PALEOLITHIC CONDITIONS
Calcium is the fifth most abundant element in the environment in which life evolved, and was present in high concentrations in the foods consumed by evolving hominids and huntergatherer humans. During the evolution of human physiology, calcium intake would have averaged apJ:rox 0.8 mmol (32 mg)/kg body weight/d. When adequate food was available, the gut would have served as a nearly continuous source of calcium. Despite low absorption efficiency, such a diet provided sufficient calcium to offset obligatory losses and to adjust to day today variability in intake and excretion. The mechanisms for dealing with a fall in ECF [Ca2+], described earlier, would have been called into play only infrequently, i.e., at times of food shortage or for long intervals between meals. Thus, PTH secretion is basically an emergency measure.
Intermittent utilization of the reserve would not have resulted in lasting effects under primitive conditions. In fact, the diet was so calcium dense that human physiology has been optimized to prevent calcium intoxication, rather than to deal with chronic shortages. This is the reason intestinal calcium absorption efficiency is low, renal conservation is weak, and dermal losses are completely unregulated. (Contrast this behavior with that of sodium, a scarce mineral in the primitive environment: sodium is absorbed at nearly 100% efficiency and, in trained individuals, both urinary and sweat losses can be reduced nearly to zero.) By contrast, under contemporary conditions, adult diets often contain less than 0.2 mmol (8 mg) Calkg body weight/d.
The reason for the fall in the calcium content of the human diet from paleolithic to historic times is that cereal grains and legumes, which constitute more than half the total food intake of agriculture-based populations, are typically poor sources of calcium. Neither food group was prominent in pre agricultural diets. Hence, the shift from a hunter-gatherer to an agricultural base for the feeding of humans produced a marked decline in calcium density of the food supply. The much lower calcium intake of contemporary diets requires that adaptive mechanisms be invoked frequently, and in some individuals, constantly. The ability to adap to low intake or high loss varies, and depends upon genetics, life stage, and environmental factors. As already noted, blacks adapt with high efficiency. Similarly, most young individuals can increase absorptive efficiency sufficiently to permit building a skeleton during growth. However, even in the young, adaptation is often not fully optimal, i.e., bone mass probably does not reach its full genetic potential on prevailing calcium intakes.
Moreover, this ability to get by on low intakes declines with age. Various policymaking bodies have recently recognized this decline by recommending higher calcium intakes for the elderly. But beyond simply recognizing a special need in the elderly, there is a sense in which the requirement in the elderly uncovers the true requirement for all ages, that is, it reveals the intake that protects the skeleton without requiring constant adaptation. It is the spectrum of responses to this constant adaptation that constitutes the basis for the role of low calcium intakes in nonskeletal disorders.
