Nutrition (16 page)
Dietary sources of iron
Rich sources include red meat; offal; shellfish; fish, especially sardines; wheatgerm; egg yolk; green vegetables; prunes and other dried fruit; fortified flour and cereals.
Dietary iron is available bound to protein as haem (from animal sources) and as inorganic salts. Absorption of haem iron is three times more efficient than that of non-haem iron, although most dietary iron is obtained in inorganic form. The presence of vitamin C in food increases absorption of inorganic iron by converting ferric iron (Fe
3+
) to ferrous iron (Fe
2+
). It’s important to know, however, that over-boiling vegetables decreases their iron availability by up to 20 per cent.
3+
) to ferrous iron (Fe
2+
). It’s important to know, however, that over-boiling vegetables decreases their iron availability by up to 20 per cent.
Deficiency
Dietary iron intakes are falling due to decreased meat consumption and reduced energy intake. As a result, iron-deficiency anaemia (IDA) is increasingly common. The most vulnerable groups include infants over the age of six months (and younger ones if exclusively breastfed), toddlers, adolescents, menstruating or pregnant women and the elderly. Vegetarians and people who have high intakes of iron-absorption inhibitors (such as phytates) are also at risk.
Lack of iron quickly leads to the production of red blood cells that are much smaller and paler (due to lack of haemoglobin) than normal. This results in iron-deficiency anaemia, with symptoms of paleness, fast pulse, tiredness, exhaustion, dizziness, headache and even shortness of breath and angina if anaemia is severe.
Other symptoms that can occur in iron deficiency include generalized skin itching, concave brittle nails, hair loss, sore tongue, cracking at the corners of the mouth, reduced appetite and difficulty in swallowing.
Other symptoms that can occur in iron deficiency include generalized skin itching, concave brittle nails, hair loss, sore tongue, cracking at the corners of the mouth, reduced appetite and difficulty in swallowing.
Worldwide, iron deficiency is the most common nutritional disease, with most cases going unrecognized. Women are more at risk of iron deficiency than men because of blood loss during menstruation. This can result in a low-grade iron deficiency that is enough to impair immunity, without causing frank iron-deficiency anaemia. If anaemia is suspected, it is important to seek medical advice before taking iron supplements, as the cause needs to be determined. Iron supplements may mask iron deficiency by correcting changes usually seen in blood tests, so that a condition such as a bowel cancer (which can cause continuous tiny losses of blood) may initially be missed.
Toxicity
Your body has no specific mechanism for excreting excess iron. High intakes accumulate in the liver as storage proteins (haemosiderin, ferritin). Excess iron intakes can cause severe intestinal damage, as well as nausea, indigestion, constipation and bloody stools. It can also lead to liver damage, shock and coma. Excess is especially dangerous for children.
HAEMOCHROMATOSIS
Haemochromatosis is an autosomal recessive genetic condition in which the absorption of dietary iron is unregulated. Excess accumulation of iron can lead to liver cirrhosis, liver carcinoma, diabetes and heart failure. In Caucasian populations, haemochromatosis affects one in 140 people. In addition, one in 100 carries a genetic mutation that also increases the risk of iron accumulation. Another one in seven people carry one copy of the defective gene and have mildly increased iron stores, although significant iron loading is rare.
Magnesium
Magnesium is a metallic mineral needed as a cofactor for the function of over 300 enzymes. Few enzymes can work without it, and it is involved in every major metabolic reaction in the body, including the synthesis of protein and genetic material; metabolism of essential fatty acids and glucose; interactions between sex hormones and nuclear receptors; the production of brain chemicals and regulation of mood.
Magnesium is also vital for the integrity of ion pumps in cell membranes, which control the flow of sodium, potassium, calcium and chloride in and out of cells. This maintains the electrical potential across cell membranes that allows for nerve transmission, brain function and a regular heartbeat.
Seventy per cent of your body stores of magnesium are in your bones and teeth.
Dietary sources of magnesium
Rich sources include seafood; seaweed; meat; eggs; dairy products; wholegrains; pulses, especially soybeans; nuts; bananas; dark green leafy vegetables; chocolate and yeast.
