The Gut: The Unobtrusive Regulator of Sodium Balance

THE GUT: THE UNOBTRUSIVE REGULATOR OF SODIUM BALANCE ALASTAIR R. MICHELL* It is almost axiomatic that the mammalian kidney is the major regulator of body sodium; unfortunately, at least in the case of herbivores, it may not actually be true. The consensus that renal physiology is the central issue in sodium metabolism has led to fragmentation of research, with little interaction between work on sodium excretion and that on behavioural regulation of salt intake [1], little interest in nonrenal aspects of excretion, and even less in factors that might regulate the major reserves of sodium in bone [2]. The importance of studying the variety of mechanisms contributing to the regulation of body sodium is illustrated by changes associated with the reproductive cycle in sheep. Unexpectedly , oestrus and oestrogens, instead of causing salt retention, are associated with a reduction of body sodium in this species; 3 days later there is sodium retention. Whatever the reason for this unusual pattern, it is achieved notjust by the kidney but by coordinated changes in both urinary and faecal sodium excretion and also in salt appetite [3]. Renal excretion of sodium is frequently eclipsed by faecal losses in diarrhoea; indeed alimentary dysfunction, certainly in animals, is a more frequent cause of dehydration and salt loss than is renal disease. Diarrhoea is fundamentally a failure of net intestinal sodium and water absorption sufficient to overwhelm the compensatory reserve capacity of the colon. Nevertheless, in healthy mammals faeces are widely regarded as a trivial route of sodium loss. Indeed, both clinically and experimentally , confident estimates of sodium excretion (and intake) are frequently based on urine samples alone. "Potassium, calcium, magnesium and phosphate are the main inorganic minerals in faeces, with only traces of sodium . . ." [4]. The purpose of this article is to suggest that more than the authorship of this familiar view is Wrong and to emphasise that *Department of Medicine, Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire, AL9 7TA, United Kingdom.© by The University of Chicago. AU rights reserved. 003 1-5982/86/2902-0483SO 1 .00 Perspectives in Biology and Medicine, 29, 2 ¦ Winter 1986 | 203 despite the importance of sodium loss in diarrhoea, little is known about the regulation of enteric sodium excretion in normality. Faecal Sodium Because sodium excretion has seemed synonymous with urinary excretion for so long, a substantial accumulation of data revealing that faecal sodium excretion predominates in many herbivores has virtually been ignored [5]. Indeed, almost invariably, even the authors have declined to comment, presumably deterred by this physiological delinquency and fearing that it might detract from more important aspects of their work. Yet, in 1748 Brownrigg was aware that "the dung of such animals as feed upon grass or grain doth also contain plenty of common salt" [6]. Faecal sodium concentration and/or output has been shown to exceed that in urine from sheep [5, 7-10], horses [11, 12], and cattle [13-16], while faecal sodium in the terminal colon greatly exceeds that in urine from cattle, sheep, donkeys, and camels [17]. Even pigs, which are not strictly herbivores, may on occasion excrete the majority of their sodium via faeces rather than via urine [18, 19]. Individuals do not universally conform to this pattern. For example, two studies show that the majority of dairy cows excrete sodium mostly via urine [20, 21]; it is notable in both studies, however, that among those on lower intakes (700 mmol/day or less), faecal excretors ofsodium outnumber urinary excretors by a ratio of 2 : 1 . Substantial amounts of sodium are needed to match the output in milk, but provided that any additional sodium does not exceed 300 mmol/day the data show that sodium is mainly excreted in faeces [20]. Even 300 mmol/day is twice the basic maintenance requirement [22]—thus, on moderate rather than exorbitant intakes of sodium, faecal excretion predominates. Similar conclusions emerge from consideration of the regression of faecal sodium excretion on sodium intake in cattle [22]. Sheep are particularly interesting because any group, regardless of diet, breed, or genetic red cell type, usually contains individuals consistently excreting most of their sodium intake renally or in their faeces. This deflects the argument that faecal sodium...