A liquid which occurs in different physical forms at different temperatures between freezing, liquid forms and gaseous steam and in association with polymers eg proteins. Water provides bulk and is a solvent for the body and is an acceptor and donor of protons.
Water is the basic chemical of life which readily dissolves a number of chemicals. Such solubility is important in biological processes, cell structure, blood and excretory systems, e.g. urine, bile. Water also allows for flexibility and movement of the body.
The body is a complex phase system with differing osmolality and solubilities. The movement of substances between these phases is important for water balance , metabolism and physiology. Water forms 50–60% of body weight. One-third is extracellular fluid, two-thirds are intracellular. These compartments are separated by cell membranes, often freely permeable to water movement, dictated by osmolality. It is essential to well-being that the body contains an appropriate amount of water. Insufficient or too much water or reduced or excessive loss result in I’ll health. Such imbalances can occur both from natural or pathological causes.
Humans satisfy their requirements for water by drinking habits which are not necessarily dependent on true thirst. The efficient thirst mechanism and regulation of the water retention depends upon the antidiuretic hormone (ADH, vasopressin) . There is also a well learnt, almost reflex taught from birth to eat and drink
Water intake includes fluid drunk and the water in food.in addition there is metabolic water produced by the oxidation of carbohydrates, protein and fat:
• 1 g starch produces 0.60 g water
• 1 g protein produces 0.41 g water
• 1 g fat produces 1.07 g water
Fluid intake is usually of the order of 2–2.5 litres/day for the average adult in a temperate climate. This intake will be affected by temperature, activity, diet and health.
During the first 2 days, daily urine output is of the order of 20 ml. By 2 weeks, daily output is 200 ml, with a milk intake of 500 ml. At 3 months the daily milk intake is 800–900 ml and urine output 300 ml. Loss by evaporation is high because of the high surface area : body weight ratio.
It is important to monitor fluid intake in the elderly as fluid requirements are unchanged.
MINERALS AND ELECTROLYTES —
To convert mmol to mg , multiply mmol by the molecular weight.
– Atomic weight 23 valency 1.
Sodium is soluble in water , is usually found as a salt with chlorine , the common sodium chloride.
The body distribution of sodium differs from that of potassium, the body pools are, however similar. Sodium is a major cation and contributor to the osmolality of the extracellular fluid of the body. Sodium is important in the transport of chemicals across cell membranes. Sodium and potassium are exchanged across nerve membranes; during nerve conduction.
The requirement for salt, or rather thirst sensation, is controlled by the adrenocortical hormone (ADH ) and renal responses to changes in plasma sodium concentration. Hyponatraemia reduces ADH secretion, which is followed by renal loss of water and correction of the imbalance. This results in thirst and an increased water intake. Changes in sodium concentration result from changes in water intake rather than the Sodium is lost in urine, sweat and minimally in faeces under a variety of physiological and pathological circumstances eg diarrhoea, heat exposure.
A commonsource of sodium, sodium chloride in the diet is table salt, added to food at the table, in cooking, and in processed foods. Sodium occurs naturally in most foods. Milk, beets, and celery also naturally contain sodium, as does drinking water, although the amount varies depending on the source.
Salt is an important food preservative The sodium content of natural food varies between 0.1 to 3.3 mmol/100 g. In contrast, processed foods have a sodium content of 11–48 mmol/100 g, partly for taste, and in part because sodium nitrate is used as a preservative.
Sodium is also added to various food products. Some of these added forms are monosodium glutamate, sodium nitrite, sodium saccharin, baking soda (sodium bicarbonate), and sodium benzoate. These are ingredients in condiments and seasonings such as Worcestershire sauce, soy sauce, onion salt, garlic salt, and bouillon cubes.
Processed meats, bacon, sausage, and ham, and canned soups and vegetables are all examples of foods that contain added sodium. Fast foods are generally very high in sodium.
Dietary sodium is usually measured in milligrams (mg). Table salt is 40% sodium; 1 teaspoon of table salt contains 2,300 mg of sodium. The dietary intake of sodium varies between populations (100–200 mmol/day in Britain).
The recommended dietary intake is not universally agreed; suggested figures include Adults 70 mmol/day..
