Nutrition: vitamins and minerals

Vitamins

Vitamin B

Vitamin B, often called the vitamin B complex, consists of a whole range of different compounds, some of which have similar functions and work together. However, unlike the families of compounds forming vitamins E and K, the B vitamins are sufficiently different from one another to be given individual names or numbers, and to be listed separately on many food labels. Except for vitamin B₁₂, the body can only store limited amounts of B vitamins and because they are all water-soluble, any excess is excreted in the urine. Their water-solubility also means that B vitamins are easily lost in cooking, and they can also be destroyed by light and exposure to air.

Thiamin (or thiamine, also known as vitamin B₁)

The deficiency disease beriberi has been known for thousands of years. The name literally means ‘I can't, I can't’ in Sinhalese (a major language in Sri Lanka), and reflects the crippling effect on its victims, who suffer from neurological symptoms, including pain, fatigue and paralysis, and cardiovascular disease. The disease was most common in southeast Asia, where white or ‘polished’ rice was a major part of the diet. The main source of thiamin is in the outer layers of the grain, the bran, which is removed during milling to produce white rice grain and white rice flour. Thiamin is added to white flour in the UK and many breakfast cereals are also enriched in thiamin. It is present in seeds, nuts and in beans and in smaller quantities in other foods such as meat, milk and potatoes. Since potatoes are eaten frequently in the UK diet, they can form a useful source, though thiamin is gradually destroyed by boiling water and it is estimated that 20% of the possible dietary intake is lost in cooking.

Thiamin is essential in many of the metabolic pathways in the body, especially in the processing of carbohydrate to provide energy. Since the nervous system relies almost exclusively on carbohydrate (glucose) for its energy, it is not surprising that the symptoms of deficiency are seen there. Because, as a water-soluble vitamin, little thiamin can be stored in the body, symptoms appear in less than a month on a diet in which it is completely absent. The early symptoms can, however, be rapidly corrected by regular intake of thiamin.

There are two forms of beriberi, known as the ‘wet’ and ‘dry’ forms. In ‘wet’ beriberi, there is swelling of the limbs, increased heart rate, lung congestion and an enlarged heart, all symptoms of heart failure, which can ultimately be fatal. The symptoms of ‘dry’ beriberi include pain, tingling and loss of sensation in the hands and feet, muscle wasting and gradual loss of function and paralysis of the legs, brain damage and eventually death. Nowadays, with better nutritional information and the addition of thiamin to many foods, beriberi is rarely seen, except in people with alcoholism, who mainly have the ‘dry’ form, in a condition called Wernicke-Korsakoff syndrome. Chronic alcoholism is often associated with poor nutrition and therefore a low intake of thiamin. Additionally, alcohol appears to interfere with thiamin absorption from the gut. The symptoms of Wernicke-Korsakoff syndrome begin with peripheral nerve damage (loss of feeling in hands and feet), then damage to the central nervous system and finally a confused mental state, or psychosis, which affects mood, language and thinking.

Because of its involvement in carbohydrate metabolism, additional thiamin may be needed during pregnancy, lactation and also in cancer patients and in people on kidney dialysis. It has been suggested that additional thiamin could be beneficial to performance in certain sports, but experiments so far have produced no evidence to support this idea.

Riboflavin (vitamin B₂)

Riboflavin or vitamin B₂, which was originally known as vitamin G, is found in a wide variety of foods, including milk and dairy products. It is more stable to heat than some of the other B vitamins, but is destroyed by exposure to sunlight. Milk in a glass bottle exposed to sun, loses 10% of its riboflavin per hour. Riboflavin plays a crucial role in the metabolism of carbohydrates and proteins and is involved in many other metabolic reactions in the body.

Although riboflavin deficiency does occur in some parts of the world, it is usually associated with deficiencies in other B vitamins and a specific deficiency syndrome is hard to identify. The clearest signs of deficiency are in the mouth, with cracks and inflammation at the corners, sore and ulcerated lips and a painful tongue. Others signs are detected in the eyes, with increased sensitivity to light and burning and itching sensations.

Niacin (vitamin B₃)

Niacin, which comprises two compounds, nicotinic acid and nicotinamide, also occurs widely in food and is added to many breakfast cereals. It is easily absorbed into the blood from the digestive system and plays a vital role in energy production in cells. It appears to reduce the levels of low density lipoproteins or LDLs in the blood and increase high density lipoproteins or HDLs, perhaps by affecting the proteins that carry the fats. This is important because LDLs are a way of transporting cholesterol around in the blood. Cells that need cholesterol take it up from LDLs. If cells contain excess cholesterol, it is returned to the blood packaged into HDLs. The higher the ratio of HDL to LDL in the blood, the lower the risk of developing heart disease. Thus, if niacin increases HDLs and reduces LDLs, this should give some protection against heart disease.

