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Shaking it up: Ground-breaking new salt study goes against the grain

Metro News logo Metro News 2017-05-16 Genna Buck - Metro

Shaking it up: Ground-breaking new salt study goes against the grain © istock Shaking it up: Ground-breaking new salt study goes against the grain

Findings released this week are shaking up the world of salt science.

It’s widely accepted in the medical and nutritional communities that eating too much salt makes you thirsty and promotes water retention — the condition where you produce less urine and feel bloated and puffy.

But the exact opposite is true, according to an extremely detailed, long-term study published in the May 2017 issue of the Journal of Clinical Investigation. 

Normally it would be impossible to control every single thing adults eat for months on end and measure their urine volume every day (who would sign up for that study?).

But scientists took advantage of a natural experiment: A group of 10 Russian cosmonauts, all healthy men, who were kept in isolation to simulate a long-haul flight to Mars.

Over the course of two separate space simulations, one of 105 days and one of 205 days, the cosmonauts were fed otherwise-identical diets of high sodium (12 g/day), medium sodium (9 g) and low sodium (6 g), for several weeks in each case.

The researchers found the men drank significantly less water when they were eating more salt. And increasing their salt intake made them pee more, not less.

It’s not the first time this finding has been reported – studies as long ago as 2000 reached a similar conclusion. But it’s unusual.

Norm Campbell, a professor at the Libin Cardiovascular Institute of Alberta, said the study was very well done, but cautioned against over-interpreting.

“It is tough to be able to fully understand the research.  It was very carefully done, but is a small study in healthy men.  It may not relate to women or older people or people with disease,” he wrote in an email to Metro. 

“I would not dismiss the study … but neither would I believe the findings relate to all of us in everyday life yet.”

An even weirder finding in the study: On the saltiest diet, the men complained of constant hunger.

The researchers think it might be because salt increases the production of hormones called glucocorticoids, which help to burn fat.

A similar study in mice, published in the same journal at the same time, showed the same result: Mice fed a high-salt diet had to eat 25 percent more food just to maintain their weight. 

On the other hand, there’s a different, large review of past studies that shows eating more salt increases weight, Campbell said – though, he added, it’s hard to say how meaningful that is, because people who eat lots of salt probably eat lots of junk food in general.

But if the latest findings are confirmed, the logical conclusion would be that it’s really, really difficult to lose weight on a low-salt diet.

Over the past few years, clinical research has cast doubt on the notion – promoted by government agencies and the World Health Organization – that everyone should strictly limit their salt intake for the sake of their heart health.  Health Canada’s recommendation is 1,500 mg/day, or about 3.75 grams of salt (that’s less than a teaspoon). Canadians’ average intake is 3,800 mg (9.5 g).

There are many, many studies showing that reducing salt intake helps people with hypertension get their blood pressure under control. But whether it’s a good idea for healthy people to follow a low-salt diet is hotly debated.

Cutting salt has negative side effects, too, including spikes in stress hormones and increased levels of fats in the blood.

A 2015 review of the state of sodium science in the Journal of Clinical Hypertension – looking at the connection between excess salt and everything from mental distress to kidney disease – concluded “many of the findings are (due) to weaknesses in the study designs rather than real effects of sodium on health.”  

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