Vitamin A

Vitamin A

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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.

Dosage:

  • RDA: 900 mcg RAE (3,000 IU) for men, 700 mcg RAE (2,333 IU) for women. These are the minimum amounts to prevent deficiency, not optimal amounts for active individuals
  • Practical supplementation range: 3,000-10,000 IU/day of preformed vitamin A (retinol or retinyl palmitate). This is a safe, well-tolerated range for most adults and covers the gap between typical dietary intake and optimal tissue levels
  • Upper limit (UL): 10,000 IU/day (3,000 mcg) of preformed vitamin A for adults. This is the threshold above which chronic use increases the risk of liver toxicity and adverse effects. Do not exceed this without medical supervision and regular bloodwork
  • Women of childbearing age should be cautious above 10,000 IU/day due to teratogenic risk (birth defects). Preformed vitamin A above this threshold during pregnancy is associated with abnormalities of the eyes, skull, lungs, and heart. Beta-carotene does not carry this risk. If you're pregnant or planning to become pregnant, stick to 5,000-8,000 IU/day max from preformed sources and get the rest from food
  • Best forms: Retinyl palmitate and retinol are the most bioavailable supplemental forms. Beta-carotene is safe and non-toxic (your body regulates conversion) but unreliable as a sole source of vitamin A due to variable conversion genetics
  • Best food sources (preformed vitamin A, the kind your body can use directly):
    • Beef liver (85g pan-fried): ~6,600 mcg RAE (~22,000 IU), 730% DV. The single most concentrated source by a wide margin. One serving per week covers most people's vitamin A needs entirely. If you don't like the taste, blending small amounts into ground beef mince or making pâté are common workarounds
    • Lamb liver (85g cooked): ~7,800 mcg RAE (~26,000 IU), 865% DV. Even higher than beef liver, though less commonly available
    • Cod liver oil (1 tablespoon / 14ml): ~4,080 mcg RAE (~13,600 IU), 453% DV. The easiest daily source if you're not eating organ meats. Also provides vitamin D and omega-3s, making it a practical all-in-one. Look for brands that list the actual retinol content rather than just beta-carotene
    • Liverwurst / liver pâté (55g / quarter cup): ~2,250 mcg RAE (~7,500 IU), 250% DV. A more palatable way to get liver into your diet for those who find organ meat unappealing
    • King mackerel (85g cooked): ~214 mcg RAE, 24% DV
    • Salmon (85g cooked sockeye): ~60 mcg RAE, 7% DV
    • Butter (1 tablespoon / 14g): ~97 mcg RAE, 11% DV. Adds up if you cook with it daily
    • Whole eggs (2 large, scrambled): ~98 mcg RAE, 11% DV. The vitamin A is entirely in the yolk, so egg whites contribute nothing
    • Whole milk (1 cup / 240ml): ~78 mcg RAE, 9% DV
    • Hard goat cheese (28g): ~87 mcg RAE, 10% DV
    • Cheddar cheese (28g): ~75 mcg RAE, 8% DV
  • Plant sources (provitamin A, requires conversion): These values are already expressed in RAE, meaning the conversion inefficiency is already factored in at the standard 12:1 ratio. If you carry BCMO1 variants (see above), your actual yield could be 30-70% less than listed
    • Sweet potato (1 medium, baked with skin): ~1,400 mcg RAE, 156% DV. The best plant source by far
    • Carrots (1 cup raw, chopped): ~1,070 mcg RAE, 119% DV
    • Butternut squash (1 cup cooked): ~1,140 mcg RAE, 127% DV
    • Spinach (1 cup cooked): ~570 mcg RAE, 63% DV
    • Kale (1 cup cooked): ~170 mcg RAE, 19% DV
    • Red bell pepper (1 large raw): ~257 mcg RAE, 29% DV
    • Mango (1 cup sliced): ~89 mcg RAE, 10% DV
    • Cantaloupe (1 cup cubed): ~270 mcg RAE, 30% DV
  • Practical note for people getting vitamin A from food only: If you eat liver once a week (even 50-85g), you're almost certainly covered for the entire week. If you don't eat liver, combining cod liver oil daily with regular intake of eggs, butter, full-fat dairy, and a mix of orange/green vegetables is the next best approach. A diet that's high in chicken breast, rice, and vegetables but low in organ meats, egg yolks, and full-fat dairy (the typical "clean" bodybuilding diet) is one of the worst for vitamin A status
  • Absorption: Vitamin A is fat-soluble, always take with a meal containing fat. Low-fat diets significantly impair absorption. Gut conditions that reduce fat absorption (Crohn's, celiac, pancreatic insufficiency) also impair vitamin A uptake
  • Pairing: Take with vitamin D and zinc for synergy, but don't mega-dose vitamin A without proportional vitamin D. A reasonable ratio to aim for is roughly 5-10:1 (vitamin A IU to vitamin D IU), though individual needs vary