Infants up to 6 months, based on calculations for breast fed babies 9–12 mmol/day. Human populations can survive at extremes of sodium intake from less than 0.2 g (10 mmol)/day of sodium in the Yanomamo Indians of Brazil to over 10.3 g (450 mmol)/day in Northern Japan. Indicating the ability of the normal human body to conserve sodium by markedly reducing losses of sodium in the urine and sweat. Under conditions of maximal adaptation and without sweating, the minimal amount of sodium required to replace losses is estimated to be no more than 0.18 g (8 mmol)/day
Healthy adults should however limit sodium intake to 2,300 mg per day whilst individuals with high blood pressure should consume no more than 1,500 mg per day. Those with congestive heart failure, liver cirrhosis, and kidney disease may need much lower amounts.
Specific recommendations regarding sodium intake do not exist for infants, children, and adolescents. Eating habits and attitudes about food formed during childhood are likely to influence eating habits for life. For this reason, moderate intake of sodium is suggested
Severe diarrhoea is or sweating in high temperatures is a cause of severe sodium loss. Oral replacement solutions i.e. water : glucose, sucrose or rice powder (30 g/l) and sodium 90 mmol/l or 30 mmol/l in infants should be used to replenish sodium levels. Endurance sports eg marathon running can result in massive sodium losses through sweat.
Excess Too much sodium may lead to high blood pressure in those who are sensitive to sodium, this has lead to recommendations to reduce dietary sodium (salt) intake.
Sodium may lead to a serious build-up of fluid in people with congestive heart failure, cirrhosis, or kidney disease. Such people should be on a strict sodium-restricted diet, as prescribed by their doctor.
– atomic weight, 39; valency 1
Potassium is a mineral involved in electrical and cellular body functions, the regulation of acid-base balance,protein synthesis , carbohydrate metabolism, muscle and for normal body growth and the normal electrical activity of the heart and other contractile organs.
The extracellular : intracellular potassium ratio is important in the membrane potential difference in cells. Most of the potassium is in the cell fluid compartment.
The body homeostasis of potassium is controlled by renal glomerular filtration and tubular secretion. Proximal tubular reabsorption is partially an active process and is complete by the end of the proximal segment.
Potassium is lost in urine or faeces under a variety of physiological and pathological circumstances.
Dietary potassium intake is from fresh vegetables and meat. In natural and processed foods the potassium content varies from 2.8 to 10 mmol/kg.
Many foods contain potassium. All meats (red meat and chicken) and fish such as salmon, cod, flounder, and sardines are good sources of potassium. Soy products and veggie burgers are also good sources of potassium. Vegetables including broccoli, peas, lima beans, tomatoes, potatoes (especially their skins), sweet potatoes. Fruits that contain significant sources of potassium include citrus fruits, cantaloupe, bananas, kiwi, prunes, and apricots.
Milk and yogurt, as well as nuts, are sources of potassium.
Infants – 0.4 grams to 0.7g/day
Children and Adolescents – 3 to 4g/day
Adults – 5g/day
Women who are producing breast milk need 1-2 g more/day.. Insufficiency
Many foods contain potassium, so potassium deficiency is rarely caused by inadequate diet. However, even a moderate reduction potassium levels can lead to salt sensitivity and high blood pressure. The recommended dietary intake of 4.7 gm or higher can slightly lower blood pressure.
A deficiency of potassium (hypokalemia) can occur in excessive vomitting and diarrhea and rare kidney and adrenal gland disorders . Commoner causes result from taking diuretics , laxatives, and steroids which occasionally may be very severe.
Common causes of hyperkalaemia are reduced renal function, an abnormal breakdown of protein, and severe infection. The most common cause of hyperkalemia is reduced kidney function, especially in people receiving dialysis for kidney failure. Certain medicines affect the body’s ability to get rid of potassium and include potassium sparing diuretics and angiotensin converting enzyme (ACE) inhibitors.
CALCIUM. – atomic weight, 40; valency, 2 .
Function Calcium is concentrated in the body in organelles and blood. Calcium is very important in the structure of the skeleton and to ensure an adequate extracellular fluid calcium concentration. A positive calcium balance is required before growth can proceed. Calcium intake and skeletal modeling and turnover determine calcium balance during growth.
