What might be called the “natural human diet” is low in calcium, as are present-day diets in most developing countries, where the daily intake amounts to about 200 to 500 mg. The calcium content of the European diet was probably the same in past centuries, giving rise to the frequent occurrence of calcium deficiency disease - rickets - in children.

The large increase in the calcium content of the Western diet came with the increasing use of cow’s milk as a staple food for all age groups. The calcium content of milk is high, and it serves the needs of infants whose rapidly growing skeletons need a com

Paratively large amount. Calves grow at 4 times the rate of human infants; hence, cow’s milk contains 4 times as much calcium as human milk. The average intake of milk and dairy products in prosperous countries is usually between Vi and V2 liter a day, containing 300 to 600 mg calcium. Three hundred mg is sufficient to convert the intake from other foods into an abundant 600 to 700 mg, and half a liter of milk into an excessive 900 to 1,000 mg. In many Western countries, the calcium intake from cow’s milk is more than that from all other foods combined.

Calcium is an essential nutrient, but in the ideal case, the dietary intake should not exceed requirements because the excretion of the surplus is a difficult task. The requirement of the human body can be estimated from the following data: The adult body contains about 1,100 grams of calcium, 99 percent of which is in the skeleton. The remaining 1 percent is needed for various essential functions, such as the generation of nerve impulses, muscular contraction, blood coagulation, and so forth. The skeleton reaches its maximum size and weight at the age of 35 years. While the skeleton is growing, it takes up a daily average of 80 mg calcium. After the age of 50 years, the skeleton begins to shrink, releasing calcium.

All body fluids contain calcium. About 100 mg is lost daily in urine and about 15 mg in sweat, though with hard physical exercise, sweating can excrete 80 mg calcium in a day. In addition, digestive fluids, such as saUva and pancreatic juice, discharge calcium into the digestive tract, and this amount is not completely reabsorbed. Allowing 150 mg calcium daily for this loss, the calcium requirement of a young adult,

Depending on physical exercise, amounts to 330 to 390 mg/day, and that of an old person about 240 mg/day The dietary intake should exceed the needs of the body by about 20 percent to allow for calcium passing unabsorbed through the intestines, making the daily requirement for a hard-working young adult 470 mg, that of old people 290 mg. When the risk of coronary heart disease is considered, we are mainly interested in the older age groups. In their case, a possible dietary calcium intake of the order of 1,000 mg is several times the quantity they need, and that from milk alone can be twice that amount.

The uptake of calcium from the small intestine is a controlled process, so that an excessive dietary intake does not necessarily mean that calcium finding its way into body fluids must also be excessive. Control is exercised by a vitamin D metabolite, chole-calciferol, synthesized in two steps in the liver and kidney. This metabolite is the signal for the synthesis of a carrier protein in the intestine, which is the actual transfer agent for calcium through the intestinal wall. If the calcium content of the plasma is already adequate, the synthesis of cholecalciferol is discontinued, and the excess calcium passes unabsorbed through the alimentary canal. However, newly bom infants do not yet possess this control mechanism. In their case, milk sugar, lactose, facilitates the absorption of calcium from the small intestine by a simple diffusion process. Under natural conditions, this facility does not involve a health hazard because when infants are weaned, lactose disappears from their diet. In a prosperous society, however, infants are never weaned, in the sense that lactose remains in their diet from birth to death. Not only is milk, therefore, high in calcium but its lactose content also enables it to bypass the control mechanism of the body (Bronner 1987).

It might be mentioned that when milk is fermented, lactose is converted into lactic acid, a biologically inactive substance. Fermented milk and its products, such as cheese, are very high in calcium but do not provide the facility for evading the intestinal control for its absorption. This is the probable reason that the correlation between mortality from coronary disease and cheese consumption is much weaker than it is with the consumption of whole milk.

If the quantity of calcium absorbed from the intestine exceeds requirements, a good excretory mechanism exists for disposing of the excess. The kidneys normally excrete about 100 mg/day. Concentration of calcium in the urine (hypercalciuria) involves the risk of stone formation in the kidneys, but a second excretory mechanism is available to support them. The surplus calcium becomes protein bound and is excreted by the Uver, not the kidneys. In individuals in prosperous countries, the concentration of protein-bound calcium in the plasma, 1.16 millimoles (mmol) per Uter (1), is nearly as high as that of diffusible calcium, 1.34 mmol/1 (Ganong 1987), demonstrating

That calcium intake in Western countries, indeed, tends to be excessive.