Here's what you can expect:

Vitamin A isn't a compound you'll "feel" working the way you might notice magnesium or zinc. Improvements are gradual and depend on where you're starting from. If you were genuinely insufficient, you may notice improved night vision and less eye dryness within 2-4 weeks. Skin changes, reduced dryness, less flaking, improved texture, and potentially fewer breakouts, tend to show up over 4-8 weeks. Immune improvements (fewer colds, faster recovery from illness) accumulate over weeks to months. If you were deficient and it was affecting your hormonal status, you might notice improvements in energy, libido, and mood over 4-8 weeks as testosterone normalises. If your levels were already adequate, supplementation within the normal range won't produce noticeable effects, which is exactly how it should work.

Side effects & risks:

  • Liver toxicity is the primary risk, and it's real but proportional to dose and duration. Vitamin A is stored in hepatic stellate cells, and chronic overconsumption causes these cells to become activated and hypertrophied, leading to fibrosis and eventually cirrhosis. The lowest daily dose documented to cause cirrhosis was 25,000 IU taken continuously for 6 years. Higher doses (100,000+ IU/day) have caused similar damage in as little as 2.5 years. At the recommended supplemental range of 3,000-10,000 IU/day, liver toxicity is extremely unlikely in people with normal liver function
    • If you're running hepatotoxic compounds (oral steroids, certain medications), be especially conservative with vitamin A. Don't stack liver stressors without monitoring
    • If you drink regularly, your risk is higher. Alcohol and vitamin A compete for the same hepatic metabolic pathways and the combination accelerates liver damage
  • Teratogenicity is the most serious risk for women. Preformed vitamin A above 10,000 IU/day during pregnancy is associated with birth defects. This applies to retinol and retinyl esters, not beta-carotene. Any woman who could become pregnant should track their intake carefully
  • Acute toxicity from a single massive dose (several hundred thousand IU) causes severe headache, nausea, vertigo, blurred vision, muscle pain, and skin peeling. This is rare and typically only happens with accidental ingestion or intentional megadosing
  • Chronic toxicity symptoms at moderately elevated doses (25,000+ IU/day for months) include dry skin, cracked lips (cheilitis), joint and bone pain, hair loss, fatigue, headaches, and elevated liver enzymes. These resolve after stopping supplementation in most cases
  • Bone density: There is some evidence that chronically high vitamin A intake (above UL) may reduce bone mineral density and increase fracture risk, though the data is mixed and mostly comes from observational studies
  • Drug interactions: Vitamin A can interact with retinoid medications (isotretinoin, tretinoin, acitretin), leading to additive toxicity. If you're on prescription retinoids, do not supplement with additional vitamin A. It can also interact with hepatotoxic medications, blood thinners (vitamin A may affect vitamin K metabolism at high doses), and orlistat (which reduces fat-soluble vitamin absorption)
  • Beta-carotene note: High-dose beta-carotene supplementation (not dietary intake) has been associated with increased lung cancer risk in smokers and former smokers in two large trials (CARET and ATBC). If you smoke, avoid high-dose beta-carotene supplements. This does not apply to beta-carotene from food
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Blood markers

Serum retinol (normal range 0.3-1.2 mg/L or 30-120 mcg/dL), check at baseline. Keep in mind that serum retinol is homeostatically regulated and won't drop until liver stores are nearly depleted, so a "normal" result doesn't rule out marginal insufficiency. Values below 30 mcg/dL indicate deficiency.
Retinol-binding protein (RBP), a more sensitive indirect marker of vitamin A status than serum retinol alone. Low RBP can indicate depleted vitamin A stores even when serum retinol looks normal. Check at baseline.
ALT + AST (liver enzymes), check at baseline and every 3 months if supplementing above 5,000 IU/day or if you have any liver risk factors (alcohol use, hepatotoxic medications). This is the most important safety marker.
Serum zinc, check at baseline. Low zinc impairs RBP synthesis and can cause functional vitamin A deficiency regardless of intake.
Thyroid panel (TSH, free T4, free T3), check at baseline if supplementing partly for thyroid support. Vitamin A can modulate TSH, so tracking this helps assess response.
For most people, serum retinol, a liver panel, and zinc at baseline is sufficient. Add RBP if you suspect marginal insufficiency despite normal retinol, or thyroid markers if that's part of your reason for supplementing. Recheck liver enzymes at 3 months.
This is not medical advice. Consult a healthcare provider before starting any new supplement regimen.