Ross & Wilson Anatomy and Physiology in Health and Illness (126 page)

Vitamin D

Vitamin D is found mainly in animal fats such as eggs, butter, cheese, fish liver oils. Humans can synthesise vitamin D by the action of the ultraviolet rays in sunlight on a form of cholesterol (7-dehydrocholesterol) in the skin (see
p. 358
).

Vitamin D regulates calcium and phosphate metabolism by increasing their absorption from the gut and stimulating their retention by the kidneys. It therefore promotes the calcification of bones and teeth.

Deficiency causes
rickets
in children and
osteomalacia
in adults (
p. 421
), due to impaired absorption and use of calcium and phosphate. The daily requirement is 10 μg and stores in fat and muscle are such that deficiency may not be apparent for several years.

Vitamin E

This is a group of eight substances called
tocopherols
. They are found in nuts, egg yolk, wheat germ, whole cereal, milk and butter.

Vitamin E is an antioxidant, which means that it protects body constituents such as membrane lipids from being destroyed in oxidative reactions. Recently, vitamin E has been shown to protect against coronary heart disease. Recommended daily intake is 10 mg for men and 8 mg for women, but this should be increased in high-fat diets.

Deficiency is rare, because this vitamin is present in many foods, and is usually seen only in premature babies and in conditions associated with impaired fat absorption, e.g. cystic fibrosis. Haemolytic anaemia (
p. 67
) occurs, as abnormal red blood cell membranes rupture. White blood cells can likewise be affected, and vitamin E supplements boost immune function. Neurological abnormalities such as ataxia and visual disturbances may occur if the deficiency is severe.

Vitamin K

The sources of vitamin K are liver, some vegetable oils and leafy green vegetables. It is synthesised in the large intestine by microbes and significant amounts are absorbed. The normal daily requirement is 1 μg/kg body weight and only a small amount is stored in the liver.

Vitamin K is required by the liver for the production of prothrombin and factors VII, IX and X, all essential for the clotting of blood (
p. 64
). Deficiency therefore prevents normal blood coagulation. It may occur in adults when there is obstruction to the flow of bile, severe liver damage and in malabsorption, e.g. in coeliac disease. Newborn infants may be given vitamin K because their intestines are sterile and require several weeks to become colonised with vitamin K-producing bacteria.

Water-soluble vitamins

Vitamin B complex

This is a group of water-soluble vitamins that promote activity of enzymes involved in the chemical breakdown (catabolism) of nutrients to release energy.

Vitamin B
₁
(thiamin)

This vitamin is present in nuts, yeast, egg yolk, liver, legumes, meat and the germ of cereals. It is rapidly destroyed by heat. The daily requirement is 0.8 to 1 mg and the body stores only about 30 mg. Thiamin is essential for the complete aerobic release of energy from carbohydrate. When it is absent there is accumulation of lactic and pyruvic acids, which may lead to accumulation of tissue fluid (oedema) and heart failure. Thiamin is also important for nervous system function because of the dependency of these tissues on glucose for fuel.

Deficiency causes
beriberi
, which occurs mainly in countries where polished rice is the chief constituent of the diet. In beriberi there is:

•
severe muscle wasting

•
delayed growth in children

•
polyneuritis, causing degeneration of motor, sensory and some autonomic nerves

•
susceptibility to infections.

If untreated, death occurs owing to cardiac failure or severe microbial infection.

The main cause of thiamin deficiency in developed countries is alcoholism, where the diet is usually poor. Neurological symptoms, which are usually irreversible, include memory loss, ataxia and visual disturbances.

Vitamin B
₂
(riboflavin)

Riboflavin is found in yeast, green vegetables, milk, liver, eggs, cheese and fish roe. The daily requirement is 1.1 to 1.3 mg; only small amounts are stored in the body and it is destroyed by light and alkalis. It is concerned with carbohydrate and protein metabolism, especially in the eyes and skin. Deficiency leads to cracking of the skin, commonly around the mouth (angular stomatitis), and inflammation of the tongue (glossitis).

