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9.3.1.1 Vitamin A
Vitamin A was the first fat-soluble vitamin identified in 1913. Beta-carotene is converted to vitamin A in the liver. It protects the eyes in case of infections and contributes to the vision in dim light. Vitamin A combines with the protein opsin to form
rhodopsin in the retinal rod cells. When vitamin A levels are inadequate, the lack of
rhodopsin makes it difficult to see in dim light (Gräslund et al. 2008). Vitamin A is
also involved in the physiological function of the epithelia and glands, and in normal cell differentiation. Vitamin A supports bone growth and the immune functions.
Being oil-soluble, it is prescribed as retinol palmate every 6 months to pre-school
children. The prevalence of vitamin A deficiency is 33.3% in pre-school children
worldwide, while in Africa it is estimated at 44.4%. In Ethiopia, 80,000 deaths
occur due to vitamin A deficiency. In this country, vitamin A deficiency prevalence
is listed at 61% in preschool children (Abrha et al. 2016). Vitamin A deficiency is
linked to the following diseases:
Vitamin A insufficiency leads to visual impairment, xerophthalmia, Bitot’s spots
(triangular, frothy, harsh and raised patches seen on the bulbar conjunctiva), keratomalacia (softening of the thickness of the cornea), follicular hyperkeratosis,
anorexia, growth retardation, respiratory and intestinal infections, and the degeneration of the myelin sheaths. The excess of vitamin A may cause nausea, vomiting,
and anorexia. The functions of vitamin A in the body include: development and
upkeep of the epithelial tissue, support of the skin, upkeep of visual sharpness (in
diminishing light), bone development and immunity. Sources of vitamin A include:
green leafy vegetables, germination cereals and pulses, meat, fish, green mango,
papaya, pumpkin, yellow fruits and vegetables. Other sources are eggs, spinach,
cabbage, carrots, amaranth, angle liver oils, cod liver oil, liver or halibut liver oil
(Abrha et al. 2016; Gräslund et al. 2008).
9.3.1.2 Vitamin D
The best-known utility of vitamin D is to control the concentration of phosphorus
and calcium in the blood. This anti-rachitic vitamin occurs in a number of forms.
Four crystalline D vitamins are isolated and at least 10 pro-vitamins D are known.
For humans, most of the vitamin D supply comes from animal sources or synthesized following sun exposure. Vitamin D 2 is formed by exposing ergosterol to ultraviolet light. Vitamin D 3 is the natural vitamin D found in fish oil and formed in the
skin of man and animals following exposure to sunlight (Nair and Maseeh 2012). It
may be formed by the irradiation of 7-dehydrocholesterol. Vitamin D 2 and D 3 are
known as vitamers. Vitamin D enhances the absorption of phosphorus and calcium
from the intestine and their deposition in bones. Vitamin D sources include exposure to sunlight, liver, eggs, butter, cheese, fish liver oil, fortified foods, milk, and
margarine. Vitamin D also stimulates the normal mineralization of bones and
increases the tubular reabsorption of phosphate. It also possesses antioxidant
9 Vitamins and Minerals: Types, Sources and their Functions
9.3.1.1 Vitamin A
Vitamin A was the first fat-soluble vitamin identified in 1913. Beta-carotene is converted to vitamin A in the liver. It protects the eyes in case of infections and contributes to the vision in dim light. Vitamin A combines with the protein opsin to form
rhodopsin in the retinal rod cells. When vitamin A levels are inadequate, the lack of
rhodopsin makes it difficult to see in dim light (Gräslund et al. 2008). Vitamin A is
also involved in the physiological function of the epithelia and glands, and in normal cell differentiation. Vitamin A supports bone growth and the immune functions.
Being oil-soluble, it is prescribed as retinol palmate every 6 months to pre-school
children. The prevalence of vitamin A deficiency is 33.3% in pre-school children
worldwide, while in Africa it is estimated at 44.4%. In Ethiopia, 80,000 deaths
occur due to vitamin A deficiency. In this country, vitamin A deficiency prevalence
is listed at 61% in preschool children (Abrha et al. 2016). Vitamin A deficiency is
linked to the following diseases:
Vitamin A insufficiency leads to visual impairment, xerophthalmia, Bitot’s spots
(triangular, frothy, harsh and raised patches seen on the bulbar conjunctiva), keratomalacia (softening of the thickness of the cornea), follicular hyperkeratosis,
anorexia, growth retardation, respiratory and intestinal infections, and the degeneration of the myelin sheaths. The excess of vitamin A may cause nausea, vomiting,
and anorexia. The functions of vitamin A in the body include: development and
upkeep of the epithelial tissue, support of the skin, upkeep of visual sharpness (in
diminishing light), bone development and immunity. Sources of vitamin A include:
green leafy vegetables, germination cereals and pulses, meat, fish, green mango,
papaya, pumpkin, yellow fruits and vegetables. Other sources are eggs, spinach,
cabbage, carrots, amaranth, angle liver oils, cod liver oil, liver or halibut liver oil
(Abrha et al. 2016; Gräslund et al. 2008).
9.3.1.2 Vitamin D
The best-known utility of vitamin D is to control the concentration of phosphorus
and calcium in the blood. This anti-rachitic vitamin occurs in a number of forms.
Four crystalline D vitamins are isolated and at least 10 pro-vitamins D are known.
For humans, most of the vitamin D supply comes from animal sources or synthesized following sun exposure. Vitamin D 2 is formed by exposing ergosterol to ultraviolet light. Vitamin D 3 is the natural vitamin D found in fish oil and formed in the
skin of man and animals following exposure to sunlight (Nair and Maseeh 2012). It
may be formed by the irradiation of 7-dehydrocholesterol. Vitamin D 2 and D 3 are
known as vitamers. Vitamin D enhances the absorption of phosphorus and calcium
from the intestine and their deposition in bones. Vitamin D sources include exposure to sunlight, liver, eggs, butter, cheese, fish liver oil, fortified foods, milk, and
margarine. Vitamin D also stimulates the normal mineralization of bones and
increases the tubular reabsorption of phosphate. It also possesses antioxidant
9 Vitamins and Minerals: Types, Sources and their Functions