Calcium constitutes a larger proportion of the body weight (about 2 per cent) than does any other of the "inorganic" elements. It is very unevenly distributed in the body, over 99 per cent of the total amount being in the bones. It is also very irregularly distributed among the staple articles of food, many of which are extremely poor in calcium, while milk contains it in abundance. The "ordinary mixed diet" of Americans and Europeans, at least among dwellers in cities and towns, is probably more often deficient in calcium than in any other chemical element.

In studying the effects of insufficient calcium, Voit kept a pigeon for a year on calcium-poor food without observing any effects attributable to the diet until the bird was killed and dissected, when it appeared that, although the bones concerned in locomotion were still sound, there was a marked wasting of calcium salts from other bones such as the skull and sternum, which in places were even perforated. Thus in adults there may be a continued loss of calcium without the appearance of any distinct symptoms because the losses from the blood and soft tissues may be replaced by calcium withdrawn from the bones. The injurious effect of an insufficient intake of calcium is of course more noticeable with growing than with full-grown animals. Abnormal weakness and flexibility of the bones (resembling the condition of rickets in children) has been produced experimentally by feeding puppies with lean and fat meat only, while others of the same litter, receiving the same food, but with the addition of bones to gnaw, developed normally. In this connection it should be remembered that no animal is literally carnivorous in nature, that is, none lives on flesh alone; the animals called carnivora always eat more or less of the bones of their prey.

According to Herter * many cases of arrested development in infancy may be due to an insufficient assimilation of calcium from the food. Such a deficiency in the amount assimilated may be due to defective digestion or to a diet inadequate in calcium content.

* On Infantilism from Chronic Intestinal Infection, New York, 1908.

Many medical writers have attributed different diseases to inadequate calcium supply or disturbance of calcium metabolism. Conclusive proof or disproof of such theories would however require more detailed and exact quantitative studies of the intake and output of calcium in health, and the amounts required in normal nutrition at different ages and under different conditions, than have yet been made.

The fact that normal urine has a low calcium content while the feces usually contain much the greater part of the calcium which has been taken in the food has often been interpreted as meaning that the absorption of food calcium is poor or that the calcium requirement of the body is low. It is now known, however, from experimental evidence, that most of the calcium which has been absorbed and carried through the metabolic processes, is normally excreted through the intestinal wall and thus leaves the body in the feces instead of the urine. When the diet is very poor in calcium and the output of this element materially exceeds the intake, the feces often contain a larger amount of calcium than was present in the food.

Observations upon Breithaupt and Cetti showed a considerable elimination of calcium in the feces during fasting. On the other hand, Benedict reports the result of a 31-day fast during which no feces were passed, but considerable quantities of calcium continued to be lost through the urine throughout the entire period.

On account of the fluctuating distribution of the calcium between urine and feces, conclusions regarding the calcium requirement can properly be drawn only from those experiments in which the amounts of this element in the food, in the feces, and in the urine have been directly determined. A compilation of such experiments has been made, and the reported results calculated to a uniform basis of 70 kilograms of body weight. On this basis, 63 experiments on 10 subjects (6 men and 4 women) show calcium outputs ranging from 0.27 to 0.78 gram and averaging 0.45 gram of calcium "per man per day." This includes the experiments which appear most reliable as indicating the actual (minimum) requirement in that the food did not furnish an excess of calcium over the needs of the subject, and the calcium balance showed a reasonable approach toward equilibrium. It will be noted that this average of 0.45 gram calcium (equivalent to 0.63 gram CaO) represents the expenditure under conditions of closely restricted calcium intake. It corresponds to the average of 49.2 grams of protein per man per day reached on page 220, and approximates the minimum of actual need rather than a normal allowance. The margin for safety should probably be larger for calcium than for protein because of the likelihood of relatively greater losses in cooking and in digestion, while there is much less danger of any injurious result from surplus calcium than from surplus protein. Nelson and Williams have recently found the calcium output of four healthy men on normal unrestricted diet to range from 0.68 to 1.02 grams of calcium (0.95 to 1.43 grams of CaO) per day. Here as in the case of protein the rate of metabolism to be expected in a normal man on unrestricted diet and well fed, according to American standards, runs from 50 to 100 per cent above the amount which would probably suffice to meet the actual requirement.

The calcium requirements of women are greatly increased by maternity. The need of an abundance of calcium for the rapidly growing skeleton of an infant is obvious. Before birth, and normally for several months after, this demand of the child is satisfied through the mother, whose calcium requirement is thus greatly increased. The weakening of the bones and teeth which is said to be a common accompaniment of pregnancy and lactation is held by Bunge to be largely due to a withdrawal of calcium from these structures to meet the nutritive requirements of the embryo or the nursling.