- Minerals are inorganic substances required by the body in small amounts for a variety of different functions.
- Minerals are involved in the formation of bones and teeth; they are essential constituents of body fluids and tissues; they are components of enzyme systems and they are involved in normal nerve function.
- The body requires different amounts of each mineral; people have different requirements, according to their age, sex, physiological state (e.g. pregnancy) and sometimes their state of health.
- The Department of Health has published Dietary Reference Values (DRVs) for minerals for different groups of healthy people.
- What are minerals?
- Requirements and recommended dietary intakes; bioavailability and absorption of minerals; deficiencies and excess intakes
- Chromium and other trace elements
The mineral content of foods and drinks can be found in McCance and Widdowson’s 'composition of foods integrated dataset' on the nutrient content of the UK food supply, which can be viewed here.
What are minerals?
Minerals are inorganic substances required by the body in small amounts for a variety of functions. These include the formation of bones and teeth; as essential constituents of body fluids and tissues; as components of enzyme systems and for normal nerve function.
Some minerals are needed in larger amounts than others, e.g. calcium, phosphorus, magnesium, sodium, potassium and chloride. Others are required in smaller quantities and are sometimes called trace minerals, e.g. iron, zinc, iodine, fluoride, selenium and copper. Despite being required in smaller amounts, trace minerals are no less important than other minerals.
Minerals are often absorbed more efficiently by the body if supplied in foods rather than as supplements. Also, a diet that is short in one mineral may well be low in others, and so the first step in dealing with this is to review and improve the diet as a whole. Eating a varied diet will help ensure an adequate supply of most minerals for healthy people.
The National Diet and Nutrition Surveys (NDNS) have revealed that some sub-groups of the population have low intakes of some other minerals, for example potassium, magnesium, zinc in men, and for women, iron, calcium, copper and iodine. Young British adults, especially young women, have particularly poor diets which are likely to put their future health at risk unless improvements are made. See our section on nutrient requirements for more information.
Most people do not show signs of deficiency but this does not mean their intakes or nutrient status are adequate. For example, adolescent girls, women of childbearing age and some vegans/vegetarians are more susceptible to low iron status as their dietary intake may not match their requirements, and therefore they are at risk of iron deficiency anaemia. There is also concern about the calcium intake of some adolescents, and young and older women and the implications for future bone health.
Requirements and recommended dietary intakes
The body requires different amounts of each mineral because each mineral has a different set of functions. Requirements vary according to age, sex and physiological state (for example pregnancy). They may also be influenced by state of health. The Department of Health has published recommendations in the form of Dietary Reference Values (DRVs) for minerals for different groups of healthy people (see Nutrient requirements). The Reference Nutrient Intake (RNI) is the amount of a nutrient that will satisfy the needs of practically all the population (i.e. 97.5%); in other words it is usually not necessary to exceed the RNI. Lower Reference Nutrient Intakes have also been established. These are levels judged to be sufficient for only 2.5% of a given population, everyone else will require more. So if, say, 10% of a population group have intakes of a nutrient below the LRNI for that nutrient, it is highly likely that the majority of these people are having insufficient for their needs.
Tables showing the mean intakes of mineral and trace elements from food sources as a percentage of Lower Reference Nutrient Intake (LRNI), by age and sex, are available in the attached pdf file below. These tables give an indicator of where intakes of minerals and trace elements are of concern.
But certain groups of people may have higher requirements for specific minerals, e.g. women with particularly heavy periods may need extra iron, and extra calcium (and vitamin D) is sometimes recommended by doctors for women at high risk of osteoporosis. In such cases, supplements may be useful but should not replace a varied and healthy diet.
The bioavailability and absorption of minerals
The bioavailability of a mineral (i.e. how readily it can be absorbed and used by the body) may be influenced by a variety of factors. Bioavailability will depend upon the chemical form of the mineral, other substances present in the diet and (for nutrients such as iron) the individual person’s needs as determined by how much of the nutrient is already stored in the body. This is because the body has sensitive mechanisms for preventing storage of nutrients that can be damaging in excess (as is the case with iron).
