What is vitamin A? 

Vitamin A (Retinol) is a fat-soluble vitamin naturally present in many foods. Vitamin A is a vital vitamin responsible for normal vision, immune system, and reproduction. Vitamin A also helps the heart, lungs, kidneys, and other organs work properly.

What are the types of vitamins A?

As a dietary source in foods, vitamin A naturally occurs in two primary forms, namely;

Preformed vitamin A (Retinol) occurs in meat, poultry, eggs, and dairy products.

Provitamin (β-carotene) occurs in yellow-red fruits and vegetables and green leafy vegetables. 

What are dietary sources of vitamin A?

Non-vegetarian sources: 

Animal liver

Fish liver oil

Organ meat

Eggs 

Milk

Vegetarian sources: 

Spirulina

Mango

Papaya

Green leafy vegetables

Carrot

Functions of vitamin A in the body?

Vision: 

Vitamin A plays a critical role in vision in the dim light in the eye's retina. According to Wald (1935), the characteristic pigment of the retinal rods and cones are rhodopsin and iodopsin. They differ 

only in respect of the protein moieties (called scotopsin and photopsin, respectively). The specific pigment common to both is a cis-isomer of retinol (vitamin A aldehyde). Vitamin A alcohol is oxidized to vitamin A aldehydes in the epithelium of the rods by alcohol dehydrogenase in the presence of NAD. The protein opsin (scotopsin) reacts with the retinal to form rhodopsin.

Rhodopsin, also known as visual purple, is located in the light-sensitive rod cells of the retina. 

When light strikes the retina, the rod cells are bleached as the rhodopsin splits to form retinal and opsin. A nerve stimulus passes through the optic nerve fibers to the brain's visual center, creating the sensation of vision.

The eye's retina also contains cone cells involved in the perception of color vision in good light. The light-sensitive opsin of the cone cells also contains vitamin A as part of their structure.

The functioning of the cone cells is not sensitive to variations in available vitamin A is the case in the functioning of the rod cells.

Growth: 

Vitamin A is essential for the average growth of bones. In vitamin A deficiency, bones become weak although thicker than average. The cavities in the skull and spinal column do not enlarge to make room for the growing nervous system. Deficiency may involve the failure of immature bone cells to mature into osteoclast, which is responsible for the breakdown of bone during bone remodeling. 

Immunity: 

Vitamin A influences several features of the immune system. The body needs Vitamin A to maintain the usual health and function of epithelial layers, which provide the first line of defense against invading microorganisms. Humoral immune response (antibodies) and cellular immune response, which involve the direct healing of infected cells, are regulated by vitamin A or its metabolites. As an immunomodulator, vitamin A reduces the severity but not the incidence of certain infections like tuberculosis, diarrhea, and malaria. 

Vitamin A may also alter the course of immune response in acute respiratory infection, pneumonia, and bronchiolitis and thus influence the outcome of the respiratory diseases. In children, there is evidence to indicate that vitamin A supplements are beneficial for reducing morbidity and mortality among HIV-infected children. 

Cell differentiation and gene expression:

The likely involvement in the development or differentiation of immature bone cells into different types of mature cells is just one example of various forms of cell differentiation that depend on the vitamin.

Many locations have Epithelial cells, including the skin, the eye, and the lining of the digestive system, genitourinary system, and respiratory tract. Those within the body normally secrete mucus and are covered by cilia hair-like structures. The cilia on the lining of the respiratory system prevent the accumulation of foreign material on the surface of the epithelial cells by their constant motion. The action of the cilia is involved in protecting the body against infections by sweeping the cell surface clear of invading microorganisms.

In vitamin A deficient keratinized cells, the cilia are lost. The keratinization and the loss of cilia leave the body more vulnerable to infection. Hence, vitamin A is known as an 'Anti-infective' vitamin.

Vitamins A is necessary for the formation of epithelial cells. Hence, a constant supply of vitamin A is obligatory for everyday health. The tissues most sensitive to vitamin A deficiency are the skin, trachea, salivary glands, cornea, and testes. 

Reproductive Health: 

The reproductive system is adversely affected by vitamin A deficiency. Degeneration of germinal epithelium leads to sterility in males.

