New Study Finds No Connection between Salt and Heart Disease
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By Ewen Callaway of Nature magazine
A controversial new study is questioning the oft-repeated connection between the consumption of too much salt and the development of cardiovascular disease. The meta-analysis, published online today in the American Journal of Hypertension1, examined the results of seven clinical studies and found no solid proof that reducing salt consumption prevents heart conditions.
The World Health Organization recommends that no more than 5 grams of salt per day should be consumed, whereas people in many Western countries typically eat twice as much. Public-health authorities are already looking at ways of cutting the salt content of foods. For instance, Britain's Food Standards Agency is working with food manufacturers to reduce sodium, and the New York City health department is spearheading a national initiative to cut Americans' salt consumption by 20% over 5 years. Nature examines the new study and its implications for such policies.
How might salt cause heart disease?
Consuming sodium causes the body to retain water, thereby increasing blood pressure, and hypertension is a risk factor for heart attack, stroke and other cardiovascular diseases.
A number of clinical trials and meta-analyses have suggested that cutting one's salt intake reduces blood pressure, says Rod Taylor, a statistician at the University of Exeter, UK, who led the new study. But his team says that it is unclear whether restricting salt intake reduces blood pressure sufficiently to protect against heart disease. A previous meta-analysis found that eating less salt reduced people's blood pressure -- but on average only slightly2.
On the other hand, many studies comparing how much salt people consume with their incidence of cardiovascular disease have come up with clearer links. A 2009 meta-analysis3 of 13 such studies, incorporating 177,000 patients, found that a high-salt diet increased the risk of stroke by 23%.
Why has it been so difficult to prove whether or not cutting salt prevents cardiovascular disease?
Observational studies, which look at the correlation between salt intake and the incidence of disease, can't directly pin reductions in cardiovascular disease on eating less salt, Taylor says. "People are choosing to reduce their salt, but it may be associated with a whole host of other healthy behaviors. They may be more active and eating less saturated fat" -- factors that also protect against cardiovascular disease.
"To inform policy and whether we should be advising people to reduce their salt, observational studies do fall short," Taylor adds. Controlled experimental trials, in which patients are placed on a low- or high-salt diet and followed over time, should offer a clearer answer, he says.
How was the new study conducted?
Taylor's team trawled through 2,600 published journal articles on the link between salt and cardiovascular disease, and came up with seven controlled studies that included a total of 6,250 patients who were tracked for 6 months or longer. Taylor's team grouped the patients into three categories-those with normal blood pressure, those with high blood pressure and those diagnosed with heart failure--and analyzed how their salt intake was associated with blood pressure, incidence of cardiovascular disease and incidence of death.
What did they find?
People on low-salt diets saw their blood pressure drop. But Taylor's team found no statistically significant difference in the subjects' rates of heart disease compared with rates in people who didn't reduce their salt intake. Furthermore, a low-salt diet was not linked to reduced death rates in people with normal blood pressure or high blood pressure. "In one trial in heart-failure patients, we rather worryingly found that reductions in salt increased risk of death," Taylor adds.
Why is this result different from those of other observational studies?
Taylor isn't sure why his team's review came to a different conclusion from previous observational studies. It could be that there is no link between cutting salt and preventing cardiovascular disease, Taylor says. But he questions that interpretation because his team noticed reductions in blood pressure.
Perhaps the study did not have look at enough patients to uncover a statistically significant effect. This possibility is raised by Francesco Cappuccio, who heads the World Health Organization Collaborating Centre for Nutrition at the University of Warwick, UK. "The only problem here is that they're not statistically significant and the reason for that is the meta-analysis is too small," Cappuccio says. He notes that low-salt diets did show a trend towards protecting against cardiovascular disease.
Taylor thinks the best explanation is that patients cut their salt intake early on in the studies but eventually allowed their intake to creep up, masking any benefit. "They're intensively followed up for a couple years, and 8 or 10 years later these people's behavior has probably reverted to what it was," he says.
Could a larger, closely supervised clinical trial get to the bottom of the link between salt and cardiovascular disease?
Taylor thinks that studies that rely on dietary advice directed at individuals, like those his team analyzed, do not do enough to cut people's salt consumption. Rather, scientists should look to studies that investigate the effects of public-health efforts, such as clearer labeling of a food's salt content, to see if these can prevent cardiovascular disease and death. "We need to design studies that are population-level interventions," he says, "essentially where we take a community of individuals and we target them in various ways to change their behavior and help them sustain that behavior, rather than just give them a pamphlet and have them sit down with a counselor for an hour."
However, Cappuccio says that such studies are costly, impractical and unnecessary. "They hold public health to ransom by asking for something that's impossible," he explains. "Salt, like many other nutritional factors, falls into the category where action has to be taken in the face of overwhelming evidence, even in the absence of a controlled clinical trial."
