The mineral content of foods is closely linked to the levels present in the soils on which they were grown or reared. Some plants preferentially concentrate some minerals, and are therefore good sources of certain trace elements. Further up the food chain, animals that eat these crops may also be good sources.
Boron is a trace element obtained from fruit and vegetables, especially nuts, apple, grapes, pears, plums, prunes, berries, avocado, broccoli, green leaves. Vegetarians have a higher average intake than non-vegetarians (around 10 mg a day compared with 0.5–1 mg in non-vegetarians) which may account for their lower risk of osteoporosis.
Calcium is obtained from milk and dairy products, eggs, broccoli and green leaves (but not spinach, whose oxalate content reduces its bioavailability). You can also obtain it from tinned salmon if you eat the bones, and from nuts, seeds, pulses and bread made from fortified flour. It is relatively easy to increase your calcium intake by drinking an extra pint of skimmed or semi-skimmed milk per day, which provides around 720mg calcium per pint (600 mls). The calcium found in milk is in the readily absorbable form of calcium lactate. Interestingly, the bioavailability of calcium from brassica (cabbage family) vegetables is higher than that of dairy products, however. As much as 61% of the calcium found in broccoli is absorbable, compared with only 32% of that in milk – the reason remains unknown. Some types of dietary fibre (phytates from wheat in unleavened bread e.g. chapatti) also bind calcium in the bowel to form an insoluble, non-absorbable salt. High-fibre diets, which speed the passage of food through the bowels, will also reduce the amount of calcium absorbed.
Chromium exists in several forms in nature. The hexavalent form of chromium (used in industry) is toxic but the trivalent form found in food is non-toxic at usual intakes and essential for health. Chromium deficiency is common as most people get less than 50 mcg from their diet, and only around 2 per cent of this is in an absorbable (trivalent) form. Sources include fruit and vegetables, chromium-enriched yeast, eggs, meat, wholegrains, pulses, black pepper, thyme and honey.
Copper is obtained from a number of dietary sources, including crustaceans and shellfish (prawns, oysters, lobster, crab), nuts, pulses, whole-grain cereals, avocado, artichokes, radishes, garlic, mushrooms and green vegetables grown in copper-rich soil. Brewer’s yeast is also a good source. The absorption of dietary copper is often low, however, as up to 70% remains bound to other bowel contents such as sugars, sweeteners, refined flour, raw meat, vitamin C, zinc and calcium.
Iodine is present in marine fish, seaweed products, iodized salt and crops or cattle reared on soils exposed to sea spray. Lack of iodine is common in many parts of the world, however.
Iron is obtained from eating shellfish, red meats (especially offal), fish (especially sardines), wheatgerm, wholemeal bread, egg yolk, green vegetables and dried fruit (especially prunes). It is also present in fortified cereals and foods made from fortified flour. The haem (heme) form found in meat is absorbed via a specific receptor two to three times more efficiently than the elemental iron present in plants. The Elemental iron in plants exists in two oxidation states: ferrous (Fe2+) and ferric (Fe3+) iron which each have separate uptake mechanisms. Ferric iron – the form in which most plant-derived iron is obtained – is less well absorbed due to low solubility in the alkaline small intestines.Vitamin C increases iron absorption by converting ferric iron to ferrous iron. Even so, the uptake of iron from a varied diet is relatively low and estimated as around 15%.
Magnesium is obtained from eating seafood, seaweed, meat, eggs, dairy products, beans (especially soy), nuts, wholegrains (eg brown rice), bananas and green leaves. Chocolate, drinking water in hard-water areas, mineral seasoning salt and yeast are also important sources.
Manganese is obtained from drinking tea (one cup provides around 1 mg on average), wholegrains, nuts, seeds, eggs, fruit, green leaves, offal, shellfish and dairy products.
Phosphorus is obtained from many food sources, including fish, meats, eggs, dairy products, beans, nuts and wholegrains. Phosphorus is also obtains from fizzy drinks such as colas, but is in the form of phosphoric acid which has negative effects on teeth and the body’s acid-alkaline balance. Aim to keep your intake of these fizzy drinks to a minimum.
Potassium is found in seafood, fruit (particularly tomatoes, bananas), vegetables, whole grains, milk & dairy products, rock salt and low-sodium, potassium-enriched table salts.
Selenium is the only trace element whose incorporation into proteins – as selenocysteine, the 21st amino acid – is under direct genetic control, underlining its importance to health. Selenium is obtained from Brazil nuts (richest source), other tree nuts, seafood, offal, meats (especially game), wholegrains, onions, garlic, broccoli, mushrooms, cabbage, radishes and yeast. The mineral content of these foods depends on the soils in which they are grown or reared. Soil levels in many parts of the world (especially the UK and other regions within Europe, New Zealand and China) are low, as it was leached out during the last Ice Age.
Zinc is obtained from red meat, seafood (especially oysters), offal, yeast, whole grains, beans, eggs and cheese.
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