Vitamin A and β-carotene

Vitamin A, or retinol, is an essential nutrient for man and all mammalian species since it cannot be synthesised within the body. Deficiency of the vitamin results in adverse effects on growth, reproduction and resistance to infection.

Vitamin A occurs as retinyl esters in foods of animal origin and in the form of provitamin A carotenoids in plant foods. Some carotenoids found in colourful fruits and vegetables are called provitamin A; they are metabolized in the body to vitamin A.

Among the carotenoids, β-carotene, a retinol dimer, has the most significant provitamin A activity in human nutrition, since its concentration in food and feed ingredients, particularly of leaf origin, greatly exceeds that of the other vitamin A active compounds. Other provitamin A carotenoids, such as α-carotene and β-cryptoxanthin, are half as active as β-carotene.

The β-carotene molecule contains two β-ionic rings. Theoretically the cleavage of that chain at –C15 = C15′– position provides two retinol molecules.

Carotenoids play a prominent role in protecting bodily cells and thereby act as powerful antioxidants. It has been observed that carotenoid pigments in all photosynthetic organisms, bacteria, algae, and higher plants, play an important role in protecting these organisms against the seriously damaging effects of photooxidation by their own endogenous photo-sensitizer, chlorophyll.

Due to its high bioactivity, β-carotene is also widely used in medicine. Among the numerous functions of β-carotene in the human body, the important one is related to provitamin A supply, further affecting embryonic development, correct growth, and sight. It is considered as an inhibitor of some genes; moreover, it exhibits anticancer and antioxidant properties.
Vitamin A and β-carotene

Vitamin A and stomach cancer

Gastric cancer is the fourth most common malignancy and the second leading cause of death due to cancer.

Study in Sweden published in The American Journal of Clinical Nutrition, Volume 85 suggest that high intakes of vitamin A, retinol, and the provitamin A carotenoids α-carotene and β-carotene may reduce the risk of gastric cancer. These results support the hypothesis of a possible protective role of vitamin A in gastric carcinogenesis. Another study shows it may cut the risk of stomach cancer by 44 percent.

The study, which analyzed dietary data from over 36,000 women and 45,000 men, also reported that similar risk reductions are obtained from high dietary intake of both alpha- and beta-carotene.

Vitamin A is a generic term referring to both preformed vitamin A (retinol and its esters) and some carotenoids. Vitamin A is well known to be important in the general growth and differentiation of epithelial tissues. In animals, deficiency of vitamin A has been shown to enhance susceptibility to various chemical carcinogens in the respiratory tract, bladder, colon, and stomach.

Study published in American Journal of Epidemiology, Volume 121 found that stomach cancer cases showed a significantly higher proportion with lowered total vitamin A intake levels (odds ratio = 1.71) which remained constant across sex and socioeconomic status groupings.
Vitamin A and stomach cancer

Carotene in carrot

Carrot juice has a number of health benefits. The carrot provides what is certainly the most important basic juice. Carrot roots are a rich source of carotenoids. The total carotenoid content in the edible portion of carrot roots ranges from 6000 to 54800 μg/100 g.

The predominant carotenoids in orange-colored carrots are β-carotene, α-carotene and γ-carotene. The proportion of individual pigments reported includes β-carotene (45-80%), α-carotene (15-40%), γ-carotene (2-10%), and others (3-6%). Another name for this form of carotene, the transform, is pro vitamin A.

Many authors write that carrots contain a lot of vitamin A. This is not actually true; what the carrot does contain is the pro vitamin. That means a substance that is converted by the body into the vitamin itself.

Β-carotene is converted to vitamin A on the body. Because of differences in uptake, storage and chemical processing, only about one-sixth of the β-carotene in a plant food ends up as vitamin A in the body.
Carotene in carrot

Roles of vitamin A in human body

Vitamin A -active compounds, defined as compounds having qualitatively the biological activity of retinal, are represented by retinoid and pro-vitamin A carotenoids.

Vitamin A is required for several essential life processes, including metabolism, haematopoeisis, bone development, pattern formation during embryogenesis, the maintenance of differentiated epithelia, and immune-competence.

These processes can be supported by all forms of vitamin A, including the pro-vitamin A carotenoids. The other vitamin functions namely in reproduction, growth, the maintenance of skin and mucous membranes and the visual process, require either retinol or retinaldehyde.

Retinol

Vitamin A is needed to process incoming light to visual images and to keep the eye’s surface healthy. Moreover, beta-carotene –a compound that the body converts to vitamin A - is an antioxidant and thus works to neutralize harmful substances known as free radicals.

As such, beta-carotene appears to help protect the body against a variety of disorders, including cancer and heart disease.

Vitamin A is known to be involved in fetal development and in the regulation of proliferation and differentiation of many types of cells throughout life. The effects of vitamin A on cellular differentiation are due to the control of gene expression by retinoic acid in selected tissues, the protein products being responsible for the effects.

It plays a role in immune function, both as a cell regulator and by helping maintain the skin and mucous membranes.
Roles of vitamin A in human body

Vitamin A

Food source of vitamin A
Vitamin A in food is found as retinol or as carotenes. Retinol is found exclusively in animal foods including eggs, milk and milk products. Vitamin A is present in the fat portion of whole milk, so it is not found in fat-free milk. Most fat-free milk and dried nonfat milk solids sold in the US are fortified with vitamin A.

Fortified food such as cereal can also be good source.

It is important to regularly eat foods that provide vitamin A or beta-carotene even though your body can store vitamin A in the liver. Stored vitamin A will help meet your needs when your intake from food is low.

Carotenoids are found primarily in plants foods whereas meats, fats and dairy products are reportedly low in carotenoid content. The richest known sources of pro-vitamin A are the palm oils.

Other food sources of carotenoid include vegetables such as carrot, tomato, sweet potato, spinach and other green leafy vegetables and fruits such as mango, papaya watermelon and apricots.

According to DRI committee, a man needs a daily average of about 900 micrograms; a woman needs about 700 micrograms.

Vitamin A deficiency
It is a leading cause of childhood blindness. Although dietary deficiency of vitamin A is rare in North America and Western Europe, it is the leading cause of childhood blindness worldwide, especially in Southeast Asia, parts of Africa and Central and South America.

In countries where immunization programs are not widespread and vitamin A deficiency is common, millions of children die each year from complications of infectious diseases such as measles.

Signs of vitamin A deficiency include night blindness, dry skin, and decreased resistance to infections.

Vitamin A deficiency interacts with other nutrient deficiencies and with infection, worsening respiratory infections and diarrhea and causing countless deaths.

Extremely dry skin, dry hair, sloughing off of skin, and broken fingernails are other common signs of vitamin A deficiency. Vitamin A deficiency also decreases resistance to infections, and may contribute to the pneumonia associated with vitamin A deficiency.

Hypervitaminosis
Symptoms of hypervitaminosis A may occur in the skin, nervous system, musculoskeletal, circulatory systems or in internal organs.

Toxicity varies with the dose, body mass, age, sex, diseases conditions, concurrent drugs being taken and environmental chemical exposure.

Toxicity compromises the tissues just as deficiency does and is equally dangerous.

The many symptoms of vitamin A toxicity include abdominal pain, hair loss, joint pain, stunted growth bone and muscle soreness, cessation of menstruation, nausea, diarrhea, rashes, damage to the liver and enlargement of the spleen.
Vitamin A