What are the policy implications of the new study?
The answer depends on whom you ask. Michael Alderman, an emeritus epidemiologist at the Albert Einstein College of Medicine in New York City, believes the study adds to growing evidence that cutting salt does not help people who consume modest amounts of the stuff. Public policies aimed at forcing salt reductions are misguided and potentially dangerous, he says. Eating less salt may reduce blood pressure, which is beneficial for the heart, but it could also increase insulin resistance, triglyceride levels and sympathetic-nerve activity -- all risk factors for cardiovascular disease.
Taylor, too, worries that policies directed at compelling people to eat less salt could have unintended health and economic consequences, and he calls for more research into the health effects of salt reduction. "Whilst intuitively reducing salt across the board appears to be a good thing, I would say we still need the evidence to prove it," he says.
Cappuccio is concerned that the new study will be used as a smokescreen, making it more difficult for public-health authorities to convince or even compel food manufacturers to reduce salt in processed and prepared foods, as these account for most of the salt in our diets. "It's creating a sense of controversy where policy is pushing forward," he explains.
This article is reproduced with permission from the magazine Nature. The article was first published on July 6, 2011.
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11 Comments
Add CommentI had a hard time associating my blood pressure with salt. At this time ,I respond well to coral calcium.At least it seems like I do. Prophesy appears to say a cme will kill the heart.
Reply | Report Abuse | Link to thisThe meta-analysis findings are not entirely surprising. However, a better understanding of the salt/health relationship, putting salt risk factors in perspective with other health factors, is in order (particularly, given a study found, "...reductions in salt increased risk of death"). Most of us have a knee jerk "salt equals bad" bias; thus, placing weight on conflicting information may be difficult. An idea that has become nearly totally ingrained will take strong evidence to shift. Because of the prevailing belief regarding this salt issue, a "chicken/egg" problem will necessarily exaggerate the health variance to a degree (simply because people tending to do healthful things, tend also to reduce salt intake). In the past I've made the similar point that people who exercise report better health because they are in better health in the first place (again, chicken/egg; when you’re in poor health you are less able to exercise etc.). Of course, we are all true believers in the benefits of exercise (if we were to find that exercise health benefits are even slightly over stated, that would likely tend to enable exercise backsliders; one wouldn’t want to make a PSA out of any such information).
Reply | Report Abuse | Link to thisIt is symptomatic of modern science that conclusions tend to support popular hypotheses & when they do not the rusted on believers do not wish to acknowledge the contra evidence. Remind anyone of another branch of science?
Reply | Report Abuse | Link to thisFirstly, I am becoming more & more convinced that there is something fundamentally wrong with the paradigm of clinical trials & the way they are interpreted, and with non-homogeneous selective meta-analyses in particular. But that is a digression.
Reply | Report Abuse | Link to thisBack to topic: When I was a youngster, people returned from the fighting in North Africa and elsewhere, convinced that a good daily dollop of salt was necessary for life & limb; "don't forget your salt tablets!". This seems to persist in the South-Eastern seaboard states.
Then the medical profession came up with one of it's occasional (& often delusional) insights that serum "salt" levels may correlate with vascular incidents & high blood pressure. Very often these fads & fashions are based on based on:
(1) we can measure it, & it must do something (though why this doesn't apply to magnesium is lost in the mists of pompous & fragile reputations), so let's do it.
(2) Let's do it because we can (applies to many investigations, however poorly performing, and even to radical procedures like arterial bypasses),
(3) "We need to be seen to be doing something".
So, what is & isn't going on?
I quite agree with the points already made by rgcorrgk, that there is a need for perspective, and that care should be taken not to miss the "alternative hypothesis" (that is, that there may be another explanation).
One alternative hypothesis might be that the people showing high sodium levels are individuals inclined to gluttony (which may be an innocent disorder of the endocrine system), and to eating badly-prepared additive-laden fast food - both of these significant risks in themselves. It's a chicken omelette thing (which came first...?).
The water retention idea is simplistic and, well, arguably wrong, whatever the text-books say.
Of greater importance is the ratio of sodium to other electrolytes - not generally considered in day-to-day office ConMed - and an constant adequate sufficiency in the system and intake.
Also, salt tolerance (and salt deficit tolerance) varies from person to person, We are not an identical herd.
Many practitioners will have come across individuals with normally low serum sodium levels. Such individuals, if they are clued-up, then have to fight off regular ConMed attempts to repeat chest X-rays to check for bronchial cancer. There must be a certain irony in this, but of course if you keep looking for carcinoma with X-rays, Heisenberg tends to take revenge (thus proving the correlation?).
