Vitamin A

Fat-soluble vitamin A includes retinoids and provitamin A carotenoids. The term retinoids refers to retinol, its metabolites (retinal, retinoic acid, and retinyl esters), and synthetic analogues that are structurally similar to retinol. The retinoids are often referred to as preformed vitamin A. Carotenoids are long-chained hydrocarbon compounds, of which more than 600 forms exist. Of the many carotenoids in nature, about 50 forms have provitamin A activity. The most prevalent provitamin A carotenoids in diets include a-carotene, b-carotene, and b-cryptoxanthin.

Vitamin A is known for its role in the visual system. The 11-cis retinal in the retina of the eye functions as a coenzyme with the protein opsin to form rhodopsin. After exposure to light, 11-cis retinal isomerizes to trans-retinal, releasing opsin. The release of opsin initiates an impulse along the optic nerve to send visual images to the brain. Another critical function of vitamin A is its requirement for cellular differentiation. Retinoic acid, a vitamin A metabolite, regulates the various gene expressions that encode for structural proteins, enzymes, extracellular matrix proteins, growth factors, and receptors. Retinoids play a role in normal reproduction, fetal development, and growth.


In addition, retinoic acid is involved in normal immune function, maintaining adequate levels of natural killer cells.5 Carotenoids have been shown to possibly have antioxidant activity, and to be associated with the following health effects: decreased risk of macular degeneration and cataracts, decreased risk of some cancers, and decreased risk of some cardiovascular diseases. However, in humans, the only clear function of carotenoids is vitamin A activity.4 Vitamin A deficiency occurs when there is an inadequate vitamin A intake, requirements are increased, or intestinal absorption, transport, or metabolism are impaired as a result of conditions such as diarrhea.6 Night blindness is one of the first signs of vitamin A deficiency. A more severe deficiency contributes to xerophthalmia (dry eye) and if untreated, permanent blindness. Other signs and symptoms of vitamin A deficiency include anorexia, immune deficiencies, follicular hyperkeratosis, and growth difficulties.7 Vitamin A deficiency is uncommon in the most developed countries.8 However, vitamin A deficiency in the United States is most often associated with strict dietary restrictions and excessive alcohol intake.9 The conversion factors for provitamin A activity of carotenoids were recently changed to reflect the findings of studies on carotenoid bioavailability. Previously, the biological activity of vitamin A was quantified by conversion of retinol and provitamin A carotenoids to retinol equivalents (REs).

 One RE was defined as 1 μg of retinol, 6 μg of -carotene, or 12 μg of other provitamin A carotenoids. However, in 2001, IOM introduced a new term, retinol activity equivalent (RAE) to express the vitamin A activity of carotenoids5 based on very low vitamin A activity of plant-derived foods in combating vitamin A deficiency despite high provitamin A carotenoid content. 10 One RAE is equivalent to 1 μg of retinol, which is nutritionally equivalent to 12 μg of -carotene or 24 μg of other provitamin A carotenoids. Currently, International Units (IUs) of vitamin A (1 IU 0.3 μg retinol or 0.6 μg supplemental -carotene) are utilized on food and supplement labels in the United States. Interconversions of currently used vitamin A units, RAE and IU, are given in Table 4.4. The recommendations for vitamin A intakes for the United States and Canada are given in μg RAE. The RDAs for men and women are 900 and 700 μg RAE/d, respectively. Preformed vitamin A is found in animal-derived foods, particularly organ meats, egg yolks, and fortified food products. Nonfat and low-fat milk products are frequently fortified with retinoids. Provitamin A carotenoids are abundant in dark green leafy vegetables and yellowish-orange fruits and vegetables. Vitamin A toxicity symptoms are variable and nonspecific. The symptoms may include nausea, vomiting, headaches, increased cerebrospinal fluid pressure, vertigo, blurred vision, muscular incoordination, and bulging fontanel in infants. Chronic hypervitaminosis A may include central nervous system effects, liver abnormalities, and bone and skin changes.5 Carotenoids are relatively nontoxic. Carotenodermia has been reported only from the large consumption of carotenoids in foods. Carotenodermia is harmless and results in a yellowish discoloration of the skin due to the elevation of carotene concentrations. The UL of vitamin A for adults is set at 3,000 μg/d. Currently, DRIs including ULs for carotenoids are not available. Physical exercise may raise oxygen consumption and increase free radical production, leading to lipid peroxidation or possible tissue damage.


 Although vitamin A is a weak antioxidant, vitamin A is probably needed for tissue repair, and an increased metabolism of the vitamin would be expected during strenuous exercise. Adult athletes reportedly are well-nourished with respect to the total vitamin A.  Some athletes reportedly have excessive vitamin A intakes, which can be harmful and potentially toxic. Studies indicate that supplementation has no beneficial effect on physical activity.


Because vitamin supplements usually contain 100% of the RDA in one dose, it is easy to exceed the UL for vitamin A by taking multiple doses along with a diet rich in animal products.10 Carotenoids have been shown to possess antioxidant properties. Thus, carotenoids may influence exercise-induced lipid peroxidation or muscle damage. -carotene has been most investigated for its antioxidant activity to reduce exercise-induced lipid peroxidation in combination with other antioxidant nutrients such as vitamins E and C. Clear evidence of beneficial roles of carotenoids in exercise-induced lipid peroxidation and in the abilities of carotenoids to improve physical performance has not yet been reported.


 Currently, no specific recommendations for either vitamin A or carotenoids have been set for athletes.