Determining Mean Requirements and Setting Safe Levels of Intake

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What is Sub-clinical vitamin A deficiency

Below are various authors' explanation about Sub-clinical vitamin A deficiency and the conclusion of FAO/WHO:

Vitamin A - Toxicity

Smith, F.R. & Goodman, D.S. 1976.Vitamin A transport in Human vitamin A toxicity. N. Engl. J. Med., 294: 805–808. 

Because vitamin A is fat soluble and can be stored, primarily in the liver, routine consumption of large amounts of vitamin A over a period of time can result in toxic symptoms, including liver damage, bone abnormalities and joint pain, alopecia, headaches and vomiting, and skin desquamation. Hypervitaminosis A appears to be due to abnormal transport and distribution of vitamin A and retinoids caused by overloading of the plasma transport mechanisms.

Dietary Sources

Rodriguez-Amaya, DB. 1997. Carotenoids and food preparation: the retention of provitamin A carotenoids in prepared, processed, and stored foods. Arlington, VA, John Snow, Inc./OMNI Project, wrote:

Preformed vitamin A is found almost exclusively in animal products, such as human milk, glandular meats, liver and fish liver oils (especially), egg yolk, and whole milk and dairy products. Preformed vitamin A is also used to fortify processed foods, that may include sugar, cereals, condiments, fats, and oils. 

Pro-vitamin A carotenoids are found in green leafy vegetables (e.g., spinach, amaranth, and young leaves from various sources), yellow vegetables (e.g., pumpkins, squash, and carrots), and yellow and orange noncitrus fruits (e.g., mangoes, apricots, and papaya). 

Population at Risk and Consequences of Vitamin A Deficiency

Definition of vitamin A deficiency 

WHO/UNICEF.  The Indicators of VAD (Vitamin A Deficiency) and their use in monitoringintervention programmes. World Health Organization, Geneva, VAD is not simply defined. 

With reference to WHO statement:

WHO defines it as tissue concentrations of vitamin A low enough to have adverse health consequences even if there is no evidence of clinical xerophthalmia.

Biochemical Mechanisms for Vitamin A Functions

Rando, R.R.  Retinoid isomerization reactions in the visual system. In: Blomhoff R, ed. Vitamin A in Health and Disease, he explained:

Vitamin A functions at two levels in the body. The first is in the visual cycle in the retina of the eye; the second is in all body tissues systemically to maintain growth and the soundness of cells. In the visual system, carrier-bound retinol is transported to ocular tissue and to the retina by intracellular binding and transport proteins.Rhodopsin, the visual pigment critical to dimlight vision, is formed in rod cells after conversion of all-trans retinol to retinaldehyde, isomerization to the 11-cis-form, and binding to opsin. Alteration of rhodopsin through a cascade of photochemical reactions results in ability to see objects in dim light.  

Role of Vitamin A in Human Metabolic Processes

FAO/WHO states that:

Vitamin A (retinol) is an essential nutrient needed in small amounts by humans for the normal functioning of the visual system; growth and development; and maintenance of epithelial cellular integrity, immune function, and reproduction. These dietary needs for vitamin A are normally provided for as preformed retinol (mainly as retinyl ester) and provitamin A carotenoids.

Dietary Diversification - Vitamin A

Reference from the joint report of FAO/WHO expert consultation on Human Vitamins and Minerals verbatim. (Chapter 2)

Vitamin A

2. FAO/WHO. 1988. Requirements of Vitamin A, Iron, Folate and Vitamin B12, Report of a joint FAO/WHO Expert Consultation. (Food and Nutrition Series, No. 23), FAO, Rome.

3. Olson, J.A. 1994. Needs and Sources of Carotenoids and Vitamin A, Nutr. Revs., 52(2II) S67-S73.

The vitamin A content of most staple diets can be significantly improved with the addition of a relatively small portion of plant foods rich in carotenoids, the precursors of vitamin A. 

For example, a usual portion of cooked carrots (50 g) added to a daily diet, or 21 g of carrots per 4.184 MJ, provides 500 μg retinol equivalents, which is the recommended nutrient density for this vitamin. The biologic activity of pro-vitamin A varies among different plant sources, and fruits and vegetables such as carrots, mango, papaya, and melon contain large amounts of nutritionally active carotenoids, (2, 3).

Vitamin A

Any of a group of substances found in very small amounts in most foods is a vitamin. A well-balanced, varied diet contains all the vitamins normally needed for health. Vitamins in excess of what the body needs do not increase health or well-being, and may actually produce illness. A poor diet cannot be corrected simply by taking vitamin concentrates.