Thursday, 5 May 2011

Is salt good for your health?

I have long been suspicious of the claim that salt is bad for human health. I suspect that the more salt you eat, the more thirsty you become and the more water you drink. The kidneys handle this all very well.

The debate has been going on a few decades. Some studies suggest that blood pressure is lower with lower intakes, others find no difference, and a few indicate low salt is bad for your health.

This week JAMA published one of the latter. The way it is written makes me wonder if the authors are apologising for the result. The study shows more deaths from heart attacks and strokes in those who consume less salt. They also claim that increasing salt over time increases blood pressure, but by a tiny amount. More about this below.

Kaplan-Meier Survival. From JAMA.

Reuters have reported on this finding, as has the New York Times. The salt police have reacted negatively to this article. It is clearly full of holes. Salt is evil and nothing should stop our crusade to ban this dangerous chemical.

The Times article talks to Peter Briss from the Centers for Disease Control who is convinced that salt is bad for your health and to Michael Alderman who is not. Briss complains that
that the study was small; that its subjects were relatively young, with an average age of 40 at the start; and that with few cardiovascular events, it was hard to draw conclusions.
Which would be true if the event rate were statistically insignificant. But as the study did find a difference this complaint is void. You can't argue that type 2 errors (false negative) are possible after a statistically significant result is found. The concern then is, have you made a type 1 error?
And the study, Dr. Briss and others say, flies in the face of a body of evidence indicating that higher sodium consumption can increase the risk of cardiovascular disease. 
Except that the evidence is conflicting. There is much data that points to salt not being a health concern.
But among the study’s other problems, Dr. Briss said, its subjects who seemed to consume the smallest amount of sodium also provided less urine than those consuming more, an indication that they might not have collected all of their urine in an 24-hour period.
Well they did exclude implausible urine volumes. But this comment doesn't make sense. People who eat less salt are likely to produce less urine on average. The high salt consumers raise the salt levels in their blood slightly which drives thirst. Increased fluid intake will associated with increased urine output. People can drink more than thirst dictates. But the average urine volume will likely be higher in the group who eat more salt. Finding the same urine volumes across the groups would be the real concern.
Lowering salt consumption, Dr. Alderman said, has consequences beyond blood pressure. It also, for example, increases insulin resistance, which can increase the risk of heart disease.

“Diet is a complicated business,” he said. “There are going to be unintended consequences.”

One problem with the salt debates, Dr. Alderman said, is that all the studies are inadequate. Either they are short-term intervention studies in which people are given huge amounts of salt and then deprived of salt to see effects on blood pressure or they are studies, like this one, that observe populations and ask if those who happen to consume less salt are healthier.
Exactly. Even if it does affect blood pressure, which is not certain, blood pressure is a proxy for other outcomes, mainly heart attacks and strokes. Blood pressure is clearly implicated in these diseases, but if less salt increases your blood pressure slightly and at the same time causes other changes that are detrimental for your heart then, on balance, it might be a bad thing.

We may not even be able to easily decrease our salt intake. Some research suggests that we closely monitor our salt intake thru specialised cells in the brain. If we are exposed to more salt we decrease our intake, and if we lack salt we actively seek it, such that the amount consumed over time remains a constant 4 grams per day.
Dr. Briss adds that it would not be prudent to defer public health actions while researchers wait for results of a clinical trial that might not even be feasible. 
So we should make recommendations and alter society in case salt turns out to be unhealthy when all the data is in, even though there is a possibility that such action may turn out to be hazardous?
Dr. Alderman disagrees.

“The low-salt advocates suggest that all 300 million Americans be subjected to a low-salt diet. But if they can’t get people on a low-salt diet for a clinical trial, what are they talking about?”

He added: “It will cost money, but that’s why we do science. It will also cost money to change the composition of food.” 
Much more sensible. Do nothing in public policy currently. Continue to do a variety of studies and gain an understanding of how salt works. Even then, such answers are physiological, they do not tell us public policy, they can at most inform us.

I am off to have some chips, for the good of my heart of course.

1 comment:

  1. bethyada,
    You write on a truly wide variety of interesting subject matter. Thanks for the efforts.

    Even then, such answers are physiological, they do not tell us public policy, they can at most inform us.

    The is/ought conundrum makes me nervous about "expert" driven public policy. Not only is there an issue as to whether or not that approach is appropriate, the experts get caught jumping the gun over and over again.

    ReplyDelete

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