Nutrition (15 page)
Gut health
Disorders that hasten gut transit time (e.g. diarrhoea from gastroenteritis, irritable bowel syndrome) will reduce mineral absorption, while a slowed gut transit time (e.g. constipation) may increase absorption due to prolonged contact between minerals and intestinal lining cells.
Gut diseases such as Crohn’s disease and ulcerative colitis can also affect absorption – either reducing uptake through active transport mechanisms or increasing passive diffusion by disrupting the integrity of the intestinal lining.
Other dietary components
Mineral absorption is influenced by the presence of other dietary compounds within the intestines. Substances that bind to minerals (chelators) such as bran and phytic acid (found especially in unleavened wheat bread) can hold them within the intestines so that they remain unabsorbed. Phytic acid, for example, has been reported to inhibit the absorption of iron, zinc, calcium and manganese but not of copper. Polyphenols, such as tannic acid found in tea and some vegetables, also chelate iron and other minerals. Similarly, free fatty acids can form insoluble soaps with calcium and magnesium to impair their absorption, while an as yet unidentified component of coffee appears to inhibit iron absorption.
Minerals can interfere with the absorption of each other, too. Iron interferes with zinc absorption, while copper uptake is impaired by the presence of zinc, iron and molybdenum.
As the previous pages have shown, simply measuring the amount of minerals (or vitamins) present in the diet is not always enough when assessing your diet, as there is no guarantee of how much of each micronutrient is absorbed. Blood testing in order to measure the micronutrient status of the body may be needed when nutritional deficiencies are suspected.
Boron
Boron is a non-metallic trace element that is abundant in oceans, rocks and some soils. Its function is not fully understood, but it interacts with several enzymes to stimulate or inhibit their action. As such, it is believed to promote chemical transmission in the brain and to boost the production of active vitamin D and the hormones oestrogen and testosterone.
Dietary sources of boron
Boron is essential for plant health and is present in nuts; fruit, especially apples, grapes, pears, plums, prunes, strawberries and avocado, and green vegetables, especially broccoli. Small amounts are also present in drinking water.
No EU RDA for boron has been set. The World Health Organization suggests intakes of between 1 mg and 13 mg per day are adequate.
Deficiency
Lack of boron is believed to contribute to a rare form of joint deformity known as Kashin-Beck disease that is also linked to selenium deficiency. Some researchers have also suggested that osteoporosis is a boron-deficiency disease, though this is controversial.
Toxicity
Symptoms of toxicity may include headache, muscle pain, nausea, vomiting, red eyes, rash and desquamation (peeling skin).
Calcium
Calcium is an alkaline metal and is the most abundant mineral in your body. Almost all your stores (around 1.2 kg) are in your bones and teeth in the form of calcium phosphate (hydroxyapatite) embedded in a collagen protein framework. The 1 per cent that is outside your skeleton is vital for life, however, as it is involved in energy production, muscle contraction (including your heartbeat), nerve conduction, immune function and blood clotting.
Absorption of calcium in the small intestine depends upon the presence of vitamin D.
If your diet is deficient in calcium, it is leached from your bone stores, which increases your future risk of osteoporosis (brittle bones). Good intakes are especially important during childhood and adolescence, when bones are developing, and in later years when bones naturally start to thin (partly due to lower levels of the sex hormones oestrogen and testosterone).
Good intakes of calcium are vital throughout life for the growth, development and maintenance of strong, healthy bones and teeth. Calcium also plays a vital role in muscle contraction, nerve conduction, blood clotting, energy production and the regulation of many important metabolic enzymes.
Dietary sources of calcium
Calcium is found in milk (skimmed milk provides as much calcium as whole milk but without the additional fat); dairy products such as cheese, yogurt and fromage frais (but not butter); green leafy vegetables (especially broccoli but not spinach, whose oxalate content reduces its bioavailability); tinned salmon (including the bones); nuts and seeds; pulses; bread made from fortified flour, and eggs.