Deficiency
People who are physically active can lose large amounts of magnesium in sweat. As magnesium is essential for the normal function of the parathyroid gland and for vitamin D metabolism, lack of magnesium markedly disturbs calcium balance and can lead to low calcium levels with muscle cramps. Deficiency has been implicated in constipation-predominant irritable bowel syndrome, migraine and hypertension.
Magnesium supplements above a dose of around 400 mg per day can cause diarrhoea by attracting water into the bowel. This can be beneficial, of course, and magnesium salts are used medicinally as a laxative.
Manganese
Manganese is a metallic element needed for the function of metalloenzymes involved in the synthesis of amino acids, carbohydrates, sex hormones, blood-clotting factors, cholesterol and some brain neurotransmitters. It is needed for the normal growth and development of bone, cartilage, collagen and structural molecules known as mucopolysaccharides. It also acts as an antioxidant.
Dietary sources of manganese
Sources include tea (1 mg per cup, on average); wholegrains; nuts and seeds; fruit; eggs; green leafy vegetables/herbs; offal; shellfish and milk.
Deficiency
The significance of manganese deficiency is currently unknown, but possible cases have been linked with reddening of black body hair, scaly skin, poor growth of hair and nails, disc and cartilage problems, poor blood clotting, glucose intolerance, poor memory and worsening intellect. It may also contribute to reduced fertility.
Toxicity
Industrial workers inhaling manganese dust (for example, during arc welding) have experienced neurological symptoms similar to Parkinson’s disease (manganism), which remains an active issue in health and safety legislation.
Phosphorus
Phosphorus is a mineral that interacts with calcium to form calcium phosphate (hydroxyapatite) – the major structural component of bones and teeth. Most of your phosphorus is in your skeleton, but that remaining outside cells is vital for the production of energy-rich molecules (ATP, ADP), genetic material and for the activation of B vitamins involved in energy production. Vitamin D is essential for the absorption of phosphorus and calcium from the gut and for their deposition in bone.
Dietary sources of phosphorus
Phosphorus is widely found in foods such as fish; meat and poultry; eggs; dairy products; wholegrains; nuts; pulses; yeast extract and soft drinks such as colas.
Deficiency
Deficiency is unusual but can develop with long-term use of antacids containing aluminium hydroxide, which impairs phosphate absorption. Phosphorus deficiency symptoms include loss of appetite, anaemia, muscle weakness, bone pain, rickets and poor coordination.
High intakes can cause diarrhoea.
Potassium
Potassium is an alkaline metallic mineral that is concentrated inside body cells. Cells accumulate potassium by swapping it for sodium via sodium–potassium pumps found in all cell membranes. Potassium is essential for normal muscle contraction, including generation of a regular heartbeat, nerve conduction and glucose control. It is also involved in the production of genetic material, proteins and energy. The kidneys regulate blood potassium levels and keep them within a fairly narrow range.
A good intake of dietary potassium helps to flush excess sodium from the body via the kidneys to help lower a raised blood pressure.
Dietary sources of potassium
These include seafood; meat; fruit; vegetables; wholegrains; milk and dairy products; rock salt and low-sodium, potassium-enriched salt.
Deficiency
People taking certain diuretic drugs to reduce water retention may lose enough potassium to become deficient. Rarely, ‘crash’ or very strict diets can also lead to potassium deficiency, especially if little fruit or vegetables are eaten. Low intakes can cause rapid and irregular heartbeat, muscle weakness, irritability and may progress to nausea, vomiting, diarrhoea, low muscle tone and even paralysis.
Toxicity
The body usually maintains potassium levels within tight limits. Raised potassium levels (for example, due to kidney disease) can occur, causing heart failure, abnormal heart rhythms and cardiac arrest.
