BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.7. Human Nutrition

8.7.9. Vitamins

Vitamins are Organic compounds required by healthy individuals in small amounts. Since they cannot be synthesized by the body, they must be obtained from the diet.

A deficiency in a particular vitamin leads to a range of symptoms known as deficiency diseases. Table 8.6 lists some of the sources and Functions of the major vitamins, as well as the deficiency diseases caused by their lack. Vitamins A, D, E, and K are classified as fat-soluble, while the remaining ones are Water-soluble.

Class="center">Table 8.6. Sources and functions of essential human vitamins, and deficiency diseases arising from their lack

Vitamin A (retinol)

The proper chemical name for vitamin A is retinol. It is found in animal-derived products. The orange pigment carotene found in carrots, along with similar pigments called carotenes that are common in plants, can be converted into vitamin A during Digestion. The structures of carotenes and vitamin A are exceptionally well adapted for Light absorption—in plants as carotenes, and in animals, where vitamin A is converted into the light-absorbing molecule retinal. Three animal groups possessing eyes (Mollusks, Arthropods, and vertebrates) use retinal as the light-absorbing moiety in photoreceptor molecules. Light triggers substantial structural changes in retinal, sufficient to generate a Nerve Impulse.

In addition, vitamin A plays a crucial role in maintaining healthy skin and other epithelial (surface) tissues, and young children require it for normal growth.

Vitamin A deficiency (HYPOVITAMINOSIS A). A shortage of vitamin A impairs dark adaptation (in other words, it disrupts the function of rod cells sensitive to light intensity). Initially, a condition known as night blindness develops, where an individual cannot see in dim light. This condition arises from a shortage of retinal in the rods. Ultimately, the rod cells degenerate. Concurrently, dryness of the conjunctiva and cornea occurs (xerophthalmia; from the Greek xerós meaning dry, and ophthalmós meaning eye), impairing their functions. Ulcers appear on the cornea (keratomalacia), which can result in blindness.

Vitamin A deficiency in children stunts their growth. Prolonged deficiency of this vitamin can be fatal. This situation remains typical in some developing countries, where vitamin A deficiency frequently leads to childhood blindness. Approximately 3 million children under the age of 10 are blind worldwide for this exact reason. In humans, vitamin A is stored in the liver, which can maintain roughly a two-year supply under A balanced diet. British dietitians recommend that the average daily intake of this vitamin should be double the body's actual requirements.

VITAMIN A HYPERVITAMINOSIS. Rare cases of toxicity are known to result from excessive vitamin A intake, typically caused by the long-term use of vitamin supplements. Hypervitaminosis A manifests as brittle bones, Hair loss, double vision, vomiting, and other complications. High doses of vitamin A (exceeding 3,300 µg per day) during pregnancy can lead to congenital birth defects. In the United Kingdom, pregnant women are advised against taking vitamin supplements containing vitamin A without consulting a physician.

Regular daily intake should not exceed 6,000 µg for adolescents, 7,500 µg for adult women, and 9,000 µg for adult men.

Vitamin D (calciferol)

For most people, The amount of vitamin D synthesized in the skin under the influence of sunlight is sufficient. Light-absorbing molecules found in the skin are synthesized from cholesterol. The active range of the spectrum affecting humans consists of ultraviolet (UV) rays. In Britain, for example, from late October to late March, there is virtually no UV radiation of the appropriate wavelength (280–310 nm). However, reserves of vitamin D synthesized during the summer months and stored in the liver typically last through the remainder of the year.

Most of the foods we consume contain little vitamin D, with the exception of oily fish (mackerel, sardines, herring), fish liver oil, and egg yolks. Today, vitamin D is added to certain food products, such as margarine (where its addition is mandated by law) and breakfast cereal blends.

Through a series of reactions, vitamin D is converted first in The Liver and then in the kidneys (or the Placenta in pregnant women) into its active form, which stimulates the intestinal absorption of calcium and phosphates. The synthesis of active vitamin D is closely linked to blood calcium levels and increases sharply when calcium drops below a certain threshold. Active vitamin D also regulates the deposition of calcium and phosphates in Bone tissue and their resorption from bones. In healthy adults, Bone Formation and resorption are normally balanced, keeping bone mass constant. This continuous bone remodeling allows the strength and shape of bones to adapt to various mechanical stresses.

During pregnancy and Lactation, the demand for calcium increases significantly to support the growth of the fetus. The vitamin D content in breast milk is relatively low; therefore, if sunlight exposure is limited, additional sunbathing is recommended.

Vitamin D deficiency. Vitamin D deficiency is extremely dangerous during childhood because this period involves particularly rapid skeletal growth. For this reason, adding this vitamin to the diet of children under the age of 4 is recommended. The deficiency disease, rickets, develops due to inadequate delivery of calcium and phosphates to the bones. This makes bones too weak and soft to support body weight, resulting in bowed legs and spinal curvature.

In adults, vitamin D deficiency leads to a condition known as osteomalacia (from osteon meaning bone, and malakia meaning softness). In this condition, calcium and phosphates are leached from the bones, reducing their structural strength. The risk of this disease is significantly higher for individuals with limited sunlight exposure. In the United Kingdom, There are two primary risk groups: the elderly and the Asian community. Many elderly individuals lead sedentary lifestyles and spend most of their time indoors. Due to cultural dietary traditions, the diets of these populations are often low in calcium and vitamin D. Furthermore, Asian women wearing traditional clothing that covers most of their bodies receive very little sunlight. Additionally, darker skin filters out UV radiation more effectively than pale skin. Asian children are particularly at high risk. In Britain, rickets is most frequently observed in certain impoverished inner-city communities, particularly those of Asian descent.

VITAMIN D HYPERVITAMINOSIS. An excess of vitamin D in the body leads to excessive calcium uptake. If this calcium is not excreted in the urine, it can deposit in the kidneys, posing a severe health hazard. This is significantly more dangerous for children than for adults.