Monday, May 20, 2013
In over 40 peer-reviewed publications, we described in excruciating detail the physiological effects of increasing and decreasing sodium intake, in many cases using single-blind randomised trial designs in hundreds of volunteers.
We not only examined the effects of salt on blood pressure but also on a wide range of physiological, metabolic and psychological parameters. We studied the effects on acid-base balance, we conducted genetic studies, we even performed in vitro studies on cells cultured from “salt-sensitive” and “salt-resitant” individuals.
In many respects, these studies left me as confused about the role of sodium on these parameters as I was before. Not that we did not report findings that helped us better understand the complex physiology of sodium homeostasis – it is just that we failed to convincingly demonstrate any major health implications of these findings. In some cases we even reported adverse consequences of sodium restriction resulting both in significant elevations in plasma lipids and insulin resistance (perhaps not surprising given that reducing sodium intake markedly stimulates both the sympathetic and renin-angiotensin systems – the very systems we seek to block to reduce cardiovascular risk).
That was almost 20 years ago – the field does not appear to be much clearer today.
Thus, although surprising to some, I must admit that I was by no means surprised by the report on sodium released last week by the Institute of Medicine, with the rather revealing conclusion that,
“…the evidence from studies on direct health outcomes was insufficient and inconsistent regarding an association between sodium intake below 2,300 mg per day and benefit or risk of CVD outcomes (including stroke and CVD mortality) or all-cause mortality in the general U.S. population.” (or any other population for that matter)
This is not to deny that despite considerable methodological problems (not least in the actual measurement of salt intake), there is evidence to support the idea that higher salt intake may affect blood pressure and possibly cardiovascular risk. However, the data is certainly far less conclusive than food bloggers and health activists would lead us to be believe.
Not surprisingly, the same activists and organisations are now up in arms stopping just short of criticizing the scientific credibility of the IOM expert committee – no doubt, the same folks would have been applauding the conclusions of this “illustrious panel”, had the findings been more in line with their own activist agendas.
What is perhaps even more infuriating to those who have always considered the issue of sodium recommendations a slam-dunk case is the statement by the IOM that, there is in fact no basis on which to draw recommendations for the general public in recognition of the fact that significant proportions of the population may require higher sodium intakes and may even be likely to suffer harm from overly enthusiatic sodium restriction.
While I have no illusions that this report will in any way put the century old debate to rest (indeed the report calls for further research), I think that there is a much bigger message in this report that should let us tread cautiously when it comes to dietary recommendations in general.
Let us remember that associations (on which so many of our assumptions about healthy diets depend) simply do not prove causality, even when backed by seemingly plausible biological hypotheses derived largely from rodent toxicology. We should also remember that fancy statistical predictions on the vast number of lives lost or saved by altering the population intake of this or the other nutrient, are generally based on sometimes rather heroic assumptions that may well explain whey they are rarely (if ever) borne out by actual interventions.
Thus, whether we are talking about salt, fat, carbs, sugar, fibre, gluten, calcium, Vit D, dairy or red-meat, a degree of humility in advocating for policies and other measures to reduce or increase this or the other is generally in order.
Seldom in the field of nutrition are things as cut and dried as some will have us believe – if only food were as simple as tobacco.
New Delhi, India
Disclaimer: I was invited to be on the IOM Expert Committee but had to decline due to other obligations.