For example, the bioavailability of iron from plant sources (non-haem iron) is relatively poor compared with iron from meat (haem iron) but absorption is increased when vitamin C is consumed during the same meal because the vitamin C converts it to a more bioavailable chemical form.
Some dietary constituents reduce bioavailability. Phytate, for example, found in products made from wholegrain cereals (especially unleavened breads such as chapattis) can bind and hence reduce the absorption of calcium, iron and zinc. Iodine absorption may be hindered by nitrates. Similarly, oxalate present in spinach and rhubarb binds any calcium present, making it unavailable for absorption. Also an excess of one mineral may hinder the absorption of another by competing for the same transport systems in the gut, e.g. excess iron reduces zinc absorption. This generally only becomes a problem when zinc intakes are already marginal.
Unlike some vitamins, minerals are fairly stable in normal food processing and storage conditions.
Deficiencies and excess intakes
Iron deficiency anaemia is the most common nutritional deficiency in the world, often affecting women and young children, and is found in the UK population too. Iodine deficiency is also commonplace worldwide. Nutritional deficiencies of other minerals are rare in the UK (although as indicated above, dietary intakes of a number of minerals are low in sub groups of the UK population and nutritional status of individuals may therefore be affected). Policy decisions about the adequacy of current nutrient intakes in maintaining appropriate status are often hindered by the limitations of existing markers of status and also by the available information on requirements, an example being selenium.
On the other hand, excess intakes of minerals are also sometimes of concern (for example sodium, one of several risk factors associated with high blood pressure). In general, excess intakes of a range of minerals have been reported to have varying effects, ranging from no effect (e.g. no adverse effects have been reported for excess iodine intakes up to 2mg iodine/day) to severe (e.g. excess fluoride can cause skeletal fluorosis). However, more information is needed about the effects of excess intakes of many of the essential minerals and trace elements. For further information see the report of the Expert Group on Vitamins and Minerals (EVM) (http://www.food.gov.uk/).
Calcium (Ca) is the most abundant mineral in the body and is essential for a number of vital functions. The body needs adequate dietary calcium (alongside vitamin D and several other nutrients such as vitamin K) to develop and maintain healthy bones and teeth. Calcium also plays a vital role in many systems including intracellular signalling to enable the integration and regulation of metabolic processes, the transmission of information via the nervous system, the control of muscle contraction (including the heart) and blood clotting. Furthermore, it has been suggested that adequate calcium intake (for example from reduced fat dairy products) may help lower high blood pressure and may help protect against colon cancer, although more evidence is needed to fully substantiate these functions.
The skeleton contains about 99% of the body’s calcium with approximately 1kg present in adult bones. The major constituents of bone are calcium and phosphate, forming hydroxyapatite, which is associated within a meshwork of collagen fibres to form a rigid structure. The body’s requirement for calcium fluctuates with the rate of bone development, so as well as protecting vital organs, the skeleton acts as a ‘bank’ of minerals from which calcium and phosphorus may be continually withdrawn or deposited to support physiological requirement.
Calcium levels in the blood are carefully regulated and blood plasma levels are maintained within narrow limits. Calcium absorption is well controlled to match the needs of the body and so calcium balance can be maintained at a variety of different levels of calcium intake. Even at low levels of intake, there is evidence from the Gambia, for example, that calcium balance can be achieved. Calcium status is maintained by balancing calcium absorption from the gut, excretion via the kidneys and mobilisation and deposition in the bone. These sites are regulated by feedback mechanisms controlled by several hormones including parathyroid hormone and the activated form of vitamin D. Plasma levels of calcium only become abnormal if there is a breakdown of this homeostatic mechanism, and not usually as a result of differences in dietary calcium intake. The body invests this effort because small variations in plasma calcium concentrations may have serious consequences to the functioning of vital organs and to health in general. Low blood calcium is called hypocalcaemia and high blood calcium is called hypercalcaemia..
For some nutrients, nutritional deficiency is identified by the existence of a low blood level of the nutrient but for nutrients such as calcium, for the reasons described above, low blood levels rarely occur. Because of the need to maintain blood levels, the impact of a poor supply of calcium is usually reflected in bone density because bone acts as a reservoir in times of need. For example, insufficient calcium in bones can result from an inadequate supply of vitamin D which is essential for absorption of calcium. In children, vitamin D deficiency results in rickets and, in adults, osteomalacia, in which bones become weak owing to lack of calcium.
