Nutrition (12 page)

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Authors: Sarah Brewer

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BOOK: Nutrition
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Boiling reduces the B1 content of foods by half as it leaches into water. It is also destroyed by high temperatures; roasting meat at 200 ºC lowers its vitamin B1 content by 20 per cent, while freezing meats reduces their B1 content by up to 50 per cent.
Deficiency
When B1 is lacking, pyruvate accumulates and is converted to lactic acid via anaerobic glycolysis as discussed in
Chapter 3
. As a result nerve, skeletal and heart muscle cells do not obtain all the energy they need, and their function is reduced.
Thiamin deficiency can develop rapidly, and is common in people with a high alcohol intake (which increases thiamin need) and in parts of the world where the main dietary staple is polished rather than brown rice. Lack of thiamin causes a disease known as
beriberi,
meaning ‘extreme weakness’, which affects the cardiovascular and nervous systems. Dry beriberi produces heaviness, weakness, numbness and pins and needles in the legs, while wet beriberi causes severe fluid retention. Severe deficiency is associated with a form of dementia, called Wernicke-Korsakoff syndrome, which leads to confusion, unstable gait and even coma if left untreated.
As vitamin B1 is water-soluble, and readily excreted in the urine, toxicity is not an issue.
Vitamin B2
Vitamin B2, or riboflavin, is a water-soluble vitamin that cannot be stored in the body, so a regular dietary intake is essential. B2 plays a crucial role in the production of energy and the metabolism of proteins, fats and carbohydrate during the citric acid cycle. It acts as a building block for the production of flavin adenine dinucleotide (FAD), which carries hydrogen and electrons to the respiratory chain to produce energy (see
Chapter 3
).
Vitamin B2 plays a key role in growth and the production of glycogen, red blood cells and steroid hormones. It helps insulin-secreting cells in the pancreas to detect the presence of glucose, and is also involved in immunity and the production of antibodies. Other roles include helping to maintain the integrity of your hair, skin, mucous membranes and nails.
Dietary sources of vitamin B2
As it is essential for cell function, vitamin B2 is found in all animal and cell-based foods, especially: yeast extract; liver; lean meats; wholegrain cereals; dairy products; eggs; green leafy vegetables and pulses.
Vitamin B2 is readily lost from vegetables into cooking water, colouring it yellow. Pasteurization of milk loses 20 per cent of its B2 content, which is further reduced by 90 per cent after two hours of sun exposure – buy milk in cartons rather than bottles. Boiling milk also reduces its vitamin B2 content by up to 25 per cent. In addition, freezing meat reduces B2 content by up to 50 per cent.
Deficiency
Severe lack of vitamin B2 (ariboflavinosis) is rare in Western countries, but is occasionally seen in vulnerable groups such as the elderly and those with anorexia or alcohol dependency. Symptoms include a scaly, eczema-like skin rash (seborrhoeic dermatitis), especially on the face and nose, sensitivity to bright light, itching, dizziness, insomnia, slow learning, weakness, sore and swollen throat and tongue, cracking and sores at the corners of the mouth (angular cheilosis), bloodshot corneas and anaemia.
Vitamin B2 is relatively non-toxic as excess is readily excreted in the urine.
Vitamin B3
Vitamin B3, or niacin, occurs in two main forms, both of which are water-soluble: nicotinic acid and nicotinamide, which is the active form. Small amounts of B3 can be made in the body from the essential amino acid tryptophan (60 mg tryptophan produces 1 mg niacin). Because of this, the vitamin B3 content of foods and supplements is sometimes given as ‘niacin equivalents’, which are equal to the amount of nicotinamide and nicotinic acid they contain plus one-sixtieth of their tryptophan content.
B3 plays a crucial role in the production of energy and the metabolism of proteins, fats and carbohydrates during the citric acid cycle. It acts as a building block for the production of nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucelotide phosphate (NADP). These act as coenzymes, which activate over 200 dehydrogenase enzymes involved in energy production and electron transport in cells as discussed in
Chapter 3
. NADP is also involved in the synthesis of fatty acids, steroid hormones and a sugar, called ribose, which forms part of your genetic material (DNA). People who are physically active therefore need more niacin than sedentary people.