Overall, less than 40 per cent of dietary calcium from other sources is absorbed from the gut. Some types of dietary fibre (especially wheat phytates from unleavened bread) bind calcium in the bowel to form an insoluble, non-absorbable salt.
Drinking a pint of skimmed or semi-skimmed milk per day provides 720 mg calcium – almost your full daily requirement – in the readily absorbed form of calcium lactate.
The bioavailability of calcium in brassica vegetables (e.g. cabbage, cauliflower, broccoli, Brussels sprouts, swede) is unusually high (61 per cent, compared with 32 per cent for milk), for reasons that are not fully understood.
Deficiency
When dietary calcium is in short supply, it is leached from the bones to maintain heart muscle contraction. Low intakes of calcium are linked with wide-ranging symptoms affecting the muscles, bones and teeth. These include muscle aches, pains, twitching and cramps, heart palpitations, high blood pressure, osteoporosis, gum disease and loss of teeth.
Toxicity
An excess of calcium can cause lethargy, confusion and even coma. People with a tendency towards kidney stones should ideally take calcium supplements together with essential fatty acids, but are advised to seek medical advice first.
Chloride
Chloride is a negatively charged electrolyte of chlorine. It is widely present in the diet as salts of sodium, potassium and calcium. Together with sodium (outside cells) and potassium (inside cells), chloride ions are involved in the regulation of your body’s fluid, electrolyte and acid/alkaline balance. It is also used to produce hydrochloric acid in the stomach.
Dietary sources of chloride
Chloride ions are present in just about every food, especially fruit; vegetables; seafood; seaweeds such as kelp; table salt and processed foods.
Deficiency
As chloride is widespread in foods, deficiency is uncommon except during periods of excessive vomiting. Excess intakes are more of a problem but during normal health, the body maintains a consistent blood level by excreting excess in sweat, urine and faeces.
Chromium
Chromium is a metallic trace element that exists in a number of oxidation states. While the hexavalent form of chromium (Cr
6+
) used in industry and shiny car bumpers is highly toxic, the trivalent form of chromium (Cr
3+
) is nutritionally essential. Trivalent chromium plays an important role in glucose control by regulating the interaction between insulin and receptors present in muscle and fat cells. It appears to do this in the form of a complex known as Glucose Tolerance Factor (GTF), which has been extracted from yeast. Although it has not been fully characterized (due to its instability), researchers suggest that GTF increases insulin binding to cells as well as the number of insulin receptors present on muscle and fat cells.
6+
) used in industry and shiny car bumpers is highly toxic, the trivalent form of chromium (Cr
3+
) is nutritionally essential. Trivalent chromium plays an important role in glucose control by regulating the interaction between insulin and receptors present in muscle and fat cells. It appears to do this in the form of a complex known as Glucose Tolerance Factor (GTF), which has been extracted from yeast. Although it has not been fully characterized (due to its instability), researchers suggest that GTF increases insulin binding to cells as well as the number of insulin receptors present on muscle and fat cells.
Dietary sources of chromium
As plants do not use chromium, their content depends on the amount and type of chromium present in the soil in which they are grown, and is highly variable. Potential sources include egg yolk; red meat; wholegrains; pulses; condiments such as black pepper and thyme; cheese; yeast; honey; fruit and vegetables.
Deficiency
Intestinal absorption is low (less than 2 per cent) except where chromium is present in the form of GTF such as that found in yeast. Low levels of chromium have been linked with poor glucose tolerance, weight loss and abnormal nerve function. A dietary lack of chromium appears to be a risk factor for Type 2 diabetes in some people.
Copper
Copper is a metallic trace element that is essential for healthy liver, brain and muscle function. Copper is a component of many antioxidant enzymes, including copper-zinc superoxide dismutase. It is involved in cholesterol, glucose and vitamin C metabolism, the production of brain chemicals and the manufacture of the tanning pigment, melanin, and the red blood pigment, haemoglobin. It is needed for the synthesis of collagen and is therefore involved in maintaining healthy bones, cartilage, hair and skin.
