Calcium Hunger : Behavioral and Biological Regulation

This book brings together the behavioral, physiological, and neuroendocrine regulation of calcium. An understanding of how the brain orchestrates whole-body demands for calcium is introduced. The approach is one in which behavior in addition to physiology serves bodily maintenance. The book links basic and clinical literature surrounding calcium homeostasis, as a wide variety of clinical syndromes are tied to calcium metabolism. Because calcium is so important during life stages particular to women, an emphasis is placed on the relevance of calcium to women's health throughout the book, though not exclusively since calcium is fundamental to both sexes.

Physiology has been characterized as a science that infers the operation of a system by poking it and observing its response. The author of Calcium Hunger, a physiologist in this classic mode, is concerned with the role of behavior in maintaining the internal integrity of organisms. This emphasis on behavior is in contrast to the narrow, traditional view that concentrates on the internal milieu and on the control systems and feedback loops that sustain it. Schulkin builds on the psychobiologic work of Curt Richter, who held that behavioral tendencies had evolved to serve homeostatic regulation and specifically, that organisms needing a certain nutrient will seek out and preferentially ingest foods rich in that nutrient.

The participation of behavior in physiologic regulation is an everyday experience. When we are hungry or thirsty, we eat or drink; when we are tired, we rest or sleep. Walter Cannon's conviction that the body knows best takes us beyond these general behaviors to more specific drives. Appetites for sodium (salt) and for phosphate, for example, have been well documented. In a previous book, Schulkin summarized the literature on sodium hunger. In the current book, he reviews the huge body of literature (cited in more than 1100 references) relating to calcium regulation.

Unlike sodium, for which the body has no reserve for use during periods of depletion, there is a vast store of calcium in the skeleton. As a result, calcium is almost never truly depleted, at least nutritionally. (Osteoporosis is a reduction in the size of the calcium reserve, but it does not compromise or threaten the metabolic activity of calcium.) Sodium is different. Salt-seeking and salt-preferring behaviors are well-documented phenomena that make sense in terms of survival. Since sodium is a trace element in most terrestrial habitats, the diets of most land animals teeter on the edge of insufficiency and so salt-seeking behavior confers a selective advantage. Calcium, by contrast, is abundant in most environments, and the principal reason for low calcium intake would be low availability of food or starvation. For this reason, specific calcium-seeking behavior is less obviously beneficial to an organism than sodium-seeking behavior. Nevertheless, Schulkin has taken on the challenge of writing a book about calcium hunger.

In the useful first chapter, Schulkin summarizes the behavioral regulation of biologic systems in general. He then reviews the literature on calcium taste, its neural representation, and its associated forms of behavior. In chapter 3, he examines a large literature on sex and reproductive effects, both on the need for calcium and on behavior during times of physiologic demand (e.g., pregnancy and lactation). In the final two chapters, he examines neural and endocrine mechanisms that may mediate calcium hunger and the changing need for calcium during the life cycle. The latter chapter is a useful, up-to-date summary of a rapidly changing field.

Calcium deprivation in animal models has often been studied after parathyroidectomy, and the equivocal behavior that follows is subject to multiple interpretations. Calcium deprivation has also been induced by low calcium intake, but even severe dietary deprivation lowers calcium ion concentrations in the extracellular fluid by no more than 1 to 2 percent, largely because internal homeostatic mechanisms call on the vast skeletal reserve. These mechanisms involve increased release of parathyroid hormone and increased synthesis of calcitriol, the hormonally active form of vitamin D, which has been thought to mediate a calcium-hunger response. As with the observations after parathyroidectomy, however, the evidence is ambiguous.

Is the evidence for a true calcium hunger persuasive? I am not certain that Schulkin persuades even himself. He considers the results of the many animal studies honestly and stops short of drawing sweeping conclusions. Particularly telling is the fact that in the concluding chapter, he emphasizes the importance of an adequate calcium intake throughout life, noting that this has become especially important as a result of the increase in life span, with the accompanying increased risk of chronic diseases. For some of these (e.g., osteoporosis), low calcium intake has a contributory role. The emphasis on the need for calcium is both correct and appropriate, but it is not an argument for, let alone proof of, calcium hunger.

The fact remains that diets low in both total calcium content and calcium nutrient density are a recent phenomenon. It would be surprising if natural selection had equipped us with abilities we did not need until after the technological revolution. Nevertheless, what is striking, and what Schulkin marshals so ably, is the substantial amount of work that has actually been done on this topic. For those wishing to learn more, this book is an essential introduction.

Robert P. Heaney, M.D.
Copyright © 2001 Massachusetts Medical Society. All rights reserved. The New England Journal of Medicine is a registered trademark of the MMS.