Inputs, Outputs, and the Requirement
On the input side, gross intestinal absorption in adults, at prevailing intakes, averages ut 30%; and because substantial quantities of calcium enter the digestive tract with gastrointestinal secretions, net absorption is generally only 10 - 15%. These calcium ovements are depicted schematically, which illustrates the relationship of gross and net sorption for an intake of 800 mg (20 mol) and approx 32% absorption efficiency. On the tput side, many dietary components interfere with renal calcium conservation, notably sodium, net acid production, and sulfur-containing amino acids. These agencies create a floor low which urinary calcium cannot be reduced, despite the effect of PTH on increasing renal ular calcium reabsorption.
Finally, resting dermal losses (i.e., without sweating) are in the range of 1.5 mmol (60 ::lg)/d, and copious sweating can cause losses 5 - 10 x larger. The net result of all these excretory forces is that total obligatory calcium losses in sedentary adults on typical diets are generally in the range of 4 - 6 mmol (160 - 240 mg)/d. 'lb stay in balance, i.e., to maintain skeletal integrity, adults must absorb at least this much calcium from ingested food. At typical absorptive efficiencies, net absorption of 160 - 240 rag requires a total intake of at least 1000 - 1500 mg/d (25 - 37.5 mmoVd).
Because absoiptive extraction is low, even low calcium diets might seem to be adequate, since, in theory, much more calcium couldbe absorbed. Awareness of this potential may explain why nutritional scientists had been slow to accept the lifelong importance of a high calcium intake for contemporary humans. If the calcium was in the diet and the body was not fully accessing it, then-it was argued-the body did not really need it.
That conclusion proved to be wrong, as was definitively shown on publication of several randomized, controlled trials that demonstrated that the body would indeed use extra calcium
and improve calcium balance if the mineral were provided by a high enough intake. Failure to retain calcium at low intakes reflected not absence of need but the fact that human absorption and conservation efficiencies for calcium are simply not up to the challenge of a low intake, particularly when the bony reserves are so accessible. As noted in the following section, there was no evolutionary need for hominids to develop the type of absorptive and excretory conservation that today's diets demand, when then-available foods provided a surplus of calcium. In brief: although our diets are modern, our physiologies are paleolithic.
