Micronutrient, Vitamin A (Retinol)

Vitamin A is a fat-soluble vitamin your body needs for good vision (especially in dim light), a healthy immune system, and maintaining the linings of your eyes, lungs, and digestive tract. Your blood level of vitamin A can be measured to check whether you are getting too little or too much. Too little vitamin A can cause difficulty seeing at night, dry eyes, and increased infections, while too much—usually from high-dose supplements rather than food—can harm your liver and bones. Because your body stores vitamin A in the liver and carefully controls blood levels, a normal result does not always rule out early deficiency, and your doctor will consider your diet, health conditions, and other test results together.

When elevated: Elevated serum retinol (hypervitaminosis A) may indicate excessive supplementation or dietary intake of preformed vitamin A, and is associated with hepatotoxicity, pseudotumor cerebri, bone demineralization, teratogenicity, and dermatologic changes such as desquamation and alopecia. When low: Low serum retinol is associated with night blindness (nyctalopia), xerophthalmia, increased susceptibility to infections (particularly respiratory and gastrointestinal), impaired growth in children, and poor wound healing; severe deficiency can progress to corneal ulceration and irreversible blindness.

Vitamin A supports immune function and mucosal integrity, which are relevant during heavy training when immune suppression can occur. It also plays a role in vision and muscle cell differentiation, though deficiency is uncommon in athletes eating varied diets with animal products or colorful plant foods.