Nutrition (13 page)
Itchy rash and diarrhoea have been reported at high doses.
Biotin
Biotin is a water-soluble B-group vitamin. It is an essential cofactor for acetyl-coenzyme A and related compounds, and for carboxy lase enzymes involved in the synthesis and metabolism of fatty acids, glucose and amino acids. Biotin may also play a role in gene expression, stimulate insulin secretion from pancreatic beta cells and promote the activity of glucose receptors (Glut-4) which transport glucose into muscle and fat cells when insulin is present. Biotin is also important for healthy hair, skin, nails and sweat glands.
Dietary sources of biotin
These include liver; meat; oily fish; egg yolk and yeast extract. Biotin is also produced by probiotic bacteria in yogurt and in your own bowel, from which you can absorb useful amounts.
Deficiency
As biotin is widely distributed in food, and is also made by bacteria in the gut, from which it can be absorbed, dietary deficiency is rare. It can occur in those taking long-term antibiotics (e.g. for acne), although probiotic supplements will help to overcome this effect. Biotin deficiency can also occur in bodybuilders who eat large amounts of raw egg white over a long period. Raw (but not cooked) egg white contains the protein avidin, that binds to biotin and prevents its absorption.
Lack of biotin can cause dry, scaly, flaky skin, a rash around the nose and mouth, brittle hair and nails, patchy hair loss, tiredness and loss of appetite. Around one in 120 people have an inherited, inborn error of biotin metabolism. This is believed to affect their immunity against yeast infections and may be linked with recurrent candida infections.
Biotin is non-toxic as any excess is excreted in the urine.
Folate
Folates are a group of water-soluble compounds that are sometimes referred to as vitamin B9. Their name is derived from the Latin word
folium
meaning ‘leaf’, as this is where the first folate was identified. Folates containing more than one glutamate are absorbed and used less efficiently in the body than the synthetic monoglutamate form known as folic acid.
folium
meaning ‘leaf’, as this is where the first folate was identified. Folates containing more than one glutamate are absorbed and used less efficiently in the body than the synthetic monoglutamate form known as folic acid.
Folic acid is involved in the synthesis and metabolism of proteins, sugar and nucleic acids. Together with vitamin B12, it is especially important for cells that are dividing rapidly. When folic acid is in short supply, dividing cells become unusually large and, when red blood cells are affected, anaemia results.
Folic acid is essential during the first few weeks of a baby’s development in the womb. Deficiency can trigger a type of developmental abnormality known as a neural tube defect (for example, spina bifida), which arises between the twenty-fourth and twenty-eighth day after conception.
Folate is also needed to process an amino acid, homocysteine, raised levels of which are associated with atherosclerosis. Two of the three enzymes that control homocysteine levels depend on folate for their activity.
Dietary sources of folate
Folate-rich food sources include green leafy vegetables, e.g. spinach, broccoli, Brussels sprouts, parsley; wholegrains; beans and soy products; liver and kidney; citrus fruit; nuts; dairy products; eggs and yeast extract.
Foods originally rich in folate typically retain less than a third of their folate content after processing and cooking. Folate is destroyed by prolonged contact with light and air but can be protected by antioxidant vitamin C. Up to 90 per cent of folate content of grain is lost during milling, 10 per cent of folate in vegetables is lost by steaming, 20 per cent by pressure-cooking and up to 50 per cent by boiling.
Women planning for a baby are advised to take daily supplements containing 400 mcg folic acid from before the time of conception and during at least the first 12 weeks of pregnancy. Considerably higher doses of 4 mg per day are advised for women with a personal or family history of conceiving a child with a neural tube defect. Folic acid supplements are generally considered safe, but long-term use of high doses can mask vitamin B12 deficiency, as it prevents the occurrence of red blood cell changes that allow a lack of vitamin B12 to be detected. Therefore, the upper safe level for long-term use from supplements is suggested as 1,000 mcg (1 mg) daily, except for certain women, during pregnancy.
