Minerals

The last class of long-recognized nutrients comprises minerals. Dried bones were known to consist mainly of calcium phosphate, and when other tissues were brought to red heat in a furnace, there was always a small residue of incombustible ash, or mineral salts.

The fact that iron salts were an effective treat­ment for chlorosis (“green sickness”) in young women was not at first regarded as the correction of a nutritional deficiency. Thomas Sydenham, for ex­ample, wrote in 1670 that a medicine made from steeping iron filings in wine and boiling the liquid down to a syrup had the effect in cases of chlorosis of “returning the pale and death-like face to a ruddy color and giving the pulse strength.” But there was no more suggestion of this being a nutritional use of iron than there was of mercury being a nutritional factor in the treatment of syphilis.

By 1800, however, it was understood that blood contained iron, and then that the level of iron was reduced in the paler blood of chlorotics. From 1830 pills containing ferrous sulfate were in standard use for the treatment of chlorosis, or anemia as it came to be known, and were considered to be generally effective.

German workers, however, who were in the lead from 1850 in carrying out quantitative balance stud­ies in model animals, became skeptical as to whether these pills actually contributed iron to the blood. When large doses of ferrous sulfate were given orally to dogs, the iron was recovered almost completely in their feces, with little or none appearing in the urine. This result contrasted with the results of the Ger­mans’ previous work on protein nutrition in dogs.

Further work showed that even iron salts injected into a dog’s tissues failed to appear in the urine. In fact, it appears that animals and humans have essen­tially no means of excreting surplus iron from tis­sues. The balance is maintained by carefully con­trolled absorption mechanisms that normally admit only as much iron through the gut wall as is re­quired by the tissues. However, even under condi­tions of acute deficiency, no more than 20 percent of iron in foodstuffs can be absorbed, and normally it may be only 1 to 2 percent. Mild anemia remains a problem in many parts of the world, particularly for women on mainly vegetarian diets. They generally eat less than men but have a greater need for iron because of their loss of menstrual blood.

Animals grazing in fields where the soil is low in particular elements are unproductive because the plants they eat are also low in such elements as copper, cobalt, and phosphorus. This problem is virtu­ally unknown in human nutrition because of the greater variety of our diets. However, iodine is one exception. This element was discovered in 1811, and by 1820 its salts were already being used in the treatment of goiter. This is a condition of gross swell­ing of the thyroid gland at the front of the neck. It had always been common in particular areas of the world, usually those well away from the ocean. Tradi­tional treatments with seaweed and the ash of sponges were explained by their high content of io­dine, and by the 1830s it was proposed that goiter, and the associated condition of cretinism in children, was due to a deficient iodine intake.

Gradually, however, it came to be understood that relatively small excesses of iodine could be toxic, but that a minimal average intake of 50 millionths of a gram per day was needed to avoid goiter. Indeed, the swelling of the thyroid produced by iodine deficiency is apparently a feedback reaction whereby the body attempts to capture more of the element in order to use it for the synthesis of hormones. The prevention of goiter is the only known role of iodine in animal organisms, but the stimulatory effect of hormones extends to all tissues. Children bom as cretins, be­cause of a lack of iodine, have irreversible brain damage.

Extensive chemical analysis of foods and water supplies have failed to explain the existence of goiter in a few areas of the world, and it is believed that other factors may reduce either the bioavailability of the iodine consumed or the effectiveness of the thy­roid hormone produced from it. Some such factors, in cabbage family plants, for example, are already known. However, even in these areas an increase in the iodine supply has greatly reduced the incidence of goiter and also of cretinism.

In most countries where the production of domes­tic salt is organized in fairly large-scale units, it has been found practicable to fortify it with up to 1 part in 10,000 of potassium iodide or iodate. This has proved an effective public means of preventing the disease. In areas where salt is not refined, those at risk are sometimes given an injection of iodized oil, which can remain effective for as long as two years. In many affluent countries, chemists must ascertain whether the consumption of iodine is excessive be­cause of the increasing use of iodine-containing chemicals for the disinfection of food-processing equipment.

Another chemical element discovered in the same decade as iodine is selenium. In the 1930s it was found to be present at toxic levels in plants grown on soils rich in selenium in Wyoming and the Dakotas, and responsible for disease among grazing sheep. But not until 1957 was it recognized as an essential nutri­ent. Farm animals have been found to be deficient in places as far apart as Sweden and New Zealand; these animals have characteristic lesions in their muscles, including heart muscle. The major cause of keshan disease in parts of China, in which children die with characteristic changes in their heart muscle, is now suspected to be selenium deficiency.