Friday, July 08, 2011

A Decided Lack Of Scientific Certainty



# 5680



Last April I blogged on a CDC Grand Rounds presentation called Sodium Reduction: Time for Choice. As the following presentation slide illustrates, it promoted the idea that as a society are endangering our health by consuming too much salt.



Or are we?


Well, not according to a recent Cochrane review called


Reduced dietary salt for the prevention of cardiovascular disease

Taylor RS, Ashton KE, Moxham T, Hooper L, Ebrahim SPlain Language Summary

Cutting down on the amount of salt has no clear benefits in terms of likelihood of dying or experiencing cardiovascular disease


In fact, among patients with congestive heart failure (who are routinely advised to reduce salt intake), those on a sodium restricted diet saw an increase in all-cause deaths.




Hang on . . .


Last year (see IOM Report On Vitamin D) the Institute of Medicine released a long-awaited report on the benefits of vitamin D supplementation.


While the IOM’s Food and Nutrition Board increased their daily recommendations for Vitamin D, they fell well short of endorsing the high daily doses that have become in vogue over the past decade.


The entire 999 page report is available online for free, or may be ordered as PDF files, or as a hardback from the National Academies Press.


Essentially, the IOM recommended an RDA (recommended dietary allowance) of 600 IU of Vitamin D for most Americans, and a maximum of 4000 IUs


They also concluded:


The IOM finds that the evidence supports a role for vitamin D and calcium in bone health but not in other health conditions. Further, emerging evidence indicates that too much of these nutrients may be harmful, challenging the concept that “more is better.”


Not exactly what the vocal proponents of this popular supplement wanted to hear. And as you can imagine, their reaction to this report has not been entirely positive.


But over the eight months since this landmark report was released, we’ve seen other studies that take issue with the IOM’s findings.   


Last month, The Endocrine Society – which consists of 14,000 clinicians and research scientists – released new guidelines that recommended higher doses than did the IOM (Endocrine Society Issues Practice Guideline on Vitamin D).


And just over a week ago, a study in the Journal of Bone & Mineral Research that looked at Vitamin D levels during pregnancy recommended:


"the current vitamin D EAR and RDA for pregnancy women issued in 2010 by the Institute of Medicine should be raised to 4,000 IU vitamin D per day so that all women regardless of race attain optimal nutritional and hormonal vitamin D status throughout pregnancy."



This week Nature News has two excellent in-depth articles on why these reports contradict one another, and how difficult it can be to draw firm conclusions from observational studies.


Nutrition advice: The vitamin D-lemma

Review adds salt to a familiar concern 


While both of these articles are highly informative, they can’t tell us which side of these two debates is correct. 


And neither can I.


For those who like their science neat and tidy, devoid of ambiguity, and rock solid . . .  dueling studies such as these are no doubt more than a little disconcerting.


And we’ve seen similar (unresolved) debates over the efficacy of Tamiflu, the value of surgical masks in preventing respiratory infections, and the effectiveness of flu vaccines.


A few previous blogs on these uncertainties include:


RCTs: All That’s Gold Standard Doesn’t Glitter
When Studies Collide (Revisited)

When Studies Collide



None of this is intended to suggest that scientific research is useless, or fundamentally flawed.


Only that gaining scientific knowledge is a process . . . one that evolves over time . . . and scientific certainty is an elusive, sometimes unobtainable goal.


So the next time you see a study cited here, or any place else for that matter, keep in mind that while it may be useful . . . and it may even be correct . . .  


Caveat lector should always apply.