The effective excretory mechanism ensures that most of the surplus calcium is eliminated, but a small fraction escapes and is ultimately precipitated in soft tissues. Under normal conditions, this process is so slow that a large dietary calcium excess can be tolerated for decades, the calcification of soft tissues becoming a health hazard only in old age. Misuse, however, can overwhelm the excretory mechanism. In the 1950s, for example, it was customary to treat gastric ulcer patients with large quantities of milk, amounting to about 2 liters per day, until a disproportionately high mortahty from coronary disease was observed among them (Briggs et al. I960). Two liters of cow’s milk contains 2.4 grams of calcium, together with lactose facilitating its absorption from the intestine. The calcium absorbed from this source by the body may amount to a daily intake of 1.8 grams, perhaps 6 times the amount needed by elderly individuals. The excretory mechanism may well be incapable of dealing with such a gross excess.

As noted, excess calcium that cannot be excreted ultimately finds its way into soft tissues. Large arteries, notably the aorta, are particularly vulnerable to calcifi-cation. The heavy calcification of the aorta in individuals who died of heart disease was already observed by the pioneers of medicine in the nineteenth century, who correspondingly called the disorder “the hardening of the arteries.”

Calcium deposits in arteries have two important pathological effects. One is the calcification of atherosclerotic plaques. A recent autopsy study by A. Fleck-enstein and his group (1990) has found that atherosclerotic lesions appear to attract calcium from their earliest stage onward. Fatty streaks already contain, at an average, 10 times as much calcium as the surrounding normal arterial tissue. Normal atherosclerotic plaques contain 25 times as much, advanced plaques in individuals who died of coronary disease, 80 times as much. Such advanced plaques are, in effect, calcium plaques. Calcium compounds, mainly apatite, constitute about half of their dry weight, cholesterol and its compounds about 3 percent. It is calcium that gives advanced plaques bulk and rigidity and makes them potential obstacles to blood flow.

Secondly, mural deposits of calcium in the aorta and other large elastic arteries encroach on their elasticity. As pointed out, these arteries constitute an elastic reservoir that is distended when the heart injects a volume of blood into it during systole, storing energy in its stretched elastic tissues. The contraction of the reservoir generates diastolic pressure and maintains blood flow in the circulatory system when the heart is at rest. As the heart compresses its own arteries when it contracts, its perfusion is entirely dependent on an adequate diastolic pressure.

If the elasticity of the reservoir deteriorates, an increasing systolic pressure is needed to maintain

Diastolic pressure at a given value. Perfusion failure in a part of the heart occurs when the aging, partly calcified elastic reservoir cannot generate sufficient pressure to force an adequate quantity of blood through narrowed and obstructed coronary arteries. Calcification is involved both in the reduction of diastoUc pressure generated by the reservoir and in the obstructions presented by advanced atherosclerotic plaques. Thus, calcium excess in Western diets may well be the most important factor in the pathogenesis of coronary artery disease (Seely 1989, 1991). If the populations of Western countries were alerted to this possibility and advised to reduce their consumption, a large reduction in mortality could well be the result.

In a recent trial (Woods et al. 1992), coronary patients were treated with magnesium sulphate with beneficial results. A possible explanation is that the excretion of the four main electrolytes - sodium, potassium, calcium, and magnesium - is an interlinked process. The most difficult task of excretion for the kidneys arises when the intake of these minerals is unbalanced, high in some, low in others. Thus in the 1960s, rats on a high-cholesterol diet also had their food unbalanced in electrolytes, with an excessive sodium-potassium and calcium-magnesium ratio (Sos 1965).The rats died of repeated, humanlike heart attacks, but their lives could be prolonged if the imbalances were moderated. Thus, if human diet has an excessive calcium content, the best remedy would be its reduction, but failing that, an increase in magnesium intake can be beneficial.

As mentioned in the section “A Statistical Study,” epidemiological studies show a positive correlation between mortality from coronary artery disease and the consumption of oats, as well as of milk. The calcium content of oats, 80 mg/100 g, is high, but the strong correlation with coronary disease would probably arise only if they also contained some substance promoting the absorption of their calcium from the intestines. An oat grass, Trisetum flavescens, is known to contain vitamin Dj, capable of causing calcinosis in grazing animals, but no data are available to show that this also applies to cultivated oats.

The apparent connection between mortality from coronary disease and climate has been noted. The countries with very high mortaUty, such as Finland, Latvia, Lithuania, and Russia, have cold climates. In warmer climates, mortality is generally lower and, in tropical countries, very low or nonexistent. This may be explained by corresponding differences in calcium excretion. In a cold climate, the amount of calcium excreted by sweating is usually small, whereas a person doing hard physical work in the tropics can lose more fluid, and possibly more calcium, in sweat than in urine.

If arterial calcification is one of the main causes of death from coronary disease - the “skeleton in the atherosclerosis closet,” as a recent article called it (Demer 1995) - this could be a blessing in disguise.

The most important source of dietary calcium is one easily identifiable food item, cow’s milk - hence, arterial calcification is preventable. The best way of achieving such prevention would be the reduction of milk consumption, particularly by elderly people. An alternative course might be the elimination of lactose from fresh milk. As mentioned, the worldwide toll of coronary disease is about two million deaths per year. The possibility deserves careful consideration.

Stephen Seely