Vitamin B3 regulates the production of triglycerides and ‘good’ HDL-cholesterol in your liver. It also combines with the mineral chromium to form a substance that is commonly known as Glucose Tolerance Factor (GTF), as it increases insulin binding and glucose uptake into cells. As a B vitamin, its other important roles include helping to maintain healthy skin and nerve function.
Dietary sources of vitamin B3
Dietary sources of B3 include: wholegrains; meat and poultry; oily fish; dairy products; nuts; dried fruit and yeast extract.
Eggs and cheese are among the richest dietary sources of tryptophan.
Deficiency
Lack of vitamin B3 produces a rare deficiency disease known as pellagra, meaning raw skin, which classically produces symptoms of dermatitis (in sun-exposed areas of the body), diarrhoea and dementia plus fatigue, sleeplessness, depression, memory loss and visual impairment. This is mainly seen in parts of Africa where the diet contains large quantities of maize, whose niacin is in a non-usable form of niacytin. In Central America, however, maize for cooking tortillas is soaked overnight in calcium hydroxide, which releases the niacin content.
Toxicity
High doses of B3, especially in the form of nicotinic acid, can produce facial flushing. Very high intakes can cause thickening and darkening of skin (acanthosis nigricans), palpitations, peptic ulceration, gout and hepatitis.
Vitamin B5
Vitamin B5, or pantothenic acid, is a water-soluble vitamin that is a vital component of coenzyme A, which is involved in the citric acid cycle as acetyl-coenzyme A and succinyl-coenzyme A. Vitamin B5 is therefore vital for many energy-producing reactions in the body involving carbohydrates, fats and proteins. It is needed to synthesize glucose, fatty acids and adrenal hormones. It also stimulates cell growth in healing tissues, increasing the number and speed of cells moving into wounds and promoting stronger scar-tissue formation.
Dietary sources of vitamin B5
Rich sources include: poultry, meat, offal; wholegrains; eggs, especially fish roes; vegetables, especially broccoli, potatoes and tomatoes; pulses and yeast extract.
Despite its wide distribution, vitamin B5 is easily destroyed by food processing. Cooking destroys up to 50 per cent of B5 present in meats, and an additional 30 per cent is lost into cooking juices. Up to 75 per cent of B5 in vegetables is lost during processing and freezing.
Deficiency
Vitamin B5 deficiency is very rare, but can include irritability, restlessness, fatigue, apathy, malaise, sleep disturbance, nausea, vomiting, abdominal cramps and symptoms of the nervous system such as numbness, tingling sensations, muscle cramps, staggering gait and burning sensations in the feet. Low blood-glucose levels and an increased sensitivity to insulin have also been recorded.
Vitamin B6
Vitamin B6 consists of three water-soluble compounds: pyridoxine, pyridoxal and pyridoxamine that are converted into the most active form, pyridoxine, within the liver. Small amounts are stored in the liver, muscle tissue and brain.
Pyridoxine is an essential cofactor for the action of over 60 enzymes involved in the synthesis of genetic material, amino acids, proteins and the metabolism of carbohydrates and essential fatty acids. It is especially needed by rapidly dividing cells such as those found in the gut, skin, hair follicles and marrow. It is involved in the production of antibodies, hydrochloric acid in the stomach, the synthesis of brain neurotransmitters and in the sodium–potassium balance. It can affect mental processing and mood, and modify the action of sex hormones. It is also necessary for breaking down homocysteine – an amino acid that can damage blood-vessel walls to hasten the hardening and furring-up of the arteries (atherosclerosis). Vitamin B6 requires the presence of vitamin B2, zinc and magnesium as cofactors to fulfil all its physiological functions.
Dietary sources of vitamin B6
Rich sources of vitamin B6 include: meat, especially liver; oily fish; egg yolk; wholegrain cereals, especially brown rice; soy products; nuts, especially peanuts and walnuts; green, leafy vegetables and yeast extract.