In terms of dietary supply, a significant proportion of young women have average calcium intakes below the Lower Reference Nutrient Intakes (8% of women aged 19-24 years and 6% of women aged 25-34 years) indicating these intakes are likely to be inadequate. An adequate calcium intake is vital for health, particularly in times of growth (in childhood, adolescence, pregnancy) to establish peak bone mass and also during lactation (breastfeeding). Supplements are sometimes recommended for those at risk of osteoporosis. See nutrient requirements for information on calcium requirements throughout the life course.
Obtaining calcium as part of a varied diet is unlikely to cause any adverse effects but taking high dose supplements sometimes causes stomach pain and diarrhoea.
Milk, cheese and other dairy products provide about half of the calcium in the UK diet. Bread is also an important source in the UK because most bread flour (though not wholemeal) is fortified with calcium by law. Calcium is also provided by some green leafy vegetables such as broccoli and cabbage (but not spinach), fortified soya products and fish eaten with the bones such as sardines, tinned salmon and whitebait. For more information about the dietary sources of calcium click here.
Calcium absorption is influenced by a number of promoting and inhibitory factors. Promoting factors include vitamin D, lactose, dietary protein, non-digestible oligosaccharides and an acidic environment in the small intestine. Calcium is most readily absorbed from milk and dairy products. Inhibitory factors include phytates (e.g. in wholegrain cereals, pulses), oxalate (e.g. from spinach, rhubarb, beetroot), use of antacids, unabsorbed dietary fats, excessive intakes of dietary fibre and large intakes of phosphoric acid (e.g. from carbonated drinks). Calcium is often less available from plant foods where the calcium may be bound by phytates and oxalates in foods, which makes the calcium unavailable for absorption from the intestine into the blood. However, absorption from some plant foods is good e.g. broccoli, although the amount present is usually lower than in milk.
The major function of phosphorus is in the formation, with calcium, of the bone component hydroxyapatite. 80% of the phosphorous in the body is present as calcium salts in the skeleton and, therefore, is essential for healthy bone and tooth structure. In the adult, there is a dynamic equilibrium between calcium and phosphate in the continual remodelling of bones.. The rest of the body’s phosphorus is distributed in all cells. It is essential for the structure of cell membranes (in phospholipids) and intercellular phosphorus contributes to a number of processes associated with energy metabolism.
Phosphorus is unlikely to be in short supply in UK diets because it is available in many foods.
Phosphorus has very low toxicity.
Phosphorus is abundant in red meat, dairy products, fish, poultry, bread, rice and oats and is usually found in foods that also contain calcium.
Magnesium is an essential mineral present in all human tissues, especially in bone. It has both physiological and biochemical functions and has important interrelationships with calcium, potassium and sodium. It is needed for the activation of many enzymes (for example enzymes concerned with the replication of DNA and the synthesis of RNA) and for parathyroid hormone secretion, which in involved in bone metabolism. It is also needed for muscle and nerve function.
Nutritional deficiency is rare and characterised by progressive muscle weakness and neuromuscular dysfunction. Mild hypomagnesaemia (low blood magnesium) is common in severely ill patients, alcoholics and those with malabsorption disorders.
In contrast, intakes of magnesium that are judged to be too low are quite common in the UK. Despite magnesium being available in a wide range of foods, one in five women aged 19-34 years and more than half of teenage girls have intakes below the LRNI (51% of 11-14 age group and 53% of 15-18 age group) and more than 20% of boys aged 11-14 years are also at risk of low intakes.
There is no evidence that large dietary intakes are harmful to humans with normal kidney function. However, regular intake of high dose supplements can result in diarrhoea and may also result in raised blood levels of magnesium with associated adverse effects.
Magnesium is present in both plant and animal cells and is the mineral in chlorophyll, the green pigment in plants, and so is widely available. Sources include green leafy vegetables, nuts, bread, fish, meat and dairy products.