Hormonal and pharmacological activation of cells, membrane function and enzyme activity may all be affected by the local concentrations of calcium. Calcium is bound within the cell to enzyme proteins, this binding alters the protein configuration and hence enzyme activity. Calcium has many roles, including that of an intracellular messenger.
The major regulating hormones for calcium are parathyroid hormone, calcitonin and vitamin D. Growth hormone, thyroid hormone, adrenal steroids, sex hormones and some gastrointestinal hormones are involved but to a lesser extent.
The most important source of dietary calcium is milk, which may provide over half of the required intake. The calcium content of milk is 35 mg/100 ml for human milk and 120 mg/100 ml for cows’ milk. Other important sources of calcium include cheese (hard 400–1200 mg/100 g, soft 60–75 mg/ 100 g).
Within Great Britain, the average daily calcium intake for adults is 940 mg (24 mmol) for men and 730 mg (18 mmol) for women. The average intake varies from country to country (range 350–1200 mg; 9–30 mmol), the range being influenced by milk intake and food fortification policies.
The skeleton of a newborn infant contains 25 g of calcium and, during the first years of life, the calcium intake increases quite quickly. In the first year, at least 6 mmol/day (240 mg) is required.
Calcium requirement for increases between the ages of 1 and 10 years,7mmol/day (280 mg) and 10.6 mmol/day (425 mg) at age 10 years.
6.3 mmol/day (250 mg) for girls and 7.5 mmol (300 mg) for boys
There are demands on the maternal skelton and calcium for calcium during pregnancy compared with non-pregnant women. However, if pregnancy occurs in adolescence, then the growth requirements of both the mother and foetus require a doubling of calcium provision.
A lactating mother will secrete some 150–300 mg (48 mmol) of calcium. Approximately an additional 14 mmol/ day (550 mg) of calcium may be required during lactation.
A sustained deficiency of dietary calcium can result in osteomalacia, a loss of calcium in the bone
A reduction in plasma calcium results in tetany, which is due to hyperactivity of the motor muscles. This results in facial spasm, spasm in the wrist, hands and feet.
– atomic weight, 56, valency 2, 3
Iron occurs in two forms, ferrous and ferric, which interchange: Fe2+ Fe3+ + e–
Iron is central to oxygen metabolism in that 2.5 g circulate in haemoglobin and 0.3g are present in myoglobin. Haemoglobin has a molecular weight of 64 .5 k Da and is formed from four haem groups which are linked to four polypeptide chains. Divalent iron (Fe2+) in haem reversibly binds oxygen for transport to tissues, while oxidation of iron to the ferric (Fe3+) state in methaemoglobin causes haemoglobin to lose its capacity to carry oxygen. Iron is present in all cells of the body and plays a key role in many biochemical reactions.
The iron must be readily available for the synthesis of essential iron proteins, e.g. haemoglobin, myoglobin and enzymes, e.g. catalase
There are many important sources of iron including meat, meat products, cereals, vegetables and fruits. Milk is a poor source of iron, black pudding rich in iron.
Iron intakes in Britain average 14 mg/day for men and 12 mg/day for women.
Infants and children
Breast-fed infants are capable of absorbing 50% of the iron present in mother’s milk, whereas iron absorption from formulated milks may be only 10%. Infants requirements at 0–3 months is 0.9 mg/day; the estimated average requirement is 1.3 mg/day By the age of puberty the intake of iron should be 1 mg/day.
Median and mean losses of blood in menstruation are 30 and 44 ml, respectively. The calculated loss of iron is therefore approximately 20 mg, averaging 0.7 mg/day. In Britain the average iron intake among fertile women is in the order of 12 mg/day.
Women on poor diets can become profoundly anaemic unless attention is paid to iron intake.
It has been estimated that the toll of pregnancy on the mother’s iron stores is in the order of 700 mg. This should be met by pre-existing body stores
Breast milk contains 7 mmol/litre at 6–8 weeks postpartum, which decreases thereafter to 5 mmol/litre during weeks 17 to 22. The total loss is of the order of 5–6 mmol/day. This secretion will be balanced by the amenorrhoea generally associated with lactation.
Iron deficiency occurs with loss of blood in menstruation, insufficient iron stores (as in the premature baby), or loss from lesions in the gastrointestinal tract. Bleeding from duodenal ulcers and colonic cancers are potent causes of iron deficiency anaemia. In pregnancy, iron deficiency is associated with low birth-weight babies and perinatal mortality. Iron deficiency results in anaemia and impaired psychomotor development and reduced cognitive function in the young and during growth. Iron deficiency also reduces work performance (particularly endurance work).