Deficiency
The body stores very little folic acid, and dietary lack rapidly causes deficiency – it is cited as the most widespread vitamin deficiency in developed countries. Lack of folic acid results in reduced DNA synthesis and impaired cell division, which is usually detected due to the resulting macrocytic (large-cell) anaemia. Symptoms of folic acid deficiency include a red, sore tongue, tiredness, exhaustion, cracking at the corners of the mouth and weakness. Vitamin B12 deficiency can be masked by taking folate supplements, so these two supplements are usually given together.
Vitamin C
Vitamin C, or ascorbic acid, is a water-soluble vitamin that cannot be stored in the body in appreciable amounts. Its chemical structure is related to glucose, but unlike most animals, which can synthesize vitamin C from glucose and galactose, humans and other primates lack the enzyme (L-gulonolactone oxidase) needed to make their own supplies.
MEMBERS OF THE EXCLUSIVE VITAMIN-C-FREE CLUB
The only animals unable to synthesize vitamin C are:
• man and other primates
• guinea pigs
• Indian fruit bat
• red-vented bulbul (an Asian songbird)
• rainbow trout
• Coho salmon
• a single strain of Japanese laboratory rat
The goat, for example, which weighs around the same as a man, produces between 2 g and 13 g of vitamin C per day depending on its levels of stress and illness.
Vitamin C is an important antioxidant in all body tissues, especially the eye lens. It also regenerates other antioxidants, such as vitamin E, which initially become free radicals themselves after performing their antioxidant function. The highest concentrations are found in the adrenal glands, pituitary gland and the retina of the eyes. Vitamin C acts as an essential cofactor for at least 300 metabolic reactions which are promoted when large quantities of vitamin C are available. It is essential for conversion of the amino acid proline to hydroxyproline during collagen synthesis. It is also involved in the metabolism of stress hormones and the regulation of immune reactions.
The presence of vitamin C increases the absorption of iron in the gut by converting ferric iron (Fe
3+
) to ferrous iron (Fe
2+
) – for more details of this process, see the next chapter on minerals.
3+
) to ferrous iron (Fe
2+
) – for more details of this process, see the next chapter on minerals.
Dietary sources of vitamin C
Vitamin C is mainly found in fruit and vegetables such as blackcurrants, berries, guava, kiwi fruit, citrus fruit, mango, capsicum peppers, green leafy vegetables and potatoes. Animal sources include kidney and liver.
Vitamin C is one of the most unstable vitamins, and up to two-thirds is lost through processing, prolonged cooking and storage. Fruit juices rapidly lose their vitamin C content when exposed to air, even if chilled – virtually all is lost within 14 days. Cooking soft fruits removes two-thirds of their vitamin C content; when boiled, vegetables lose up to 50 per cent of vitamin C into the cooking water and during storage; and root vegetables lose around 10 per cent of vitamin C content per month. Some vegetables, such as asparagus, lose 90 per cent of their vitamin C content after just one week.
The EU RDA for vitamin C was recently increased from 60 mg to 80 mg, based on new understandings of average metabolic needs. In the US, the recommended intake is higher for adult males (90 mg) but slightly lower for women (75 mg). Smokers are advised to obtain 35 mg per day more, due to the oxidative damage caused by toxins in cigarette smoke, which lower blood levels of vitamin C.
Deficiency
The absorption and metabolism of vitamin C varies depending on the amount consumed. At intakes of up to 200 mg as a single dose, absorption of vitamin C is almost complete through an active transport process. At single doses of over 500 mg, it is also absorbed through a process of diffusion, which is less efficient so that only around half of a 1.5 g dose is absorbed and only 16 per cent of a massive 12 g dose.
Mild deficiency of vitamin C is relatively common, and is associated with non-specific symptoms (sometimes known as pre-scurvy syndrome) such as frequent colds and other infections, lack of energy, weakness and muscle and joint pain.
More severe lack of vitamin C leads to a deficiency disease known as scurvy. A minimum daily intake of 10 mg vitamin C is needed to prevent this, although 20 mg per day is needed for normal wound healing. In scurvy, reduced conversion of the amino acid proline to hydroxyproline (an important component of collagen) results in poor wound healing; dry, rough, scaly skin; broken thread veins in skin around hair follicles; easy bruising; scalp dryness; misshapen, tangled, corkscrew, brittle hair; hair loss; dry, fissured lips; inflamed, spongy, bleeding gums and loose teeth; bleeding skin, eyes and nose and muscle weakness.