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Reply | Report Abuse | Link to thisThen there is the question of what else is in the cooking salt apart from NaCl.
A National newspaper recently ran a hugely uninformed article on this subject, apparently totally ignorant of the importance of trace elements (commonly in sea & rock salts) to the animal constitution. What the motivation for this article was, one can only guess, though it may have a connection to the wider Codex Alimentarius agenda.
Exclusive use of refined salt will probably make you ill. Appropriate moderate use of rock salts from ancient, uncontaminated seas are likely to be helpful.
But even then, not in constant excess (it can lead to a condition similar to anorexia).
And that is not to mention whether we actually know what commercial salt additives actually do to us.
The Food Chemicals Codex Sodium Chloride Monograph (2008) specifies that salt may contain up to 2% (!! in relation to trace elements!!) of "anti-caking, free-flowing, or conditioning agents" like sodium ferrocyanide and ferric ferrocyanide, dextrose, etc.
To be fair, the cyanides probably enhance longevity by promoting apoptosis, but it is curious that they are permitted in common table salt, when the use of B17, laetrile is severely discouraged in many ConMed circles (again, one can only conjecture why). The iron is probably useful, too, now that we cook in non-iron pots.
But the main conclusion must be that sub-acute toxicity of salt is most likely just another delusion of crowds, a passing 'scientific medical' fad, like the calorie myth (try eating coal, see where it gets you), the aspirin-a-day myth (more harm than good, as I've been saying for years), the various cholesterol myths, the water flouridation myth ......
FYI - The U.S. Army was still distributing salt tablets to its ground troops in Viet Nam, anyway - to prevent dehydration in temperatures exceeding 110.
Reply | Report Abuse | Link to thisNow that I take so many medications to manage my heart condition (not related to salt uptake), manually managing their considerable side-effects, I'm being pressured to also reduce my salt intake. In retrospect, since high blood pressure has not been a problem for me, I now intend to ignore that medical advice and continue to enjoy seasoning my food as usual!
An additional issue, pertains to dietary iodine insufficiency; iodine is necessary for synthesis of thyroid hormones which have myriad metabolic and neurologic functions. Moreover, maternal thyroid hormone orchestrates fetal neurodevelopment, beginning in very early gestatations. Iodized salt is an important source of dietary iodine for many U.S. populations, In particular for residents of inland areas.
Reply | Report Abuse | Link to thisThe biggest fallacy is that salt has some negative bearing on a healthy person that does NOT have high blood pressure. On the contrary when I was taking care of HS football players in the Arizona desert in August, I had many kids cramping and passing out, their moms dutifully kept them from salt and the coaches (and the other idiot doctor in town) had them replacing the white crystally stuff that was caked on their uniforms the next day ( salt) with sugar water and a few 'lytes (some formula from Florida gators). Salt tablets or liberal use of salt at the table usually took care of the problem. In case you all haven't notice, two things you can take home, NO health advice applies to everyone; and if the government says something is bad no matter what the studies say, it probably ain't no thing so forget about it.
Reply | Report Abuse | Link to thisWhat seems so surprising to me is the continued notion that salt intake strongly drives serum sodium level. Are we to believe that our bodies are so unsophisticated that simply ingesting salt raises the serum sodium level. Of course not! As silly as suggesting that an extra glass or two water will raise blood volume and therefore blood pressure. What I read is that a hormone (insulin) tells the kidneys to retain/excrete sodium. So in the presence of high insulin the kidneys retain sodium and as insulin falls, so does serum sodium. And what drives insulin, you may ask? Carbohydrate intake. According to Taubes, if I remember my source correctly, one of the early arguments against a low carb diet was "blood pressure becoming too low". Of course that info has only been used to (incorrectly) attack low carb diets, not to help hypertensives lower their blood pressure.
Reply | Report Abuse | Link to thisYes, Creation Science and Global Warming Denialism. Interestingly has anyone been able to show that high blood pressure *causes* heart disease as opposed to just *indicates* heart disease. In other words, is it possible to have increased blood pressure caused by benign factors that do not increase risk of heart disease? Could salt be one of those benign factors? Have we been making the logical fallacy of "affirming the consequent" all this time?
Reply | Report Abuse | Link to thisI've never been convinced that 'salt is bad', noone reasonable ever has since salt is quite necessary. And salt is found liberally in most fatty foods, so that makes distinguishing heart problems with salt and fats difficult.
Reply | Report Abuse | Link to thisBUT, why not be moderate? If you exercise in the heat, obviously you need more salt and potassium. But otherwise 5 grams is quite enough salt.
Furthermore, salt does cause ones kidneys to have to work harder (along with quite alot of overly consumed items like sugar). Why overwork a necessary organ?
But sure, fine, eat whatever you want because scientists are still working through the details.