A nutritional iron overload can occur when the dietary iron intake exceeds 40 mg/day.
TRACE ELEMENTS —
Trace elements are present in the diet in less than milligram amounts.
Trace elements are important in the body by virtue of:
• establishing potential differences across membranes; such differences have to be maintained by energy-requiring reactions, e.g. Na+, K+, Ca2+
• allowing oxidation-reduction (redox) enzymatic reactions to take place, e.g. iron, copper
• acting as cofactors in enzymatic reactions, e.g. zinc
• maintaining the structure (conformation) of proteins, e.g magnesium
Trace elements are involved in skeletal structure, e.g. fluorine, magnesium and phosphorus in bone and teeth, and silicon in cartilage.
• maintaining the structure of hormones, e.g. iodine in thyroxine and vitamins eg cobalt in B12
Trace elements are required in small amounts, though their concentrations in the diet will vary. In communities totally dependent upon locally grown food the trace element content of the local soil and drinking water is of great nutritional importance. /span>
The amount and chemistry of dietary inorganic constituents affect the efficiency of absorption of the essential elements. An example is the inhibition of calcium and trace metals absorption, e.g. zinc by dietary phytate.
Some trace elements are toxic, e.g. lead, aluminium, mercury and arsenic. These may reduce intellect in the young and elderly. The function of some trace elements has still to be established, e.g. vanadium.
INDIVIDUAL TRACE ELEMENTS
– atomic weight, 27; valency, 3.
The only biological role for aluminium is as part of the succinic dehydrogenase-cytochrome C system.
Aluminium is found in tea and orange juice, herbs, processed cheeses, baking powder and pickles.
Of lesser risk are cooking pans made from aluminium.
Nutritionally not defined
But; Aluminium might be involved in the development of Alzheimer’s disease dementia and is present in increased amounts in the brains of sufferers. Drinking Water with a concentration of aluminium more than 0.10-0.20 mg/ liter is associated with an increased risk of Alzheimer’s. The intake of aluminium in USA is 7-9 mg / day.
– atomic weight, 122; valencies, 3,,
Antimony does not appear to be an essential element.
Dietary Intake 2-10 μmol/day
– atomic weight, 75; valencies, 3, 5,
Arsenic is present in many plant and animal foods but is not an essential element in the diet.
There is widespread contamination of water wells both in Bangladesh and parts of India
Arsenic is extremely toxic and an upper limit of tolerable daily intake is 2 μg/kg body weight.
– atomic weight, 11; valency, 3,
Boron acts as a ligand and forms complexes with organic compounds such as sugars, polysaccharides, adenosine-phosphate, pyridoxine, riboflavin, dehydroascorbic acid, pyridine nucleotides and steroid hormones.. There is no evidence of a role in animal metabolism.
Human diets provide 2 mg/day. Toxic effects are experienced with intakes 50 times this amount.
– atomic wt 80, valency 1,3
Bromine has no known physiological function, but is found widely in the environment. Bromine is concentrated in the thyroid gland. Bromine formerly was used as a sedative and by the Army to reduce sexual drive in soldiers.
– atomic weight, 112; valency ‘2
Cadmium is not an essential nutrient. Cadmium exposure results from industrial waste or from inhaling cigarettes smoke contaminating water, so that small amounts are absorbed, accumulate in tissues.
During pregnancy smoking increases cadmium uptake which could affect the foetus.
– atomic weight, 52; valencies, 2, 3,
The role of chromium in human nutrition is uncertain, but may act in an organic complex which influences and extends the action of insulin.
Chromium is present in all organic matter. Wheat (1.8 μg/g) and wheat-germ (1.3 μg/g) are especially good sources of chromium, as is molasses (1.2 μg/g)
a reasonable intake for adults would be 0.5 μmol/day and between 2 and 19 nmol/kg/day for children and adolescents.
– atomic weight, 59;valencies, 2,3, Function
Cobalt is a constituent of vitamin B12, the only biological function of the element.
Wheat, especially wholemeal flour (0.5–0.7 μg/g), and seafoods (1.6 μg /g) are good sources of cobalt.
