Salt satisfaction

Sodium serves a vital purpose in the human body as it helps nerves and muscles to function correctly, and it is an important compound involved in maintaining fluid balance. Most of our dietary sodium intake is provided through the consumption of sodium chloride (common or table salt). About 80 per cent of this would come from processed foods and 20 per cent from salt used at the table or in home cooking. Table salt is made up of just under 40 per cent sodium by weight, so a 6 g serving (1 teaspoon) contains about 2,400 mg of sodium (note that some of the calculations below use the more exact 39 per cent of sodium).

Apart from table salt, it has been estimated that a further ten per cent of dietary sodium intake would be provided from naturally occurring sodium or sodium-containing food additives.

So far so good, but unfortunately high intakes of sodium can increase blood pressure, and high blood pressure can increase the risk of developing heart and kidney problems. To limit the negative effects of consuming too much sodium the World Health Organization (WHO) recommends that daily sodium intake should not exceed 2,000 mg (equivalent to 5 g of salt).

Initial reassuring results for the Australian population

Estimates of salt intake is most often made using a 24 h diet recall methodology. Using this method, Food Standards Australia New Zealand (FSANZ) in 2008 estimated that daily intake of salt across all Australians was 5.5 g. This amount of salt would deliver an average of around 2,150 mg sodium per day. Taking into account non-salt sources of sodium and salt added by the consumer, FSANZ assumed a probable daily sodium intake of around 2,500 mg. Even if small reductions in sodium intake can lead to big health benefits, this was considered to be close enough to the WHO recommendations. However, because this is an average, there would be a lot of Australians who eat more than this and more than the recommended maximum intakes.

A repeat diet recall survey to update the estimate of mean daily salt intake was performed in 2011-12 through the Australian Health Survey (AHS) with slightly higher results at 6.2 g for the Australian population, equivalent to 2,404 mg of sodium. In this case the amount included sodium naturally present in foods as well as sodium added during processing, but excluded the discretionary salt added by consumers in home prepared foods or at the table. Adjusting for the latter might indicate a daily sodium intake of 2,600 mg per person, which is still only marginally higher than the WHO’s recommended maximum of 2,000 mg and therefore little cause for alarm.

But now for the bad news.

Now new research shows that the dietary recall method used by the AHS and others likely substantially underestimate mean population salt consumption. The researchers suggested that Australians are consuming 50 per cent more salt than what was previously thought.

Researchers at George Institute for Global Health analysed the salt intake of 419 residents of Lithgow in NSW using two methodologies – the previous 24 h diet recall questionnaire but also 24 h urine samples. Based on the diet recall method, which was used by AHS, the average Lithgow resident consumed 6.8 g a day when including discretionary use. But based on 24 h urine collections, the researchers were able to conclude that the average was actually 9.0 g per day, equivalent to 3,545 mg of sodium. Correcting for salt lost through non-urinary excretion, the average was adjusted to 9.9 g per day or 3,900 mg of sodium.  This is almost double the WHO’s target for daily salt and sodium intake and suggests a several folds greater disease burden attributable to excess salt consumption in Australia.

The lesson

This is but one example of the challenges involved in estimating food intake. The 24 h recall method is considered superior to the food frequency questionnaire method so often used in epidemiological surveys. Still it showed a considerable bias. Again I suggest that epidemiological correlations between food intake and disease should be taken with a grain of salt (pun intended).