25 August 2014

New research hints that sodium guidelines need to be revised

A new research study from the University of Gothenburg in Sweden on how much salt, or rather sodium, people can safely consume has been making the rounds. According to the university, the study is one of the largest of its kind and indicates that today’s recommended maximum daily intake may be wrong. 

A sprinkling of salt on nuts.

The Prospective Urban Rural study Epidemiology (PURE) study followed over 100,000 persons from 17 countries* for four years and made a number of measurements, including their intake levels of sodium and potassium and how they are related to blood pressure, mortality, heart disease and stroke. Two reports from the global study mentioned by the university were published 14 August in the New England Journal of Medicine.

"This is one of the most ambitious studies ever conducted on the effects of people’s salt intake over time," says Annika Rosengren, Professor of medicine at the University of Gothenburg and one of the researchers behind the study.

Too much dietary sodium has been known as a serious health risk because it is clearly linked to increases in blood pressure and incidence of stroke. The results of the new study show that more sodium in the diet does indeed increase a person’s blood pressure, in particular among those with a daily sodium intake exceeding 5 grams (which corresponds to 12.5g of table or cooking salt), those who already suffer from high blood pressure, and those who are 55 years of age or older.

However, one of the two reports from the University of Gothenburg study shows that the recommended level may be too low for good health, said the university. Although a low intake of sodium was in fact associated with a somewhat lower blood pressure, low levels of sodium in the diet can also be harmful. A 2012 review of existing research in the European Journal of Heart Failure concurs, showing that too little sodium in the body, called hyponatraemia, is a good predictor of subsequent death due to heart failure. 

The findings are controversial as other studies, such as this one in 2013, have found that lower sodium intake is linked to a reduced risk of stroke and fatal coronary heart disease in adults. The PURE study found the lowest risk of cardiovascular events and death among those who consumed moderate amounts of salt, rather than cutting salt to a minimum, and that the health risks increased both above and below this interval. The statement from the University of Gothenburg did not specify how much salt was considered 'moderate', and while it may make sense to say that low-sodium diets are likely to lead to hyponatraemia, hyponatraemia can also be caused by some medicines and diseases too.

"The scientific support for today’s recommendations is very weak. At the same time, however, a high sodium intake remains associated with increased risk for both high blood pressure and cardiovascular disease," says Rosengren.

Another study in the August 14 issue of The New England Journal of Medicine, from Tufts University, noted that more than 1.6 million cardiovascular-related deaths per year can be attributed to sodium consumption above the World Health Organization’s recommendation of 2g per day. An analysis evaluating populations across 187 countries found the average level of global sodium consumption in 2010 to be 3.95g per day, with all regions of the world above recommended levels.

In their meta-analysis of controlled intervention studies, the researchers found that reduced sodium intake lowered blood pressure in all adults, with the largest effects identified among older individuals, blacks, and those with pre-existing high blood pressure.

“These 1.65 million deaths represent nearly one in 10 of all deaths from cardiovascular causes worldwide. No world region and few countries were spared,” said Dariush Mozaffarian, Dean of the Friedman School of Nutrition Science and Policy at Tufts University, who led the research while at the Harvard School of Public Health. 

Mozaffarian also chairs the Global Burden of Diseases, Nutrition, and Chronic Disease Expert Group, an international team of more than 100 scientists studying the effects of nutrition on health and who contributed to this effort. “These new findings inform the need for strong policies to reduce dietary sodium in the US and across the world.”

“We found that four out of five global deaths attributable to higher than recommended sodium intakes occurred in middle- and low-income countries,” added John Powles, last author and Honorary Senior Visiting Fellow in the department of public health and primary care at the University of Cambridge. “Programmes to reduce sodium intake could provide a practical and cost effective means for reducing premature deaths in adults around the world.”

A December 2013 study authored by a number of scholars including both Powles and Mozaffarian found that sodium intakes were highest in East Asia, Central Asia and Eastern Europe (mean >4.2g/day) and in Central Europe and Middle East/North Africa (3.9–4.2g/day). Regional mean intakes in North America, Western Europe and Australia/New Zealand ranged from 3.4 to 3.8g/day.

According to the World Health Organisation (WHO), sodium is found naturally in foods such as milk and cream (approximately 0.05g of sodium per 100g) and eggs (approximately 0.08g/100g). Bread contains about 0.25g/100g), snacks such as pretzels, cheese puffs and popcorn have approximately 1.5g/100g, while soy sauce would have 7g/100g), and bouillon or stock cubes, approximately 20g/100g.

A July 2014 study in the UK found that halloumi cheese (2.71±0.34g/100g) and imported blue cheese (2.71±0.83g/100g) contained the highest amounts of salt, while cottage cheese (0.55±0.14g/100g) contained the lowest amount of salt.

Health Xchange, a portal set up by Singhealth, has listed nutritional content of common Chinese street (hawker) foods in Singapore. Fishball soup tipped the scales at 2.9g of sodium for a serving of 798g. Soup dishes dominated the sodium stakes. Other dishes with over 2g of sodium per serving included lor mee (540g), which contains over 2.5g of sodium; kway chap with 2.3g of sodium; prawn mee soup, 2.4g of sodium; 569g of Penang laksa, with 2.2g of sodium, and 528g of ban mian with almost 2.2g of sodium.

*This appendix on another aspect of the PURE study lists participating countries. High-income countries include Canada, Sweden, and the UAE; upper-middle income countries were represented by Argentina, Brazil, Chile, Malaysia, Poland, South Africa, and Turkey, while China, Colombia, and Iran were the lower-middle income countries. Bangladesh, India, Pakistan, and Zimbabwe contributed data for low income countries.