Sodium is responsible for regulating body water content and electrolyte balance. The control of blood sodium levels depends on a balance between sodium excretion and absorption at the kidneys, which is regulated by nerves and hormones. Sodium is also required for the absorption of certain nutrients and water from the gut. Sodium is a component of common salt, known as sodium chloride (NaCl).
As with some other minerals, sodium levels in blood and tissues are under homeostatic control. The kidneys tightly regulate sodium concentration and can make the urine almost salt-free or excrete sodium in urine when supply is excessive. Sodium intakes in the UK are considered to be too high and so deficiency of sodium is unlikely but under some circumstances losses can occur:
- Excess sweating:, e.g. due to exercise in a hot environment, may cause some sodium depletion.
- Diarrhoea can cause fluid loss and dehydration leading to some sodium depletion.
- The kidneys normally act to protect the body’s stores of sodium, but in Addison’s disease failure to produce aldosterone (hormone that allows the kidneys to retain sodium and water) leads to the kidneys inability to conserve sodium.
- Renal failure: The kidneys may also lose sodium in some types of renal failure.
Drugs: Diuretic drugs may remove large amounts of sodium in the urine.
High sodium intakes, along with obesity and high alcohol intake, are considered to be among the risk factors for high blood pressure (hypertension), which is a risk factor for cardiovascular disease and stroke. A low salt diet may be used in the treatment of hypertension.
Most raw foods contain very small amounts of sodium chloride (salt). But salt is often added during the processing, preparation, preservation and serving of foods. The Food Standards Agency’s 2008 urinary sodium survey assessed salt intakes in the general adult population in the UK and showed that some progress had been made towards the 6g/day target for adults.. The survey showed a reduction in the UK’s average daily salt consumption from 9.5g to 8.6g since the National Nutrition and Diet Survey (NDNS) in 2000/01. Work continues to reduce the amount of salt present in the food supply. To date much of the emphasis has been on foods sold through supermarkets but food consumed outside the home is also beginning to be targeted by the FSA’s activities.
In May 2009, the FSA published its revised salt targets for 2010 and 2012, following a review of the existing targets during 2008. The FSA emphasised the reductions that had already been achieved across the food industry: the average amount of salt in pre-packed bread is down by a third; there have been reductions of the order of 44% in the salt content of branded breakfast cereal; in 2007 alone there was a 13% reduction in the salt content of standard crisps, 32% in extruded snacks and 27% in pelleted snacks (some standard crisps have been reduced by up to 55%); 30% reductions in popular brands of cooking and pasta sauces and 25% in leading brands of soups; and salt in cheeses has been reduced e.g. 50% less in a range of soft white cheeses and 32% in some standard cheese slices. In addition retailers have achieved salt reduction across a wide range of own brand products.
The public also has a role to play in restricting the addition of salt to their food in the kitchen and at the table. About 20% of salt consumed is added at home during cooking and at the table.
For more information on salt: http://www.salt.gov.uk/
Potassium is essential for water and electrolyte balance and the normal functioning of cells, including nerves. Increased dietary intakes of potassium have been associated with a decrease in blood pressure, as it promotes loss of sodium in the urine. It is suggested that an increase in potassium intakes may offset the impact of some of the sodium in the diet, therefore helping to protect cardiovascular health.
Low blood potassium levels (hypokalaemia) can result from severe diarrhoea. Symptoms include weakness, mental confusion and, if extreme, heart failure.
Low dietary potassium intakes have been observed in the UK: in the NDNS of adults about 1 in 5 women had intakes below the LRNI and in common with some other minerals, potassium intakes were lower among younger women. In the NDNS of young people, 10–15% of boys had intakes below the LRNI but among girls, about 1 in 5 11-14 year olds and about 2 in 5 15–18 year olds had intakes below the LRNI.
High supplementary doses of potassium can be harmful especially if the kidneys are not functioning properly.
Potassium is present in almost all foods but fruit (particularly bananas), vegetables, meat, fish, shellfish, nuts, seeds, pulses and milk are useful sources. Processed foods typically contain less than raw foods.
Iron is essential for the formation of haemoglobin in red blood cells; haemoglobin binds oxygen and transports it around the body. Iron is also an essential component in many enzyme reactions and has an important role in the immune system. In addition, it is required for normal energy metabolism and for the metabolism of drugs and foreign substances that need to be removed from the body.