Vitamin B
₃
(niacin)

This is found in liver, cheese, yeast, whole cereals, eggs, fish and nuts; in addition, the body can synthesise it from the amino acid tryptophan. It is associated with energy-releasing reactions in cells. In fat metabolism it inhibits the production of cholesterol and assists in fat breakdown. Deficiency occurs mainly in areas where maize is the chief constituent of the diet because niacin in maize is in an unusable form. The daily requirement is 12 to 17 mg and it is fairly stable.

Pellagra
develops within 6 to 8 weeks of severe deficiency. It is characterised by:

•
dermatitis – redness of the skin in parts exposed to light, especially the neck

•
anorexia, nausea, dysphagia and inflammation of the oral mucosa

•
delirium, mental disturbance and dementia.

Vitamin B
₆
(pyridoxine)

This stable vitamin is found in egg yolk, peas, beans, soya beans, yeast, meat and liver. The daily requirement is about 1.2 to 1.4 mg and dietary deficiency is rare, although alcohol and antituberculous drugs antagonise the vitamin and can induce deficiency states. Vitamin B
₆
is associated with amino acid metabolism, including the synthesis of non-essential amino acids and molecules such as haem and nucleic acids.

Vitamin B
₁₂
(cobalamin)

Vitamin B
₁₂
consists of a number of
cobalamin compounds
(containing cobalt). It is found in liver, meat, eggs, milk and fermented liquors, and is destroyed by heat. The normal daily requirement is 1.5 μg.

Like folic acid, vitamin B
₁₂
is essential for DNA synthesis, and deficiency also leads to a megaloblastic anaemia, which is correctable with supplements. However, vitamin B
₁₂
is also required for formation and maintenance of myelin, the fatty substance that surrounds and protects some nerves. Deficiency accordingly causes irreversible damage such as peripheral neuropathy and/or subacute spinal cord degeneration. The presence of intrinsic factor in the stomach is essential for vitamin B
₁₂
absorption, and deficiency is usually associated with insufficient intrinsic factor.

Folic acid (folate)

This is found in liver, kidney, fresh leafy green vegetables and yeast. It is synthesised by bacteria in the large intestine, and significant amounts derived from this source are believed to be absorbed. It is destroyed by heat and moisture. The daily requirement is 200 μg, and, as only a small amount is stored in the body, deficiency is evident within a short time. It is essential for DNA synthesis, and when lacking mitosis (cell division) is impaired. This manifests particularly in rapidly dividing tissues such as blood, and folate deficiency therefore leads to a type of megaloblastic anaemia (
p. 67
), which is reversible with folate supplements. Deficiency at conception and during early pregnancy is linked to an increased incidence of spina bifida (
p. 182
).

Pantothenic acid

This is found in many foods and is associated with amino acid metabolism. The daily safe intake is 3 to 7 mg and no deficiency diseases have been identified. It is destroyed by excessive heat and freezing.

Biotin

This is found in a wide range of foods including yeast, egg yolk, liver, kidney and tomatoes and is synthesised by microbes in the intestine. It is associated with the metabolism of carbohydrates, lipids and some amino acids. The daily safe intake is 10 to 20 μg, deficiency is rare and it is a stable compound.

Vitamin C (ascorbic acid)

This is found in fresh fruit, especially blackcurrants, oranges, grapefruit and lemons, and also in rosehips and green vegetables. The vitamin is very water soluble and is easily destroyed by heat, ageing, chopping, salting and drying. These processes may predispose to the development of
scurvy
(deficiency). The daily requirement is 40 mg and after 2 to 3 months deficiency becomes apparent.

Vitamin C is associated with protein metabolism, especially the laying down of collagen fibres in connective tissue. Vitamin C, like vitamin E, acts as an antioxidant, protecting body molecules from damaging oxidative reactions. When scurvy occurs, collagen production is affected, leading to fragility of blood vessels, delayed wound healing and poor bone repair. Gums become swollen and spongy and the teeth loosen in their sockets.

Minerals, trace elements and water

Learning outcomes

After studying this section, you should be able to:

list the commonest mineral salts required by the body

describe their functions

explain the distribution of water within the body

describe the functions of water within the body.