Average intakes of cobalt are approximately 0.3 mg/day and the total body content about 1.5 mg.
Cobalt can have serious toxic effects, including causing goitre, hypothyroidism and heart failure.
– atomic weight, 64; valencies, 1,2,
Copper is an important component of enzymes including cytochrome oxidase and superoxide dismutase. Copper may also have a role in iron metabolism, when Fe2+ released from ferritin is oxidised to Fe3+ for binding to transferrin.
Sources of copper include green vegetables, fish, oysters and liver, at approximately 4 μmol/kcal
The normal adult diet provides 1.5 mg/day.
The Requirement for an infant is 0.3 mg/day. For children, is 0.7–1.0 mg/day and for adults of 1.2 mg/day is required.
Pregnancy: Increasing requirements during the first, second and third trimesters necessitate respectively, 0.033, 0.063 and 0.15 mg/day.
Lactation, an increment of the order of 0.4 mg/day is required.
Copper deficiency has not been reported in humans
Copper can accumulate excessively in the body in the rare, genetically determined condition Wilson’s disease
– atomic weight, 19; valency, 1 ,
Fluoride forms crystalline calcium fluorapatite in the structure of teeth and bone. When drinking water contains 1 mg/litre (1 ppm) there is a coincidental 50% reduction in tooth decay in children.
The prime dietary source for fluoride is fortified drinking water. Other sources are carrots, turnip and beet greens, sunflower seeds, garlic, spinach, green leafy vegetables, nuts (especially almonds), turnip greens. Tea is an important source of fluoride; food provides only 25 % of the required intake.
There is no known requirement for fluoride and there is no recommended intake is recommended.
It has been suggested that adults should have a mean dietary intake of 95 μmol/day or 150 μmol from fluoridated water.
High intake of fluoride, that is in excess of 1 mg/ litre, results in mottling of the teeth: In concentrations well in excess of 10 parts per million, fluoride poisoning can occur, with a loss of appetite and sclerosis of the bones of the spine, pelvis and limbs. There may be ossification of the tendon insertion of muscles.
– atomic weight, 73; valencies, 2, 4,
present in the diet at trace levels and is consumed at the rate of 1 mg/ day and rapidly excreted in urine.
Consumption of germanium of the order of 50–250 mg/day (0.7–3.4 mmol) over a prolonged period may result in morbidity and even death.
– atomic weight, 127; valencies, 2, 4 ,
Iodine is required for thyroid hormones, thyroxine 3,5,3′,5′-tetraiodothyroxine (T4), 3,5,3′-triiodothyroxine (T3).
Most foodstuffs except seafood are poor sources of iodine. Fruit, vegetables, cereals, meat and meat products may contain up to 100 μg /kg depending on the soil content where grown. Milk is a major source of iodine with a content of the order of 0.2–23 μmol/kg. Iodine-supplemented cattle feed, iodinated casein, is a lactation promoter in cows. The contaminations of milk from iodophors, the sterilising agents, used in milking is an important source of iodine. In areas where iodine is deficient and where there is endemic goiter, providing iodinated salt and oil isimportant.
The body content of iodine is 20–50 mg (160–400 μmol). About 8 mg is to be found in the thyroid gland.
Adults An intake of 70 μg/day has been suggested. Infants and children require a minimum 40–50 μg / day .
Iodine Maternal deficiency is associated with a high incidence of still-births, spontaneous abortions, congenital abnormalities and cretinism, Iodine deficiency in children causes goitre. The incidence of goitre increases with age and is maximal in adolescence, particularly marked in girls. The problems of iodine deficiency may be reversed by iodised oil.
A slight increase in the incidence of hyperthyroidism has been described following iodised salt programmes.
Correction of iodine deficiency
People in deficient areas can receive iodine as an additive to food or water as iodinated salt or by direct administration of iodised oil, potassium iodide or iodine in Lugol’s solution.
– atomic weight 207 Valency 2, 4
There is no evidence that lead is of any importance physiologically. In an industrialised society, 1–2 μmol are ingested in the food daily, and 90% of this is not absorbed. Lead is deposited in bone and excreted in bile.
Lead poisoning may lead to anaemia, peripheral neuropathy or encephalopathy. This has been a problem in lead workers, e.g. plumbers, A blood concentration of more than 1.4 μmol/ litre is undesirable. .