A lack of dietary iron depletes iron stores in the body and this can eventually lead to iron deficiency anaemia. In particular, women of child bearing age and teenage girls need to ensure they consume adequate dietary iron because their requirements are higher than those of men of the same age. Also, loss of blood due to injury or large menstrual losses increases iron requirements in the short term. Data from the NDNS indicate that average daily iron intakes from foods are below the RNI for women in all age groups, except for older women (over the age of 54 years). A very significant proportion of younger women (2 out of 5) have intakes below the lower reference nutrient intake (LRNI) i.e. intakes that are likely to be inadequate.
Currently, there are no recommendations for increasing iron intake during pregnancy as the extra demand should be offset by pre-existing body stores, lack of menstrual blood loss and the increased intestinal absorptive capacity of the mother during the second and third trimesters of pregnancy.
More than 2 billion people worldwide suffer from iron deficiency anaemia, making it the most common nutritional deficiency condition.
As with some other minerals, under normal circumstances absorption of iron is tightly controlled as iron can have adverse effects owing to its ability to generate oxygen free radicals. However, 1 person in 200 of northern European descent is genetically predisposed to the iron loading disease haemchromatosis.
Dietary iron is found in two basic forms. Either as haem iron (from animal sources) or non-haem iron (from plant sources). Haem iron is the most bioavailable form of iron. However, the predominant form of iron in all diets is non-haem iron, found in cereals, vegetables, pulses, beans, nuts and fruit. Absorption of non-haem iron is affected by various factors in food. Phytate (in cereals and pulses), fibre, tannins (in tea) and calcium can all bind non-haem iron in the intestine, which reduces absorption. However, vitamin C, present in fruit and vegetables, aids the absorption of non-haem iron when eaten at the same time, as does meat.
Liver, red meat, pulses, nuts, eggs, dried fruits, poultry, fish, whole grains and dark green leafy vegetables are all sources of iron. Since the 1950s in the UK, all wheat flours (other than wholemeal) have been fortified with iron and many breakfast cereals are also fortified with iron and so contribute to iron intake, However, the nature of these foods imposes limitations on the type of iron that can be used as a fortificant and so low bioavailability may be an issue, as suggested in the recent draft report on iron from SACN here.
A draft review of iron and health has recently been published by the Government’s advisory committee, SACN here.
The major function of zinc in human metabolism is as a cofactor for numerous enzymes. Zinc has a key role as a catalyst in a wide range of reactions. It is directly or indirectly involved in the major metabolic pathways concerned with protein, lipid, carbohydrate and energy metabolism and is also essential for cell division and, therefore, for growth and tissue repair and for normal reproductive development. In addition, zinc is required for the functioning of the immune system and in the structure and function of the skin, and hence plays a vital role in wound healing.
In some countries, delayed puberty and small stature have been linked to zinc deficiency, though it is not certain that this is due to zinc deficiency alone.
Excess zinc in the body from very high doses can interfere with copper metabolism.
Zinc is present in many foods and is most readily absorbed from meat, which provides about a third of zinc in the UK diet.. It is also present in milk, cheese, eggs, shellfish, wholegrain cereals, nuts and pulses. For cereals and pulses, zinc’s availability is limited by phytates.
Iodine is an essential component of the thyroid hormones, thyroxine and triidothyronine, which are vital regulators of metabolic rate and of physical and mental development.
Iodine deficiency results in lethargy and swelling of the thyroid gland in the neck which forms a goitre. Iodine deficiency is relatively rare in the UK but is still prevalent in many areas of the world, where it remains a major nutritional public health issue.
Infants born of severely iodine deficient mothers may be mentally retarded (cretinism).
With regard to low dietary intakes in the UK, data from the NDNS indicate that 12% (1 in 8) young women have intakes below the LRNI. This may be associated with low intakes of milk and milk products.
Excess iodine is not absorbed so toxicity is unlikely in healthy individuals.
The amount of iodine in plant foods such as vegetables and cereal grains is determined by the amount of iodine in the growing plant’s environment, and the amount in the soil or water can vary dramatically. The only rich sources of iodine are seafoods (sea fish, shellfish and seaweed), but milk is also a source. In some countries certain foods, e.g. salt and bread, are fortified with iodine.