The deficiency disease associated with lack of niacin is pellagra. Its symptoms are the four Ds – diarrhoea, dermatitis, dementia and death, normally experienced in that order! The term ‘pellagra’ was first used in 1771 to describe the disease that was endemic at that time in poor populations in southern Europe. ‘Pellagra’ is from the Italian words pelle meaning ‘skin’ and agro meaning ‘sour’ and refers to the thickened, roughened skin, or dermatitis, which is characteristic of the disease.

It was noticed that people with pellagra subsisted on a diet that was based on maize and contained very little meat. By 1900, the disease had spread to France, Egypt and England, and in 1902 it was first reported in America. For the next 20 years, it reached epidemic proportions in the southern USA. Again, poverty and the consumption of large quantities of maize (corn) appeared to be the risk factors. Although it was soon realised that the deficiency disease could be prevented by the inclusion of meat in the diet, it was not until the late 1930s that the explanation of the link with eating a lot of maize was understood.

This link involves a molecule called tryptophan. Tryptophan is an amino acid, one of the molecules that make up proteins, and is commonly found in animal proteins. Niacin can be synthesised in the body from tryptophan. In fact, in the average UK diet, there is probably sufficient protein to satisfy all the niacin requirement of the body, and dietary niacin is therefore not essential. However, maize contains so little tryptophan that there is insufficient for the body to convert to niacin. Additionally, any niacin present in the maize itself is so tightly bound to molecules in the maize which are not digested, that it cannot be absorbed by the body. In the indigenous populations of Mexico and Central America, who also subsist on a diet of maize, there have been almost no occurrences of pellagra. It seems that their tradition of soaking the maize in an alkaline solution of lime before cooking it, releases the bound niacin, freeing it for absorption by the body. Poor peasants of the Deccan Plateau of India, however, do suffer from pellagra, although their staple carbohydrate is millet (sorghum) rather than maize. This grain contains sufficient tryptophan but it also contains very high levels of leucine, another amino acid that has been found to prevent niacin synthesis in the body, so symptoms of its deficiency occur. People with HIV infection can also suffer from a pellagra-like condition, since the infection causes the tryptophan levels in their blood to be very low. High doses of niacin can reverse the pellagra condition.

Vitamin B₆ (pyridoxine)

Vitamin B₆ is composed of pyridoxine and two closely related compounds. It is found in small quantities in many foods, though it can be destroyed in the cooking process. No clear deficiency disease has been recognised in humans as being directly caused by lack of this vitamin, since it is often found in conjunction with other B vitamins and their absence has greater effects. Its main role is in the conversion of some amino acids into other ones, depending on the requirements of the body. It also plays a role in fat metabolism (in the conversion of linoleic acid to arachidonic acid) and in carbohydrate metabolism. Thus, deficiency causes generalised problems such as anaemia, dermatitis and neuromuscular problems such as headaches, aching muscles and difficulty in walking. There is some evidence that low doses of vitamin B₆ can be helpful in improving the symptoms of premenstrual syndrome (PMS), even when there is no evidence of deficiency in the diet.

Folate (folic acid, vitamin B₉)

Folate is a generic name for a group of related compounds. The name ‘folate’ was based on the word ‘foliage’, after it was identified in a crude extract from spinach, though it is also found in liver, other green vegetables, oranges and potatoes and it is often added to breakfast cereals (usually listed as folic acid). Folate is less sensitive to heat than many of the B vitamins, though it is destroyed if food is reheated or kept hot for long periods. Folate is involved in amino acid metabolism, but its crucial role is in cell division, since it is used in DNA synthesis. So deficiency of folate has its major effect on dividing cells, especially those in the bone marrow (which produces red blood cells) and those lining the digestive system. Failure of normal cell division in the cells lining the digestive system can lead to loss of appetite, nausea and diarrhoea, and soreness in the mouth. Failure of normal cell division in the bone marrow leads to a type of anaemia called megaloblastic anaemia, where large, immature blood cells which do not have the normal oxygen-carrying capacity, are released into the circulation. After iron deficiency, folate deficiency is the next most common cause of anaemia.

