Vitamin A (retinol) is a fat-soluble vitamin essential for vision, growth, reproduction, cell division, and the integrity of the immune system. The most well-known function of vitamin A is its role in vision. As part of the membrane-bound visual protein rhodopsin, vitamin A is essential in the eyes for converting light into nerve impulses. Night blindness and xeropthalmia are signs of moderate to severe vitamin A deficiency. Xerophthalmia is a common cause of preventable blindness in many underdeveloped countries around the world. Vitamin A is essential for normal cellular differentiation. Its metabolite, retinoic acid, assumes a central role in gene activation and transcription. As a result, vitamin A status has profound effects on all rapidly dividing tissues in the body, such as immune cells and intestinal cells, and affects fertility, fetal development and growth. Cellular differentiation is crucial for normal immune response, and vitamin A deficiency can start a vicious cycle affecting the immune system. During vitamin A deficiency, immune function is impaired, which puts the body at increased risk for infections. Acute infections further deplete the body of vitamin A, which leads to an even more impaired immune function and an even lower resistance to infections. While a few studies have found that vitamin A status in developed populations may be adequate, many studies showed that marginal vitamin A deficiency is quite common. Women often have lower vitamin A levels than men – who tend to consume more meats high in vitamin A. The elderly and immune-compromised are also at increased risk for marginal vitamin A deficiency. The intestinal absorption of vitamin A is associated with fat absorption. Therefore, some dietary fat must be present for efficient vitamin A absorption to occur.