Sums and Assumptions about Salt

SUMS AND ASSUMPTIONS ABOUT SALT ALASTAlR R. MICHELL* Formal attitudes toward nutrition have only relatively recendy been based on scientific evidence. The foundations ofdietary customs remain cultural rather than scientific and rest to varying degrees on religion, superstition, and folk wisdom. With many aspects of nutrition, lack of knowledge and economic resources are formidable obstacles to progress, especially in poorer countries. The long-term effects of nutritional innovations may be difficult to assess within the time span imposed by the need to solve urgent medical problems. This theme can readily be elaborated , but some of the difficulties of achieving improved standards of nutrition are revealed in even clearer perspective by examining much simpler situations. It would be expected, for example, that with a dietary constituent like salt, deliberately consumed by man and animals for thousands of years and (at least in modern times) widely available and inexpensive, we should have a very firm idea of both requirement and safe upper limits of intake and little difficulty in putting sensible nutritional standards into practice. Less obvious factors, however, can be equally intractable, and their importance may become more evident from consideration of an aspect of nutrition which ought to pose no problem in wealthy and well-educated countries. This paper therefore examines the sums and assumptions which influence our intake of sodium. Salt, Nutrition, and Health "Good nutrition" is an evasive idea to define. It must derive from knowledge of adequate levels of intake and the effect of inadequacy or excess. Additional factors determine whether it is widely practised. First, there must be dispersal ofknowledge to those who advise both the public and the food industry. Second, public awareness of the available in- *Department of Medicine, Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire, United Kingdom.© 1984 by The University of Chicago. AU rights reserved. 0031-5982/84/2702-0370$01.00 Perspectives inBiology andMedicine, 27, 2 ¦ Winter 1984 | 221 formation is required; even the mildest proposals for modifications of food habits arouse hostility unless the reasons are convincing. Third, however persuasive the arguments may be, no recommendations will succeed unless their implementation is practicable, convenient, and cheap. Furthermore, it is no use suggesting alteration ofintake unless its quantification, or at least its estimation, is easily achieved among the common foodstuffs. Nor can success be expected if the changes involve more expensive or less palatable ingredients. Finally, all these rational factors must stand the test of prejudice or find the support of motivation . It ought to be easy with salt. It is cheap, simply measured, and a long-standing,dietary constituent for both humans and animals, and it would be hard to suggest an element having a better-documented body of physiological and clinical information than sodium. If it were totally harmless, "good nutrition" might amount to little more than indulging individual preference. But it is nearly 80 years since Ambard and Beaujard [1] first suggested a possible link between salt consumption and hypertension. Subsequent work by Allen [2], Dahl [3], and their successors has meant that for at least 20 years there has been a strong suspicion that excessive sodium intake might contribute to a considerable proportion of the clinically detected cases ofhypertension. Controversial as this view remains, the fact that hypertension is so widespread and so damaging should provide sufficient reason to examine the basis of salt consumption and explore its possible safe upper limits; sodium intake ought not to be a matter of indifference. It is therefore all the more discomforting to realise that, despite copious physiological, analytical, and nutritional data, we still do not know, for either man or animals, whether sodium should be regarded as a potential micronutrient, macronutrient, or even—within the range of customary intake—a toxin. Moreover, whatever knowledge we have is frequently resisted rather than applied, even by those best placed to give advice. Part of the problem relates to patterns of scientific thought, but part seems as deep-rooted as the instinct to take salt. Indeed, in the field of physiological psychology, this instinct has been the most extensively studied aspect of"nutritional wisdom," that is, the ability to select dietary components according to bodily need [4, 5]. The concept...