Due to the huge amount of cell division that goes on in the first few months of pregnancy, pregnant women need as much as five times more folate than the normal daily requirement. Up to 25% of women would show changes in their bone marrow that are characteristic of folate deficiency if they did not increase their intake. Folate also appears to be important around the time of conception. For this reason, women planning to become pregnant are now encouraged to take folate supplements for about three months before conception and for the first three months of pregnancy. There appears to be a link between lack of folate and neural tube defects such as spina bifida, where the spinal cord does not develop correctly in the early fetus. Several studies have shown that giving folate supplements to women who have previously given birth to a child with a neural tube defect can reduce the risk of the same problem arising in a subsequent pregnancy by almost 75%.

There is some evidence that folate deficiency is also linked with increased risk of cardiovascular disease and with cancer, but more work is needed in both these areas. Alcohol affects the uptake of folate from the digestive system into the blood; so alcoholics are at risk of folate deficiency for this reason as well as because their diet may be lacking in folate. Other population groups who do not have a balanced diet, due to poverty, poor food choices, or illness, may also be at risk. Some commonly used drugs, including aspirin, indigestion remedies and the contraceptive pill, together with some antibiotics and anti-epilepsy drugs, may affect folate uptake too, and smokers may need additional folate. Chemotherapy drugs used in cancer treatment can also cause folate deficiency. In fact, folate deficiency is probably the most common vitamin deficiency seen in the developed world.

Vitamin B₁₂ (cobalamin)

Vitamin B₁₂ is yet another group of compounds, this time with an atom of the metal called cobalt (present in only trace quantities in the body) in their structure, hence the alternative name ‘cobalamin’. Vitamin B₁₂ works alongside folate and if levels of it are low, folate deficiency symptoms occur too. It is stored in the liver and in general the body does not appear to need a regular intake. Many people have enough B₁₂ stored in their liver to last for up to 30 years. Unlike most vitamins, vitamin B₁₂ is found only in foods obtained from animals. In ruminant animals such as cattle and sheep, the bacteria in their stomachs synthesise vitamin B₁₂, hence its presence in their meat, milk and dairy products. They too store it in their liver and hence eating liver is a rich source of the vitamin. Vegetarians are likely to take in sufficient vitamin B₁₂ due to contamination of their food by yeasts and bacteria, but strict vegans may need to supplement their diet to ensure sufficient intake. If the diet contains excess amounts of vitamin C, this can bind to vitamin B₁₂ and limit its availability. Vitamin B₁₂ is essential for the formation of the protective coating of myelin, which is found around some nerve fibres (neurons) and so its deficiency can lead to malfunction of the nerves and eventual paralysis and dementia. Like folate, vitamin B₁₂ is also vital for cell division, especially in the bone marrow, since it also plays a role in DNA synthesis.

Deficiency of vitamin B₁₂ due to dietary insufficiency is uncommon, but this vitamin can be deficient due to a condition that prevents its normal absorption. Because the B₁₂ molecule is particularly large, in order to be absorbed by the body it has to be linked to a protein known as ‘intrinsic factor’, which is produced by the lining of the stomach. The combined ‘complex’ is then absorbed into the blood when the food reaches the small intestine. If the cells that produce intrinsic factor are destroyed, or the intrinsic factor is inactivated, vitamin B₁₂ cannot be absorbed. This situation may occur due to an autoimmune disease in which the body produces antibodies against the cells that produce the intrinsic factor or against the intrinsic factor itself. Women are more commonly affected than men and this type of autoimmunity tends to run in families. The intrinsic factor-producing cells can also fail to function efficiently in a patient with ulcers, stomach cancer or other conditions such as Crohn's disease that affect the digestive system. The condition that results in all these cases is called pernicious anaemia, for which the treatment is regular injections of vitamin B₁₂.

Pantothenic acid (vitamin B₅)

The name ‘pantothenic acid’ is derived from the Greek pantothen which means ‘from all sides’, indicating that it is widely distributed in the diet. It plays a vital role in metabolism, particularly in the production of energy in cells. Naturally occurring pantothenic acid deficiency is very rare, since it is so widespread in the diet. However, during World War II, prisoners in the Philippines, Burma and Japan suffering from severe malnutrition did experience numbness, tingling and painful burning in their feet, which was relieved specifically by pantothenic acid. In mice and rats, pantothenic acid deficiency led to their fur turning grey, and on the basis of this finding, pantothenic acid has been added to some shampoos, in the hope that it might prevent grey hair in humans. There is as yet no evidence that it does.

Biotin (vitamin H)

Although biotin is usually considered to be a member of the B vitamin complex, it is also sometimes known as vitamin H. Like pantothenic acid, it plays a major role in metabolism. Deficiency is not normally seen, though it can be induced in rats and people by them eating large quantities of raw egg white. This binds biotin and prevents it being absorbed. Various symptoms result, including hair loss, dermatitis, depression and lethargy.