The main function of fluoride in the body is in the mineralisation of bones and teeth. Fluoride also protects the teeth from dental caries (tooth decay) and is now routinely added to most toothpastes.
In rare cases, very large amounts of (non-dietary) fluoride can cause fluorosis. Symptoms may be mild such as mottling and crumbling of the teeth, or more severe causing skeletal changes such as calcification of ligaments and tendons which leads to muscle, joint and bone problems.
Fluoride is found in fluoridated water, tea and fish. The diet provides only about 25% of total intake. The addition of fluoride to toothpaste is important in those areas where the water supply is low in fluoride.
Copper is the third most abundant dietary trace metal after iron and zinc. It is a component of many enzymes and is needed to produce red and white blood cells. The body also needs copper to utilise iron efficiently and it is thought to be important for infant growth, brain development, the immune system and for strong bones.
Dietary induced copper deficiency is extremely rare due to the plentiful supply in the diet and the high efficiency of absorption. However, a rare genetic condition, known as Menke’s disease, results in the inability to absorb copper and leads to severely impaired mental development, failure to keratinise hair and skeletal and vascular problems.
As with some other minerals, under normal circumstances absorption of copper is tightly controlled so overload of copper is very rare. However, Wilson’s disease, another genetic condition, leads to the inability to excrete excess copper in bile and results in copper accumulation in the body, especially the liver and brain, with consequent pathological damage.
Sources of copper include shellfish, liver, kidney, nuts and wholegrain cereals (about a third of intake in the UK is from cereals).
The main function of selenium is as a component of some of the important antioxidant enzymes (e.g. glutathione peroxidase), and therefore to protect the body against oxidative damage. It is also necessary for the use of iodine in thyroid hormone production, for immune system function and for reproductive function.
The best characterised selenium deficiency condition is Keshan disease, a heart condition that affects children and women of child-bearing years in rural China where soils are deficient of selenium, leading to continuing low levels in the food chain.
Selenium intakes in the UK are below the DRVs but the implications of this are uncertain because of the lack of reliable biomarkers for selenium status and requirements.
In excess selenium is exceedingly toxic. Symptoms of selenosis (selenium excess) include brittle nails and hair, skin lesions and garlic odour on the breath.
Selenium is found in a variety of foods, especially Brazil nuts, bread, fish, meat and eggs. The selenium content of cereals is directly proportional to the selenium content in the soil. In the UK selenium intakes have fallen with the decline in import of North American selenium-rich wheat and the increased use of European cereals which are less rich in the mineral. However, some bread manufacturers still import wheat from North America.
Manganese is required for bone formation and for energy metabolism. It is also a constituent of an antioxidant enzyme, which helps prevent free radical-mediated damage to cells.
Manganese deficiency is rarely seen.
Manganese toxicity is not a problem because blood levels are carefully controlled.
It is present in plant foods such as vegetables, cereals and nuts. Tea is also a rich source. In the UK, 50% of manganese intake is derived from cereals and cereal products.
Chromium (III) is the active form of this nutrient and its main functions appear to be linked with carbohydrate and lipid metabolism. This form of chromium is thought to promote the action of insulin, the hormone which controls glucose levels in the blood. Subjects with adequate dietary chromium have improved control over blood glucose and a better blood lipid profile.
One significant characteristic of chromium deficiency is impaired glucose tolerance, which can be improved by chromium supplementation. However, chromium supplementation does not improve insulin action for people who were not initially deficient.
Chromium is not known to show toxicity.
Sources of chromium include meat, nuts, cereal grains, brewer’s yeast and molasses.
Other trace elements
There are other minerals which are needed in tiny amounts and which appear to be essential in the diet, e.g. molybdenum, boron. Others occur in the diet, but whether they are essential is unclear, e.g. nickel, lithium, antimony, aluminium and lead.
For more information on trace elements see the report from the Expert Group on Vitamins and Minerals on Safe Uppler Levels for Vitamins and Minerals (2003): http://www.food.gov.uk/multimedia/webpage/vitandmin/
Page created July 2009