– atomic weight, 7; valency, 1
There is no known role for lithium in normal physiology. But has an important role in the prophylactic management of manic depressive psychosis.
MAGNESIUM. – atomic weight, 24; valency, 2, Function
Magnesium is complexed with ATP in ATP-dependent enzyme reactions, e.g. glycolysis and Krebs cycle, adenyl cyclase in cAMP formation, phosphatases, and in protein and nucleic acid synthesis.
Magnesium is an important cofactor for cocarboxylase and is involved in the replication of DNA and synthesis of RNA.
Magnesium is present in most foods, particularly those of vegetable origin containing chlorophyll, e.g. green leaves and stalks. Typically, a diet contains 200–400 mg/day.
The whole body content of magnesium is about 1 mol (25 g). Almost two-thirds of body magnesium is to be found in bone in association with phosphate and bicarbonate. The remaining 30% is found intracellularly in soft tissues, bound to protein.
Adults: Magnesium balance is achieved with an intake of 50 mg/day.
Infants: Human milk contains approximately 0.12 mmol per 100 ml, so that infant intake is approximately 1 mmol/day (25 mg). By 3 months the intake is 6 mg (0.25 mmol)/kg/day.
Pregnancy: The foetus requires approximately 8.0 mg/day over 40 weeks, so that the maternal requirement is of the order of 16 mg/day.
Lactation: The magnesium content of breast milk is approximately 1.2 mmol/litre, producing about 25 mg/day. It is suggested that the lactational increment should be 50 mg/day.
Magnesium deficiency is manifested by progressive muscle weakness, failure to thrive, neuromuscular dysfunction, tachycardia, ventricular fibrillation, coma and death. Alcohol abuse and diuretics are important causes of a low serum magnesium.
– atomic weight, 55; valencies, 2, 3, 4, Function
Manganese is important in enzyme activity, e.g. pyruvate carboxylase, mitochondrial superoxide dismutase and arginase. It may also activate other enzymes, e.g. glycosyl transferases, hydrolases, kinases, prolinase and Sources
Wheat: especially wholemeal, 50 μg/g, wheat-germ, 130 μg /g and nuts, 17 μg/g, tea , cereals, legumes and leafy vegetables.
The average intake in Britain is in the order of 4.6 mg per person per day, half of which is derived from tea. The content of manganese in breast milk in the first 3 months postpartum is 1.9 μg /day and 1.6 μg / day thereafter.
Healthy infants fed cows’ milk will have an intake of 28–42 μg /kg/day
Manganese deficiency has not been reported in humans.
– atomic weight, 200; valencies, 1, 2 ,
Mercury is not an essential element
Mercury is present in food in trace amounts, possibly because it has important uses in industry and therefore is a widespread contaminant. .
Mercuric poisoning may occur with contamination of food eg sprayed grain, fish, in excess of 145 μmol/kg
MOLYBDENUM. – atomic weight, 96; valencies, 2, 3, 4,
Molybdenum is essential for the enzymes xanthine oxidase/dehydrogenase, aldehyde oxidase and sulphite oxidase, important in the metabolism of DNA and sulphites.
The amount of molybdenum in plants is dependent on where they are grown and upon the soil content.. Important dietary sources are wheat flour and germ (0.7 and 0.6 μg /g respectively), legumes (1.7 μg /g and meat (2 μg /g
These are 50–400 μg /day for adults. For breast-fed infants a requirement of 0.5–1.5 μg / kg/day has been suggested.
– atomic weight, 59; valencies, 2, 3. , •
Nickel may be essential in some animals and birds but deficiency in humans has never been proven.
Plasma concentrations are 2–4 μg/100 ml, some bound to albumin, the remainder in free solution. Nickel is excreted in urine.
Deficiency and excess
As yet, there have been no reports of molybdenum deficiency in humans.
– atomic weight, 31; valencies, 3, 5 ,
Phosphorus is an important component of the crystalline structure of the bony skeleton with calcium. Phosphorus is important in oxidative phosphorylation as part of adenosine triphosphate (ATP). Under normal physiological conditions, electron transport is tightly coupled with phosphorylation. ATP generation depends upon electron flow, which only occurs when ATP can be synthesised.
