Mineral nutrition and metabolism
This chapter focuses on mineral nutrition and metabolism. Although carbon, hydrogen, oxygen, and nitrogen together account for 96% of the body weight, at least 24 other elements are present; as calcium and phosphorus account for about 2–5%, the rest must be present in very small amounts. However, the quantity in which an element is present neither gives any indication of its biochemical importance nor does it bear any close relationship to dietary requirements. The functions of the mineral elements are many and various. Some act as essential structural components, some fulfil a catalytic or regulatory role, while some do both. Most proteins contain sulfur, some contain phosphorus, some iron, and one, thyroglobulin, contains iodine. Other elements, notably sodium, potassium, chlorine, and phosphorus, play an essential part in the maintenance of osmotic and acid/base balance, and many ions are components of enzyme systems. Minerals are constantly being lost from the body, chiefly in the urine, and must continually be replaced. The approximate amounts of the seven principal minerals that are required daily to keep the adult human body in balance are sodium, potassium, calcium, magnesium, chlorine, phosphorous, and sulfur.
Although carbon, hydrogen, oxygen and nitrogen together account for 96% of the body weight (Table 11.1), at least 24 other elements are present and, since calcium and phosphorus account for about 2·5%, the rest must be present in very small amounts. However, the quantity in which an element is present gives no indication of its biochemical importance, nor does it bear any close relationship to dietary requirements.
|%||Amount per 70 kg|
The functions of the mineral elements are many and various. Some act as essential structural components, others fulfil a catalytic or regulatory role, while some do both. Calcium and phosphorus are notable examples. Most proteins contain sulphur, some contain phosphorus, some iron and one, thyroglobulin, contains iodine. Other elements, notably sodium, potassium, chlorine and phosphorus, play an essential part in the maintenance of osmotic and acid/base balance and many ions are components of enzyme systems.
Of the 26 elements known or claimed to be essential for animal organisms, 11 are considered as major elements, namely: C, H, O, N, S, P, Ca, K, Na, Mg, CI. Two others, namely iron and zinc, are present in small but appreciable amounts. The remaining 13 elements are found only in trace amounts and some of these have not been conclusively shown to be essential. These 13 elements are: I, Cu, Mn, Co, Mb, Se, Cr, Ni, Sn, Si, Vd, As and F.
Evidence of a requirement for the last six rests on the effects on the growth and reproductive ability of experimental animals fed purified diets and reared in conditions in which atmospheric contamination by these elements was minimized.
Minerals are constantly being lost from the body, chiefly in the urine, and must continually be replaced. The approximate amounts of the seven principal minerals that are required daily to keep the adult human body in balance are:
|Sodium 6·0 g||Chlorine 9·0 g|
|Potassium 4·0 g||Phosphorus 0·8 g|
|Calcium 0·6 g||Sulphur 1·0 g|
|Magnesium 0·35 g|
A good average diet should supply these without difficulty but some mineral constituents are inefficiently absorbed. The minerals which are nutritionally of most importance because their intake may be less than the requirements are calcium, iron, iodine and fluorine. However, defective absorption, resulting from general malnutrition or a variety of other causes, may be responsible for secondary deficiences of substances present in the diet in adequate amounts.
Salt, i.e. sodium chloride, an indispensable body constituent, is present in most natural foods but whether humans get enough from these sources or need additional amounts is a matter of dispute. Certain primitive tribes survive on what is naturally present in their food but, on the other hand, wars have been fought over sources of salt, and for centuries its trade was more important than that of any other commodity. Efficient mechanisms exist within the body of normal subjects both for the conservation and for the excretion of salt and a wide range of intakes is compatible with the health of such individuals. There is undoubtedly a minimum requirement for salt for the maintenance of electrolyte balance because, although its excretion in the urine can be reduced to an almost negligible amount, salt is also lost in sweat. At the other extreme, provided it is accompanied by an adequate intake of water, large amounts of salt can be excreted by normal subjects so that how much, if any, extra salt one adds to one’s food resolves itself into a matter of taste.
The current suggestion that a high salt intake leads to hypertension and cerebrovascular disease is largely based on epidemiological evidence. Any such relationship is complicated by the fact that blood pressure is known to be influenced by many other factors, e.g. body weight, temperament, physical activity, stress, external temperature and smoking habits. Nevertheless, comparisons of populations show a general correlation between the average salt intake and the blood pressure level. The Japanese consume more salt than any other nation and have the highest incidence of hypertension and cerebrovascular disease, although not of ischaemic heart disease. However, within the various populations some individuals are much more sensitive to salt than others. This is also true for rats, some, but not all of which, develop hypertension in response to a high-salt diet. Salt sensitivity and essential hypertension in humans are believed to be caused by an inherited renal abnormality that makes it difficult for the kidneys to excrete Na+ and this difficulty is compounded as the salt intake rises. Restriction of the sodium intake significantly reduced the arterial blood pressure of about 60% of people suffering from essential hypertension suggesting that a high salt intake does not always account for the development of this condition. It seems possible that, at least in some instances, hypertension may result from a deficiency of the newly discovered natriuretic hormones. These are a series of peptides which have been isolated from the atria of the heart and which cause a marked increase in sodium and water excretion. In the rat the predominant peptide contains 28 amino acid residues and is known as cardionatrin I. The human form differs from this by only one amino acid.