Dietary sources of copper
Rich sources include crustaceans, e.g. prawns; shellfish; offal; nuts; olives; pulses; wholegrain cereals and green vegetables.
Vegetable sources depend on the copper content of the soils in which they are grown.
Up to 70 per cent of dietary intake remains unabsorbed because it is bound to other bowel contents.
Deficiency
Lack of copper can occur in some conditions such as Crohn’s or coeliac disease in which general nutrient absorption is reduced, and in people with a hereditary inability to process copper properly. Copper deficiency mainly affects the cardiovascular system and is associated with anaemia, low white-cell count, increased susceptibility to infection, fluid retention, loss of taste sensation, raised blood cholesterol levels and heart failure. It has also been linked with abnormal structure and pigmentation of skin and hair, osteoporosis and joint pains.
The risk of copper deficiency is greater when zinc intakes are high as the two interact to reduce copper’s bioavailability. The ideal ratio of copper to zinc in both the diet and food supplements is 1:10.
Toxicity
Excess copper (for example, from drinking water supplied through copper pipes) is toxic at relatively low levels – just twice as high as normal intakes – causing restlessness, nausea, vomiting, colic and diarrhoea. Long-term high intakes can lead to copper-induced cirrhosis of the liver and brain damage.
Fluoride
Fluoride is a mineral that binds to bone to produce calcium fluoroapatite, which, in low concentrations, is more resistant to reabsorption than normal bone hydroxyapatite which may protect against osteoporosis (brittle bones). Similarly, it can strengthen tooth enamel so that it is more resistant to decay.
Dietary sources of fluoride
These include tea leaves (which provide 70 per cent of average intakes); fluorinated water supplies; seafood (especially oysters); pulses and wholegrains.
No EU RDA is currently set for fluoride. Intakes of between 1.5 mg and 4 mg have been suggested as desirable. Fluorination of drinking water provides between 1 mg and 3 mg fluoride a day.
Deficiency
Lack of fluoride is associated with weakened teeth and bones, leading to an increased risk of tooth decay and osteoporosis. Paradoxically, excess fluoride also promotes formation of abnormal, weakened bone and discoloured teeth (fluorosis) due to the presence of too much calcium fluoroapatite.
Iodine
Iodine is a non-metallic trace element that is essential for the production of two thyroid hormones, thyroxine and triiodothyronine. These regulate the basal metabolic rate – the speed at which energy and heat are produced in the body.
Dietary sources of iodine
These include marine fish; seafood; seaweed; iodized salt; milk from cattle receiving iodized feed; crops and cattle reared on soils exposed to sea spray.
Deficiency
Lack of iodine during pregnancy affects fetal brain development, leading to severe mental retardation (cretinism). Although this is uncommon in developed countries, it is a serious problem in some parts of the world, especially Indonesia. In endemic areas, this devastating condition can easily be prevented by administering injections of iodized oil to expectant mothers before the sixth month of pregnancy – preferably during the preconceptual period.
In adults, gross iodine deficiency leads to swelling of the thyroid gland (goitre).
Other symptoms that have been linked with iodine deficiency include an underactive thyroid gland, with associated symptoms of tiredness, lack of energy, weight gain, muscle weakness and susceptibility to cold.
Selenium plays a role in the metabolism of thyroid hormones, and the effects of iodine deficiency are made worse by low selenium intakes.
Toxicity
Up to 3 per cent of people are allergic to concentrated iodine solutions that are applied to disinfect the skin (for example, before a surgical operation). Excess iodine can cause a metallic taste in the mouth, oral sores, headache, diarrhoea, vomiting, rash and – as with a deficiency – can also lead to thyroid swelling (goitre).
Iron
Iron is an essential metallic mineral needed for production of the red blood pigment, haemoglobin and the red muscle pigment, myoglobin, both of which act as oxygen carriers. Two-thirds of your iron stores are circulating within the haemoglobin of your blood. Many enzyme systems, including cytochrome enzymes in the liver, also rely on iron for the production of energy, liver detoxification and immunity.
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