Other critical roles are in nucleic acids through the phosphorylation of sugars as a base-sugar-phosphate nucleotide, Phosphorus is important in the control of enzyme activity through phosphorylation.
Phosphorus is present in all natural foods. The usual diet in Britain provides 1.5 g of phosphorus daily.
Phosphorus requirements are estimated as equimolar to calcium
– atomic weight, 79; valencies, 2, 4,
Selenium is an essential nutrient. Selenoenzymes, selenium-dependent enzymes, e.g. iodothyronine, deiodinase and glutathione peroxidases, protect the cell from peroxidative damage. Selenium is part of the active site of both enzymes as selocystine. Selenoprotein activity includes the hepatic microsomal deiodination of thyroxine.
The main sources are cereal, meat and fish: meats, 2 μg /g, seafoods, 0.5 μg/g, nuts, 0.7 μg /g, and wheat flour, 0.3 μg /g.
For adult approximately 40-70 μg /day in a British adult;
Pregnancy and lactation Fertility is dependent upon an adequate selenium intake. There are adaptive changes in metabolism during pregnancy. In lactation, the concentration of selenium in colostrum varies from 50–80 ng/ml. The concentration in breast milk during the first month of lactation is between 18–30 ng/ml. This demands an increase in dietary intake of about 15 μg/day
Infants Breast-fed infants receive approximately 5–13 μg /day. Intakes from formula feeds are generally lower, of the order of 2–4 μg /day. The non-breast-fed infant’s is of the order of 1.5 μg /kg/day at 4–6 months.
Children of the order of that for the adult per kg body weight.
There is an overlap between selenium deficiency and vitamin E deficiency. This is because selenium is involved in glutathione peroxidase which destroys lipid hydroperoxides and is important in stabilizing lipid membranes by inhibiting oxidative damage. Selenium deficiency results in a decrease in glutathione peroxidase activity.
– atomic weight, 28; valency, 4 ,
The role of silicon in human nutrition is unclear. Silicon may be important in the proteoglycans of cartilage and of the ground substance of connective tissue. The human aorta, trachea, lungs and tendon are rich in silicon. The aortic silicon content may decline with age, particularly in the presence of atherosclerosis
Cereal grains, other forms of dietary fibre and drinking water (2–12 μg /ml) important sources of silicon.
Silicic acid in foods and drink is absorbed quickly. The body storage pool is approximately 3 g in a 60-kg man.
The dietary requirements of silicon are not known.
– atomic weight, 108; valency, 1
Silver occurs in low concentration in soils, plants and animal tissues. It may interact with copper and selenium but has no known essential function in humans.
STRONTIUM. – atomic weight, 88; valency, 2,
Strontium has no known function in man
Strontium is widely distributed in the environment and in plants, particularly in wheat bran, rather than the endosperm of grains, and in the peel of root vegetables. The strontium content of drinking water varies from 0.02–0.06 mg/litre, though higher values have been recorded.
Strontium is present in foods which are rich in calcium, for example milk and fresh vegetables, and is stored in bone. The concentration is approximately 1000 times less than that of calcium. Following the open testing of atomic bombs in the middle years of the 20th Century there was a real problem with radioactive strontium 90 in the environment cancer of the bone becoming a problem.
The dietary intake is of the order of 1–3 mg/day.
– atomic weight, 32; valencies, 2, 4, •
Sulphur occurs in tissues as the sulphate, a component of the proteoglycans which are important in extracellular matrices, cartilage, vascular and reproductive systems as dermatan sulphate, chondroitin sulphate and keratin sulphate. Disulphide cross-linkages are also important in the specific three-dimensional folding of proteins.
Sulphate is involved in the hepatic enzymatic detoxification phenols, alcohols and thiols, in part by increasing water solubility and facilitating excretion. Sulphate is involved in an active form of phosphoadenosine and phosphosulphate and is derived from cysteine and methionine using the molybdenum-dependent sulphite oxidase system. Sulphur is part of glutathione and some coenzymes including coenzyme A. Sulphur is also present in sulphur-containing amino acids. l-Methionine is metabolised by transmethylation and trans-sulphuration.
sulphur is obtained from proteins in the diet. The essential amino acid methionine is one of several sulphur-containing components of protein.