Selenium
Selenium is a metallic trace element that is so important to health that its incorporation into proteins (as selenocysteine, a recently identified amino acid) is directly controlled by your genes. Selenium is present in at least 30 human proteins (selenoenzymes) that are essential for cell growth, antioxidant protection, antibody synthesis and the activity of natural killer cells, which target virally infected cells and cancer cells. It is involved in the regulation of thyroid hormones and liver detoxification of cancer-causing chemicals (carcinogens). In parts of the world where selenium intakes are low, the incidence of cancer increases by two- to sixfold. Those with the lowest selenium intakes have the highest risk of developing leukaemia or cancers of the colon, rectum, breast, ovary, pancreas, prostate gland, bladder, skin and lungs.
Dietary sources of selenium
These include Brazil nuts (the richest dietary source); seafood; offal; meat (especially game); wholegrains; onions and garlic; broccoli, cabbage, mushrooms, radishes and celery and selenium-enriched yeast.
A healthy diet can no longer provide adequate intakes of selenium in many parts of Europe. Between 1975 and 1994, selenium intakes almost halved in the UK from 60 mcg to 34 mcg per day – mostly because the UK switched from using selenium-rich flour from the US and Canada to using flour produced in Europe.
Deficiency
Selenium is important for muscle cell function. In parts of China, selenium intakes are low enough to cause a form of heart failure (Keshan disease) and an unpleasant, deforming type of arthritis known as Kashin-Beck disease. These risks are even higher where intakes of other antioxidants such as vitamins A, C and E are also low. Symptoms and signs that have been linked to a lesser selenium deficiency include age spots, pale fingernail beds, increased susceptibility to viral infections (especially influenza) and some cancers.
Toxicity
Selenium toxicity can occur with intakes above 800 mcg daily, leading to a garlic odour on the breath, salivation, fragile or black fingernails, a metallic taste in the mouth, dizziness, nausea, vomiting and hair loss.
Silicon
Silicon is a non-metallic element that, in its pure form, is biologically inactive. As soluble silicate it is an essential trace element that strengthens collagen and elastin fibres and contributes to tissue elasticity. It increases mineralization of growing bones and is needed for the formation of cartilage.
Dietary sources of silicon
These include rice bran; wholegrains; green leafy vegetables; capsicum peppers; root vegetables; nuts and seeds.
There is no EU RDA for silicon. Intakes of 10 mg to 30 mg a day are believed to be adequate.
Deficiency
As the average diet supplies up to 50 mg per day from food and 10 mg per day from water, intakes are usually more than adequate. Silicon deficiency has been associated with bone and joint deformities and loss of bone mineralization (osteoporosis) in animals, but this has not been confirmed in humans.
Sodium
Sodium is a metallic mineral that is vital for regulating body-fluid balance. Most body sodium is present in tissue fluids as it is actively pumped out of cells in exchange for potassium ions. Sodium–potassium pumps in cell membranes transport three positively charged sodium ions out of a cell for every two positively charged potassium ions transported inside. This produces an electrical potential across cell membranes in which the inside of the cell is negatively charged compared with the outside. This electrical potential is vital for life, as it allows nerve and brain cells to conduct electrical signals and muscle cells to contract. In fact, the active transport of sodium and potassium ions in and out of body cells is one of the main energy-using processes contributing to your basal metabolic rate. It is estimated to account for 33 per cent of energy used by cells overall, and 70 per cent of energy used by nerve cells alone.
Dietary sources of sodium
These include table salt (sodium chloride); salted crisps; bacon; salted nuts; tinned products (especially those canned in brine); cured, smoked or pickled fish and meats, meat pastes and pâtés; ready-prepared meals; packet soups and sauces; stock cubes and yeast extract.
There is no EU RDA for sodium as most people take in too much. The UK reference nutrient intake for adults aged 19 to 50 years is 1,600 mg sodium (equivalent to 4 g salt) daily, and the average diet supplies 2,880 mg salt per day from food (not counting salt added during cooking or at the table). Variable amounts are obtained from drinking water, as sodium is added during softening of hard water. Soft water contains up to 25 mg sodium per litre, and hard water up to 75 mg per litre. Contamination of wellsprings with seawater can also affect its sodium content.
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