Up to 70 per cent of vitamin B6 in meat is lost during processing and cooking. Sterilization reduces pyridoxine in milk by 20 per cent, canning lowers vegetable pyridoxine by 20 per cent and a further 40 per cent is lost into water when frozen vegetables are thawed and cooked.
Deficiency
Lack of vitamin B6 is uncommon but has been associated with depression, irritability, a greasy rash on the forehead and around the nose plus cracking of the lips and tongue. Deficiency has also been linked with carpal tunnel syndrome, and symptoms similar to those occurring in premenstrual syndrome (PMS) such as anxiety, irritability, mild depression, bloating and tender breasts. Although this is controversial, supplements are sometimes recommended for women experiencing debilitating PMS symptoms.
Toxicity
High doses of vitamin B6 may cause headache, acne, skin reactions, nausea, abdominal pain, loss of appetite and can cause abnormal results in liver-function tests. There have been some suggestions that prolonged high doses, of above 10 mg daily, may cause reversible nerve symptoms such as pins and needles, but this is controversial. The risks associated with taking vitamin B6 at doses between 10 mg and 200 mg long term are unclear, but are probably low.
Vitamin B12
Vitamin B12, also known as cobalamin, is a micronutrient that contains cobalt – the only known requirement for this metal in human physiology. Although water-soluble, you can store enough in your liver to last several years.
Vitamin B12 acts as a cofactor for enzymes involved in fatty-acid and amino-acid metabolism and the processing of homocysteine – a potentially harmful amino acid linked with atherosclerosis. It is essential when genetic material (DNA) is synthesized during cell division, and if in short supply, newly formed cells are larger than usual. When this affects red blood cells, pernicious anaemia results. When it occurs during pregnancy, developmental abnormalities such as spina bifida are possible. Pregnant women have recently been advised to take vitamin B12 supplements together with their folic acid. Other key roles for vitamin B12 include helping to maintain normal nerve function, immunity and healing.
Dietary sources of vitamin B12
B12 is only found in consistent amounts in animal-based foods such as: liver and kidney; meat and poultry; oily fish, especially sardines; eggs and dairy products.
Vegetarian sources include blue-green algae, extracts from bacterial cultures and synthetic B12 in fortified breakfast cereals. Around 20 per cent of the B12 in food leaches out into juices during cooking.
Deficiency
Vitamin B12 is absorbed in the lower part of the small intestine, but only if a carrier protein, gastric intrinsic factor, is present (which is made by cells in the stomach lining). Vitamin B12 deficiency becomes increasingly common with age due to loss of the cells that produce intrinsic factor, and/or reduced absorption in the small intestine because of bowel disease. People with autoimmune conditions such as Type 1 diabetes and thyroid disease are especially likely to develop B12 deficiency, due to autoimmune loss of intrinsic-factor function. B12 absorption also depends on the presence of calcium.
Lack of vitamin B12 causes the production of cells that are larger than they should be. In the case of red blood cells, this leads to a particular form of anaemia known as pernicious anaemia. As this develops slowly and gradually, the symptoms, including paleness (often with a lemon tinge to the skin), tiredness, weakness and shortness of breath, are often not recognized until the anaemia is advanced.
Vitamin B12 is also needed for healthy nerve function and if not recognized and treated, deficiency can damage nerves in the spinal cord, leading to a rare condition known as subacute combined degeneration of the spinal cord. Other symptoms that may be caused by a vitamin B12 deficiency include a smooth, sore tongue, tiredness, exhaustion, menstrual disorders, numbness, tingling, trembling, clumsiness, difficulty walking (especially in the dark when you can’t see where you are going), poor memory, lack of concentration, confusion and depression.
Because the symptoms are so variable, vitamin B12 deficiency should be considered in all spinal-cord, nerve and psychiatric disorders. Many people who are lacking in B12 do not develop obvious symptoms for several years, however, and they tend to creep up on you slowly in an insidious way – hence the name of the vitamin B12 deficiency disease, ‘pernicious’ anaemia. Although vitamin B12 treatment for pernicious anaemia traditionally involves intramuscular injections, it can be given sublingually (under the tongue) in high doses as absorption through oral mucus membranes overcomes the lack of intestinal intrinsic factor.

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