Vitamin A

Vitamin A is a group of fat-soluble retinoids that are essential for numerous physiological functions. It exists in two primary forms in the diet: preformed vitamin A (retinol and retinyl esters) found in animal products, and provitamin A carotenoids (primarily beta-carotene) found in colorful fruits and vegetables.

The body converts carotenoids to retinol as needed, making vitamin A status dependent on both dietary sources and conversion efficiency. Preformed vitamin A is more bioavailable and efficiently utilized, while carotenoid conversion varies significantly among individuals based on genetics, gut health, and dietary factors.

Vitamin A plays critical roles in the visual cycle, particularly in low-light vision. In the retina, retinal combines with opsin proteins to form rhodopsin (rod cells) and photopsins (cone cells), enabling light detection and signal transduction to the brain. Without adequate vitamin A, night blindness develops as the first clinical sign of deficiency.

Beyond vision, vitamin A is essential for maintaining epithelial tissue integrity throughout the body, including skin, respiratory tract, and gastrointestinal lining. It regulates immune cell development and function, supporting both innate and adaptive immunity. Vitamin A is also crucial for embryonic development, organ formation, and reproductive health.

While deficiency is a major public health concern in developing countries, excess preformed vitamin A can be toxic, making proper dosing and monitoring important, particularly for pregnant women and those taking high-dose supplements.

Vitamin A functions through several key mechanisms. In vision, retinal (an aldehyde form of retinol) combines with opsin proteins in photoreceptor cells to form visual pigments. When light strikes these pigments, retinal changes shape (isomerization), triggering a signal cascade that sends nerve impulses to the brain. This process requires constant recycling of vitamin A between the retina and retinal pigment epithelium.

As retinoic acid, vitamin A regulates gene expression by binding to nuclear receptors (RAR and RXR families). These receptor complexes bind to specific DNA sequences, controlling transcription of genes involved in cell differentiation, growth, and organ development. This mechanism explains vitamin A's role in epithelial cell maintenance and immune function.

Vitamin A supports immune function by maintaining integrity of mucosal barriers (respiratory, gastrointestinal, genitourinary tracts) that block pathogen entry. It also regulates development and differentiation of immune cells, including T cells, B cells, and natural killer cells. Vitamin A deficiency impairs both antibody responses and cell-mediated immunity.

Absorption of preformed vitamin A (retinol) occurs in the small intestine via specific transporters, with efficiency typically 70-90%. Carotenoids require conversion by intestinal enzymes (dioxygenases) before absorption, with conversion rates varying from less than 10% to over 50% depending on the carotenoid and individual factors. Vitamin A is transported in blood bound to retinol-binding protein (RBP) and stored primarily in the liver.

Preformed vitamin A (retinol) is found in animal products, particularly liver, fish liver oils, and dairy products. Provitamin A carotenoids (beta-carotene, alpha-carotene, beta-cryptoxanthin) are found in colorful fruits and vegetables, particularly orange and dark green varieties. Beta-carotene has the highest vitamin A activity among carotenoids.

Vitamin A deficiency primarily affects vision and immunity. Night blindness (nyctalopia) is the earliest sign, progressing to xerophthalmia (dry eye), Bitot spots, corneal ulceration, and potentially blindness. Deficiency also increases infection susceptibility, particularly measles and diarrheal diseases in children.

Vitamin A requirements are expressed as Retinol Activity Equivalents (RAE) or International Units (IU). The RAE accounts for different bioactivities of retinol and carotenoids. 1 mcg RAE = 1 mcg retinol = 12 mcg beta-carotene = 24 mcg other provitamin A carotenoids = 3.33 IU retinol = 10 IU beta-carotene.

Vitamin A supplements come as preformed vitamin A (retinol, retinyl palmitate, retinyl acetate) or provitamin A carotenoids (beta-carotene). Preformed vitamin A is more bioavailable but carries toxicity risk. Beta-carotene is safer but conversion varies.