It's time to step up to the plate
For more than a quarter of a century, the Department of Health and Human Services and the Department of Agriculture have jointly published a new version of the "Dietary Guidelines for Americans" every five years. In 2010, the sixth edition of this publication will come out.
With reference to the consumption of salt, the Dietary Guidelines has served the public poorly. The fact of the matter is that the figures are based entirely upon expert opinion, not scientific trials. This was confirmed in the paper delivered at the Institute of Medicine workshop "The Development of DRIs 1994-2004: Lessons Learned and New Challenges" held in Washington September 18-20, 2007 by Dr. Peter Greenwald, Director of Cancer Prevention at the National Cancer Institute of the National Institutes of Health . Dr Greenwald described how most of the figures behind the recommended dietary intakes were based upon expert opinion - the lowest quality of medical evidence - rather than randomized controlled double blind clinical trials - the highest level of evidence.
The next iteration of the Dietary Guidelines will be the sixth in the series. Are we destined to continue basing our dietary recommendations on the lowest level of evidence? What would happen if Americans actually adhered to the dietary guidelines? The only evidence we have thus far is not particularly reassuring. In two studies on healthy young adults, carried out in very different geographic locations, college students were limited to 2300 mg Na/day - the Upper Limit for sodium recommended by the Institute of Medicine (1), (2). In both cases, all the students taking part in the trial experienced elevated aldosterone levels, and in the case of the Shapiro et al. study, they all showed evidence of arterial stiffness.
These are the only experimental results we have relating to the Institute of Medicine recommendations for sodium intake. Surely, it behooves us to test whether the IOM's recommendations are safe for Americans or not. If they once more appeared in the Dietary Guidelines, based solely on expert opinion, consumers would be justifiably bound to abandon confidence in the science behind these recommendations.
It is time to step up to the plate and have the Institute of Medicine recommendations tested experimentally
We need to establish a trial with a significant group of healthy young adults who are comprehensively counseled on how to limit their sodium intake to 2300 mg/day. As a lead-in to the trial, their baseline blood pressures, 24-hr urinary sodium, and plasma renin and aldosterone levels would be accurately determined. Once the trial began and the young adults maintained their sodium intakes at 2300 mg/day, these parameters will be checked on a weekly basis. The trial will continue for 30 days and a final analyses of blood pressures, urinary sodium, renin and aldosterone would be taken.
Such a trial would give us an excellent understanding of the impact of maintaining a consumption level of 2300 mg/day of sodium or less - on blood pressure as well as renin/aldosterone levels. It would be the first large-scale trial into the effect of adhering to the IOM recommendation for sodium and will be critically important in establishing the next edition of the Dietary Guidelines. Such a trial would not be overly costly and would, for once, remove considerable doubt regarding the validity of the IOM recommendations.
(1) Kodama N, Nishimuta M, Suzuki K., "Negative balance of calcium and magnesium under relatively low sodium intake in humans," J Nutr Sci Vitaminol (Tokyo), 2003, Jun;49(3):201-9.
(2) Shapiro, Y., Boaz, M., Matas, Z., Fux, A., & M. Shargorodsky, "The association between the renin-angiotensin-aldosterone system and arterial stiffness in young healthy subjects," Clinical Endocrinolog,. 2008 Apr;68(4):510-2.