Whereas a high intake of sodium seems in some subjects to promote the development of high blood pressure, a high intake of potassium has the reverse effect and it is now thought that the Na/K ratio may be of greater significance than the Na intake alone.
According to the NACNE Report (1983) the average intake of sodium chloride in the UK is about 12 g per day which is in excess of the requirement even of people who undertake strenuous physical exercise and lose large quantities by sweating. The Report recommends that the average intake should be reduced to half or even a quarter of this amount since a high salt intake has not been shown to be of any benefit. There seems little doubt that hypertensive subjects should reduce their salt intake but whether it is necessary or desirable for others to do so is open to question. If an individual likes salty food, has no reason to believe he or she is at risk of hypertension and maintains an adequate water intake, there is no compelling evidence to suggest that he or she would benefit from a reduction in salt intake. An increased consumption of fruit and vegetables and the sparing use of foods and manufactured products which have a high sodium content would, however, result in a decrease in the Na/K ratio of the diet which might have a health-promoting effect. (It is important to distinguish between the salt (NaCl) intake and the sodium (Na) intake: 12 g of NaCl = 4·7 g of Na.)
The importance of calcium in body structure and as a participant in and regulator of body processes can hardly be overestimated. Calcium is no less important from the nutritional point of view. Because of (1) the large reserve of calcium in the bones and its ability to maintain a long-term buffering effect against calcium loss, (2) the difficulties inherent in calcium absorption which are discussed below, and (3) the ability of the body to adapt to a low calcium intake, there is considerable controversy regarding calcium requirements. On the basis of balance studies in which the dietary intake was compared with the amount of calcium lost via the kidneys, intestine and skin, the recommended daily intake was for many years given as 1 g. However, as a consequence of global surveys the World Health Organization reduced this figure to 0·5–0·6 g for adults, infants and older children and recommended an intake of 1·0–1·2 g during pregnancy and lactation.
Special attention needs to be given to the requirements of child-bearing women, since the mother’s skeleton yields up the calcium that the fetus and young infant require and the effect of repeated pregnancies on a woman receiving inadequate amounts of calcium and vitamin D can lead to osteomalacia (page 157). There is, however, no convincing support for the view that in normal well-fed women the teeth tend to become decalcified during pregnancy and lactation or that they are more prone to dental decay. Thus the maxim ‘for every child a tooth’ is not well founded.
A puzzling feature of calcium metabolism is that adaptation to much lower dietary calcium intakes than those which are generally believed to be desirable seems to be possible. Thus growing children in Sri Lanka were found to maintain a positive calcium balance on intakes of about 200 mg per day. Furthermore, experiments in animals have shown that, on a low calcium diet, calcium is absorbed more efficiently and the concentration of intestinal calcium-binding protein (page 445) increases. Whether, at the same time, calcium excretion is reduced has not been established.
Few foods are rich sources of calcium although fish such as whitebait and sardines, the backbones of which can be eaten, may provide up to 400 mg/100 g. Otherwise the best sources are milk and milk products such as cheese (Table 11.2). Green vegetables, cereals and pulses also contain appreciable amounts, but their calcium is less well utilized than that of milk. Since cow’s milk contains about 0·12 g of calcium per 100 ml, half a litre or 1 pint (568 ml) should provide the daily requirement. Calcium deficiency is thus mainly a hazard in countries where milk and cheese are not regularly consumed.
|Wholemeal bread (100% extraction)||25|
|White bread (70% extraction fortified)||100|
Elderly people, particularly women, tend to develop the condition of osteoporosis in which complete loss of bone tissue occurs in small areas within the bones which become porous and brittle. There is no clear-cut evidence of a relationship between the calcium intake and the rate of bone loss in humans. Nevertheless, it would be unwise to let the calcium intake fall below the recommended level. The practice of adding calcium salts to flour to minimize the likelihood of calcium deficiency would seem to be a sound one, particularly in view of the negative association between the hardness of water and mortality from cardiovascular disease.
Phosphorus is closely bound up with calcium from both metabolic and nutritional viewpoints. Phosphorus occurs abundantly in plant and animal tissues and, if the other nutritional requirements are satisfied, the diet should contain adequate amounts. Meat, eggs, dairy products and cereals are all good sources.
The absorption of calcium from the gut is always far from complete. About 70% of that ingested is excreted in the faeces. Many calcium salts are insoluble at physiological pH values and tend to be precipitated in the gut lumen. Spinach and rhubarb, although they contain appreciable amounts of calcium, may have an adverse effect on the calcium balance since their excess oxalate may precipitate calcium derived from other foods.
Phytic acid, which is present in cereal grains and is rich in phosphate, also tends to form insoluble salts with calcium, magnesium and iron and to render them unavailable. Since it is concentrated in the outer husk of the grain, high extraction flours (page 175