Dietary intake is of the order of 0.7 mg/day.
– atomic weight, 119; valencies, 2, 4 ,
The biological function and nutritional requirements of tin have yet to be identified
Human diets may contain 150–200 μg /day, though how much is from food and how much from tin cans is not clear. The lacquering of the interior of cans reduces the amount of tin available for absorption.
High intakes of inorganic tin lead to gastro-intestinal symptoms.
The upper limit permissible in a canned food is 2.1 mmol/kg. Average intakes in Britain have been estimated at 190 μg /day for an adult, but 99% of the tin is excreted in faeces.
– atomic weight, 51; valencies, 2, 3, 4
The biological function and nutritional requirements of vanadium have yet to be identified
Vanadium is present in most human foods, particularly shell-fish, mushrooms and peppers. The daily intake in the American diet may be 25 μg.
These are of the order of 1–2 μg /g.
– atomic weight, 65; valency, 2,
Zinc isessential in the human diet.
Zinc is required for many enzymatic functions, DNA synthesis, cell division and protein synthesis. Zinc is involved in enzyme activity, including carbonic anhydrase, alcohol dehydrogenase, alkaline phosphatase, lactate dehydrogenase, superoxides, dismutases and pancreatic carboxypeptidase. The role of the zinc ion is to stabilise a highly reactive hydroxide ion , so that an activated nucleophile is available for catalysis.
It has long been believed that zinc is important for wound healing.
The adult body content of zinc is over 2 g (30 mmol). The prostate gland, choroid of the eye and semen have high concentrations of zinc, though the greatest amount is in bones (about 200 μg /g.). Red cells contain 13 μg /ml, hair 120–250 μg /g both in newborns and adults.
Dietary sources of zinc are meats (3–5 mg/ 100 g), whole grains and legumes (2–3 mg/100 g), and oysters (70 mg/100 g).
The recommended intake varies between 7 and 15 mg/day, though only 20–30% of this is absorbed.
Adults – Adult requirements of zinc are of the order of 2–3 mg/day (30–40 mmol
Infants – Human milk is not a rich source of zinc and the infant depends very much on the stores accumulated in the last 3 months of intrauterine life. Assuming a daily requirement of 1 mg/day and an absorption efficiency of 30%, an intake of 3-4 mg/ day has been suggested.
Children – of the order of 4-7 mg/day .
Pregnancy – Extra zinc is required during pregnancy. Zinc intake of the order of 8 mg is needed by the foetus during the last 3 months of gestation, requiring an additional maternal intake of 2 mg/day, assuming an absorption efficiency of 20–30%. This means that the dietary requirement is 6–14 mg/day.
Lactation It is not known if there is an increased dietary need during lactation.
Supplementary zinc may be given as zinc sulphate.
Zinc deficiency is rare. The clinical syndrome of growth retardation, male hypogonadism, skin changes, mental lethargy, hepatosplenomegaly, iron-deficiency anaemia and geophagia has been reported from studies of male dwarfs in Iran.
The dermatological signs of severe zinc deficiency include progressive bullous-pustular dermatitis at the extremities and the oral, anal and genital areas, combined with paronychia and generalized alopeciae. These respond to dietary supplementation of zinc sulphate.
Zinc deficiency has been reported in alcoholics and with steatorrhoea and, zinc malabsorption, or when there is loss of zinc protein complexes into the intestinal lumen in malignancies. Extensive burns may also result in reduced plasma zinc concentration, due to loss of zinc through the skin. In renal disease an excess zinc may be lost in the urine, with a consequent reduction of plasma and tissue concentrations.
Genetic disorders Acrodermatitis enteropathica is a potentially lethal autosomal recessive trait which manifests itself in infancy after weaning. Acrodermatitis enteropathica consists of pustular and bullous dermatitis, alopecia and diarrhoea. Zinc supplementation is necessary.
Summary of activity of trace elements —
|Enzyme co-factors||Aluminium, copper, magnesium, manganese,molybdenum, selenium, zinc|
|Protein ligand||sulphur, phosphorous|
|Skeletal structure||fluorine, magnesium, phosphorous, silicon,sulphur|
|No evident nutritional function||bromine, cadmium, caesium, chromium, germanium, lead, lithium, mercury, nickel, silver, strontium, tin , vanadium|