suggests oral contraceptives can alter retinol metabolism.
Muscle stem cells and performance: Recent research has shown that vitamin A, through retinoic acid signalling, maintains muscle stem cell (satellite cell) quiescence and mitochondrial function. In mice, removing dietary vitamin A caused mitochondrial dysfunction and cell cycle disruption in muscle stem cells that mimicked old age. The receptor for retinol uptake (Stra6) decreases with age, which may partly explain age-related muscle recovery decline. In separate studies, mice on low vitamin A diets ran significantly less distance and at slower speeds, had reduced force generation, and lower glycogen stores in muscle tissue. This doesn't mean mega-dosing vitamin A will make you stronger, but it does mean that insufficiency directly impairs muscle function and recovery.
Cofactor relationships: Vitamin A works closely with several other nutrients. Vitamin D shares the RXR nuclear receptor with vitamin A, meaning they can compete for the same signalling pathway. Very high vitamin A intake can antagonise vitamin D signalling, and vice versa. The practical takeaway is to keep them in reasonable balance rather than mega-dosing one without the other. Iron enhances vitamin A mobilisation from liver stores and improves its utilisation, which is why the puberty study above used both together. Iron deficiency can impair vitamin A metabolism even when vitamin A intake is adequate. Zinc is required for the synthesis of retinol-binding protein, the carrier that transports vitamin A from the liver to tissues. Without adequate zinc, vitamin A can be stuck in storage and unavailable to cells. This is a common stack problem: you can be eating plenty of vitamin A and still be functionally deficient if your zinc is low.