Vitamin A Deficiency symptoms: 

Symptoms of a vitamin A deficiency can differ in severity. Some people may have more severe complications than others. Below are several possible symptoms you may experience:

Night blindness: This causes you to have trouble seeing in low light. It will eventually lead to complete blindness at night.

Xerophthalmia: With this condition, the eyes may become very dry and crusted, damaging the cornea and retina.

Infection: A person with a vitamin A deficiency can experience more frequent health concerns as they will not fight off infections as quickly.

Bitot spots: This condition is a buildup of keratin in the eyes, causing hazy vision.

Skin irritation. People experiencing vitamin A deficiency could have problems with their skin, such as dryness, itching, and scaling.

Keratomalacia: This is an eye disorder involving drying and clouding the cornea — the transparent layer in front of the iris and pupil.

‌ Keratinisation: This is a process by which cells become filled with keratin protein, die, and form formidable, resistant structures in the urinary, gastrointestinal and respiratory tracts.

Stunted growth: Not having enough vitamin A could delay development or cause children to experience slow bone growth or stunted growth.

Fertility: A deficiency in vitamin A may cause challenges when trying to conceive a child, and in some cases, infertility.

How to diagnose vitamin A deficiency?

TYPICAL FINDINGS ASSOCIATED WITH CATEGORIES OF VITAMIN A STATUS
Range	Plasma Retinol	Clinical Signs	Vulnerable groups/ Most Common Situations
Deficient	<0.35 μmol/L	Night blindness; other ocular manifestations common	Preschool-age children and pregnant or lactating women with low vitamin A intakes; inflammation and poor nutritional status
Marginal	0.35–0.70 μmol/L	None or minimal (positive plasma response to vitamin A)	Children, pregnant women in vulnerable populations, often with high rates of infection
Adequate	>1.05–3.00 μmol/L	None	Typical of a well-nourished general population
Excessive	Upper normal to >3 μmol/L	Not apparent or mild; may have elevated liver enzymes in plasma indicative of liver damage	Long-term supplement use; frequent intake of foods (e.g., liver) high in preformed vitamin A
Toxic	Similar to above, with circulating retinyl esters in fasting plasma	Headache; bone or joint pain; elevated liver enzymes and clinical signs of liver disease; very high vitamin A in liver and increased levels in extrahepatic tissues	Food faddists and users of high-dose vitamin A supplements; patients treated with retinoids

Very rarely, plasma retinol may be low because of hereditary familial low retinol-binding protein (see text); 0.35 mol/L  10 g retinol/dL.

The range 1.05 to 0.70 mol/L indicates marginal status, and 0.70 mol/L indicates vitamin A deficiency. These ranges may be more appropriate for adults whose median plasma retinol levels are higher than children.

Positive relative dose response (RDR) or modified (MRDR) test

Low plasma retinol in inflammation may indicate an acute phase response associated with reduced RBP production rather than deficient vitamin A Storage.

Retinol and RBP are standard, whereas total retinol increases due to retinyl esters, and total liver retinol exceeds approximately 300 g/g.

Retinoid administration typically reduces, not increases, plasma retinol levels.

How much vitamin A I need to take daily?

RDA* or Recommended Dietary Allowance proposed by NIN & ICMR for Indians.

These recommendations help us know how much vitamin A we need to prevent insufficiency or deficiency in our daily diets. Let us check the ranges in detail below.

Age Group	RDA of Retinol. (per day/ mcg)	RDA of Beta-Carotene. (per day/ mcg)
Men	1000	6000
Women (Non pregnant, Non lactating)	840	5040
Pregnancy	900	5400
Lactation	950	5700
Infant
0-6 months	350	2100
6-12 months	350	2100
Children
1-3 years(Boys & Girls)	390	2340
4-6 years (Boys & Girls)	510	3060
7-9 years (Boys & Girls)	630	3780
10-12 years  (Boys)	770	4620
10-12 years  (Girls)	790	4740
13-15 years  (Boys)	930	5580
13-15 years  (Girls)	890	5340
16-18 years  (Boys)	1000	6000
16-18 years  (Girls)	860	5160
Old age
≥ 60 years (Men)	1000	6000
≥ 60 years (Women)	840	5040

 

*RDA- Means Recommended Dietary Allowance per day as per an individual’s age group.