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By Laura Crowley 22-May-2008 - Low sodium levels may increase the risk of cardiovascular disease, say researchers in an observational study, contradicting recent evidence from intervention trials indicating the dangers of too much salt.

The observational study was carried out by researchers from the Albert Einstein College of Medicine at Yeshiva University in the US and published in the Journal of General Internal Medicine.

It concluded that those participants in the lowest quartile of salt consumption were 80 per cent more likely to die from cardiovascular disease (CVD) than those in the highest quartile.

The researchers also said there is no evidence that high sodium intake can independently cause, or lead to a higher risk of, heart disease - bringing a new argument to the ongoing debate.

However, numerous scientists would disagree, as many are convinced that high salt intake is responsible for increasing blood pressure (hypertension), a major risk factor for CVD, which causes almost 50 per cent of deaths in Europe.

Sodium is an essential nutient, but in light of concerns about the effects of excess salt (sodium chloride) on health and about the amount of salt in processed foods, governments have been leading salt reduction initiatives.

The UK's Food Standards Agency (FSA) recommends a limit of six grams of salt per day for the general population, and less for children. Sodium levels can be found by dividing the amount of salt by 2.5.

Dr Hillel Cohen, lead author of the study and associate professor at Einstein, said: "Our findings do again raise questions about the usefulness or even safety of universal recommendations for lower salt diets for all individuals, regardless of their blood pressure status or other health characteristics."

The study

The data for the study came from the third National Health and Nutrition Examination Survey (NHANES III), an American study used to determine patterns of nutrition and health.

The research team investigated 8,699 subjects over the age of 30, and sodium intakes were taken from answers on consumption in the 24-hour period before taking the survey.

The respondents were placed into four brackets of sodium consumption - less than 2.060g; 2.060 to 2.921g; 2.922g to 4.047g; and 4.048 to 9.946g.

After adjusting for CVD risk factors such as smoking, diabetes, and blood pressure, the lowest quartile were found to be 80 per cent more likely to die from CVD compared to the highest quartile.

Additionally, the risk of death from any cause was found to be 24 per cent greater for those consuming lowers salt levels. The scientists said this difference was not substantial enough to dismiss the role of chance in the link between salt and CVD.

However, there are limitations from the study, as pointed out by Carrie Bolt, a nutritionist for The Consensus Action on Health and Salt (CASH).

She said: "These results are quite surprising and I would question the methodology as 24-hour recall is notoriously inaccurate as people always underestimate how much food they have eaten. Twenty-four hour urine analysis is a much more exact way to determine someone's sodium intake."

Still, Wouter Lox, managing director for EU Salt, told FoodNavigator.com that such research indicates the importance of being cautious: "We should be cautious about salt reductions programmes. When installing a long term health policy, we should be doing it step by step and all the time reflecting on whether it is the right step."

He also said lifestyle is a major contributing factor in the development of CVD, and raised concerns that focusing on one nutrient could mislead consumers that simply cutting down on salt can prevent the onset of such diseases.

Opposing views

There has been much evidence suggesting high sodium intake carried health risks. For example, last April, research published in the British Medical Journal said cutting salt intake could slash the long-term risk of CVC by up to 35 per cent.

The study included over 3,000 participants aged between 30 and 54 with high-normal blood pressure (pre-hypertension). Researchers intervened in their diet by reducing sodium, helping weight loss, and managing stress.

They found participants who cut back on salt during the trials tended to stick to a lower salt diet compared to those who had been in the control group. In total the researchers obtained information from 2415 (77 per cent) participants, 200 of whom had reported some sort of cardiovascular problem.

The results showed these pre-hypertensive individuals were 25 per cent less likely to develop cardiovascular problems over the course of the 10-15 years post-trial, with a 20 per cent lower mortality rate.

Commenting on the implications of this study, Prof. Graham MacGregor from CASH said: "With the accumulating evidence now available to us, and the fact that the majority of a person's salt intake comes from processed foods, every manufacturer now needs to act immediately to reduce the salt added to their foods," he added.

As awareness has grown on the possible health risks of salt, pressure has been mounting on food manufacturers to reduce the salt content of their foods.

According to research by TNS Worldpanel released earlier this year, reformulation in the food industry reduced British salt intake by 2,000 tonnes last year.

Sources

Journal of General Internal Medicine
Published on-line ahead of print, doi: 10.1007/s11606-008-0645-6
"Sodium Intake and Mortality Follow-Up in the Third National Health and Nutrition Examination Survey (NHANES III)"
Authors: HW Cohen, SM Hailpern, MH Alderman

British Medical Journal
Published on-line ahead of print, doi: 10.1136/bmj.39147.604896.55
"Long term effects of dietary sodium reduction on cardiovascular disease outcomes: observational follow-up of the trials of hypertension prevention (TOHP)"
Authors: N.R. Cook, J.A. Cutler, E. Obarzanek, J.E. Buring, K.M. Rexrode, S.K. Kumanyika, L.J. Appel, P.K. Whelton et al.