One of salt's major functions is to regulate blood volume and
pressure including the flexibility of the blood vessels. The human heart
is a big pump. When it contracts, it forces blood through the arteries of the circulatory
system; that pressure is "systolic," the "top" number. Between
heartbeats, the heart relaxes. Pressure measured between heartbeats is
"diastolic," the "bottom" number. When blood volume increases or the
blood vessel walls don't expand enough, blood pressure increases. Normal blood pressure is
less than 130/85 according to the National Heart, Lung and Blood Institute. In a population, blood pressures are a good
indicator of the incidence of
cardiovascular events like heart attacks and strokes.
As long ago as 2,000 B.C. when the famous Chinese "Yellow
Emperor" Huang Ti recorded salt's association with a "hardened pulse," we
have known of a relationship between salt and blood pressure. That’s not news. Nor is the fact that manipulating sodium intake
can change blood pressure in sensitive individuals, those termed "salt sensitive" (a condition with roots in both genetics and lifestyle ( 1 2). For a
century, medical researchers have been able to measure sodium and have documented that by
increasing or decreasing sodium in the body, many people’s blood pressure moves up
or down in small but often-detectable amounts. What
is more newsworthy is that over the past quarter-century, we’ve learned that the body
often makes physiologic
adjustments to “correct” for such changes and preserve blood pressure at the
“proper” level (e.g. changes in renin system response). And all this leads to the final point about salt
and blood pressure: the only rationale
offered for reducing salt to reduce blood pressure (in some people) is that it will lessen
their risk of a heart attack or stroke. The
news today is that not a single study has shown improved health outcomes for populations
on reduced sodium diets.
There continues a
controversy among medical researchers
about the appropriate public health response to these facts and whether it's time to reconsider the "conventional
wisdom" that reducing dietary sodium intakes is advisable. Some argue that public policy should demand
reduced sodium intakes to lower population blood pressure and, with it, the risk of heart
attacks and strokes. Others point out that
even significantly reduced sodium intakes produce very small population blood pressure
reductions compared to other interventions such as avoiding
obesity, and that dietary advice should be targeted towards “salt sensitive”
individuals. Yet others review the medical
literature and show no
documented health outcomes improvement for reduced-sodium diet. Some consider those advocating sodium reduction to
be basing their case on "junk
science." Trying to put the debate in perspective, the founder of the
American Society of Hypertension, Dr. John Laragh, in April 2001, summarized the situation
as follows:
“Is there any proven reason for
us to grossly modify our salt intake or systematically avoid table salt? Is this a proven
healthy thing to do, that is, will it save us from the major goals of antihypertensive
therapy, such as a later heart attack or stroke or kidney failure? Generally speaking the
answer is either a resounding no, or that, at best, there is not any positive direct
evidence to support such recommendations. And equally relevant, what are the new risks you
might be taking on by avoiding salt?”
Among the points of argument from those who squabble about
hypertension – as opposed to those who dismiss the arguments as irrelevant in the
absence of documented health outcomes benefits – are the following:
In any case, the researchers ( 1 2 3 ) and dietitians continue to debate about hypertension, but a new emphasis on evidence-based medical decision-making ( 1 2 3 4 5 6 7 8 ) arising in Canada and the UK – both for pharmacologic and non-pharmacologic interventions – suggests that the future debate may shift away from impacts on blood pressure itself to examine the more relevant policy end-point: whether a public health intervention – be it drugs or diet – actually has proven health outcomes benefits. While the public is only now hearing about "EBM" -- evidence-based medicine -- the popular press has been covering the scientific debate now for several years ( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 ) and the public is listening. Hopefully, we will be able to reduce the length of time converting research findings into practice and into public health policy. EBM works and will save billions in health care costs. For more information about the importance of focusing health care solutions on health outcomes and the use of evidence-based medication, you may wish to visit the Cochrane Collaboration, Oxford University's Centre for Evidence-based Medicine, the Health Information Research Unit at McMaster University or the Canadian Centres for Health Evidence.
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