Vitamin A

What vitamin A actually is (and why the name is misleading)

Vitamin A isn't one molecule. It's a group of fat-soluble compounds that your body uses for vision, immune defense, skin maintenance, and gene regulation. The two dietary forms matter: preformed retinol (from animal foods like liver, eggs, and dairy) and provitamin A carotenoids (beta-carotene, alpha-carotene, and beta-cryptoxanthin from orange and green vegetables). Your body converts carotenoids into retinol, but the conversion rate is poor. It takes roughly 12 molecules of dietary beta-carotene to produce one molecule of retinol ^[1]. This distinction between retinol and beta-carotene has real consequences for how you plan your intake.

How vitamin A affects aging

Retinoic acid, the active metabolite of vitamin A, binds to nuclear receptors (RAR and RXR) that regulate over 500 genes involved in cell differentiation, immune function, and tissue repair. That's the textbook version. The longevity-relevant part is that carotenoid status tracks directly with biological age markers. A study of US adults found that higher serum levels of alpha-carotene, beta-carotene, and beta-cryptoxanthin were associated with 5-8% longer leukocyte telomeres compared to the lowest quartile ^[2]. A 2024 NHANES analysis confirmed that higher dietary carotenoid intake correlated with decelerated biological age acceleration across multiple epigenetic clocks ^[3].

The mechanism is straightforward: carotenoids neutralize reactive oxygen species that damage telomeric DNA. But this benefit comes from food-sourced carotenoids, not high-dose supplements. That distinction matters a lot.

The supplement paradox

Two landmark trials changed how we think about vitamin A supplementation. The ATBC study (29,133 male smokers) found that 20 mg/day beta-carotene supplements increased lung cancer incidence by 18% ^[4]. The CARET trial confirmed this, showing 28% more lung cancers in smokers and asbestos workers taking beta-carotene plus retinol ^[5]. These results apply specifically to isolated high-dose beta-carotene supplements in smokers. Dietary beta-carotene from fruits and vegetables shows the opposite pattern: a meta-analysis of prospective studies found that higher dietary beta-carotene intake reduced all-cause mortality by 17% ^[6].

Preformed retinol carries its own risk. The tolerable upper intake level is 3,000 mcg RAE (10,000 IU) per day for adults. Chronic intake above this threshold can cause liver damage, bone loss, and birth defects ^[7]. A single serving of beef liver contains 730% of the daily value, so people who regularly eat organ meats should monitor their intake.

Vitamin A and your immune system

Your gut lining depends on vitamin A more than on almost any other nutrient. Retinoic acid maintains tight junction proteins between intestinal cells, supports mucin production, and stimulates secretory IgA, the antibody that patrols mucosal surfaces ^[8]. Deficiency breaks down these barriers, increasing intestinal permeability and systemic inflammation. Vitamin A also shapes immune cell behavior: it promotes regulatory T cells and dampens overactive inflammatory responses, which is why deficiency is linked to higher susceptibility to respiratory and gastrointestinal infections.

Recent research shows that gut bacteria themselves produce retinoids. Commensal bacteria like Lactobacillus species can convert dietary precursors into active retinoid forms, creating a feedback loop between your microbiome and vitamin A status ^[9]. This means gut health and vitamin A status are more intertwined than previously understood.

Practical guidance on intake

The RDA is 900 mcg RAE for men and 700 mcg RAE for women. Most people in developed countries get enough from food, but vegans and people with fat malabsorption conditions may fall short. The best food sources of preformed retinol are liver (one serving covers a week's worth), eggs, and full-fat dairy. For carotenoids, sweet potatoes, carrots, spinach, and kale deliver the highest concentrations. Cooking and adding fat to carotenoid-rich vegetables increases absorption from roughly 10-15% to 30% or more. If you supplement, stick to mixed carotenoid formulas rather than isolated beta-carotene, and keep retinol supplements well below the 3,000 mcg upper limit.