Toxicity
The safety of vitamin C supplementation has been researched and established over a long period of time. Some vitamin C is metabolized to oxalic acid, but claims that large doses could trigger oxalate kidney stones have proved unfounded. However, those known to be recurrent stone-formers who have a defect in ascorbic acid or oxalate metabolism and people with renal failure should restrict daily vitamin C intakes to approximately 100 mg.
Large doses may trigger indigestion or diarrhoea. This is largely due to the acidity of vitamin C rather than a specific sign of toxicity, and some people are more sensitive to the acidity of vitamin C than others.
WARNINGS
Individuals with the iron-storage disease haemochromatosis should not take vitamin C supplements except under medical advice.
Suddenly stopping high-dose supplements can cause temporary symptoms of vitamin C deficiency (rebound scurvy), when enzymes activated by high levels of vitamin C are suddenly deprived of the extra vitamin C they need to work properly.
Vitamin D
Vitamin D, or calciferol, is the collective term for five different fat-soluble vitamins, of which the most important are vitamin D2 (ergocalciferol, derived from plant foods), and vitamin D3 (cholecalciferol, derived from animal foods). Dietary vitamin D3 is significantly more effective in maintaining blood vitamin D levels than vitamin D2.
MAKING IT ON YOUR OWN
Vitamin D3 can be synthesized photochemically in your skin from a reaction between 7-dehydrocholesterol and UVB ultraviolet sunlight (290 nm to 315 nm spectrum). This only occurs when the UV index is greater than 3 which, in the UK, is achieved during spring and summer. You cannot produce enough vitamin D to meet your needs during autumn and winter, when your blood levels of vitamin D naturally tend to fall. Used properly, a sunscreen with a sun protection factor of SPF8 reduces vitamin D production in the skin by 95 per cent, while SPF15 reduces vitamin D production by 99 per cent. The development of a tan suggests enough UVB radiation has struck the skin to stimulate production of melanin pigment and some vitamin D, regardless of sunscreen use.
Cholecalciferol is classed as a prohormone, as your liver and kidneys convert it into the hormone calcitriol (1,25-dihydroxy-cholecalciferol). Calcitriol regulates calcium and phosphate metabolism via the kidneys, small intestines and bone by increasing calcium and phosphorus absorption in the small intestines, regulating calcium reabsorption in the kidneys as well as the secretion of parathyroid hormone, which releases calcium from bones.
In addition, vitamin D regulates the balance between production and breakdown of bone and joint connective tissues such as collagen and elastin during bone growth and remodelling. It stimulates the production of proteoglycans (cartilage building blocks) within joints and stimulates immune cells that fight infection and target cancer cells.
Recent research also suggests that vitamin D plays a direct role in learning, memory and mood – especially in older people – as well as reducing calcium deposition in arteries to reduce hardening of the arteries (arteriosclerosis).
Dietary sources of vitamin D
Vitamin D is mainly obtained from eating: oily fish (sardine, herring, mackerel, salmon, tuna); fish-liver oils; liver; eggs; fortified milk; fortified margarine and butter.
The EU RDA for vitamin D is 5 mcg. People over the age of 50 usually need at least 10 mcg per day as blood levels fall with increasing age and synthesis in the skin becomes less efficient. The upper safe level for long-term use from supplements is suggested as 25 mcg per day in the UK, although this is generally considered too low. An upper safe level of 50 mcg has been suggested by the US Food and Nutrition Board and the European Com mission Scientific Committee on Food, while an upper safe level of 80 mcg per day has been set in Australia and New Zealand.
Vitamin D amounts are sometimes expressed as a measure of activity (International Units, or IU) rather than as a measure of weight (micrograms) as different molecules contribute to vitamin D status in the body. Each 1 mcg of vitamin D is equivalent to 40 IU. The EU RDA of 5 mcg is therefore equivalent to 200 IU per day.
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