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Thursday, April 24, 2014

US Salt intake ~ No change in 40 years

In 1977, The US Senate Select Committee on Nutrition and Human Needs issued the 1st edition of Dietary Goals for the United States. One of the recommendations was to: “Reduce salt consumption by about 50 to 85 percent to approximately 3 grams per day”. This equated to 1,200 milligrams of sodium, which is the element of salt (sodium chloride) implicated as a contributory factor for the development of hypertension. This is slightly below the current sodium recommendations of the American Heart Association of <1,500 milligrams per day but is half what the Institute of Medicine set out as a target for the US population in its most recent report on dietary guidelines (2,300 milligrams per day). Whatever the figure, which the public is blissfully ignorant of, a campaign to lower salt intake has been in operation now for almost 50 years. Two recent papers have looked at the pattern of salt intake in the US over that period.

Measuring salt intake in our diet is very difficult because, over the short periods that dietary surveys are completed (1-4 days) salt intake can fluctuate dramatically. An equally important limitation in this area is the accuracy of food composition tables as to salt levels in foods. It should be noted that over 80% of salt intake comes from foods and not the saltcellar. Thus an alternative to measuring salt intake is to measure salt excretion, specifically sodium excretion, since the body does not normally accumulate sodium and thus the quantity excreted over 24 hours should roughly equal the amount ingested over the same period.

Collecting all ones urine over a 24-hour period is very difficult. The subjects have to carry a 5 liter plastic container with them throughout the day and everywhere they go and they have to bring the container to the bathroom to collect all of the urine excreted at each urinary event. Thus, not surprisingly, such studies tend to have relatively small numbers. Researchers at the Harvard School of Public Health searched the literature for all studies that involved a 24-hour urinary sodium excretion measure among US citizens over the period 1957 to 2003[1]. They found 38 studies. Of these, 5 were large with an average of 2,900 subjects but the remaining 33 were relatively small with an average of <400 subjects per study. The average daily output of sodium in milligrams per 24 hours was 3,526 prior to 1980, 3,418 across the ‘80s, 3,499 across the ‘90s and 3,849 post-2000. Thus over the 40 years from the 1970’s, there was no significant change in daily sodium excretion and the estimated average daily intake of sodium was 3,526 milligrams per day, well above any of the dietary guidelines issued.

Recently, another study on trends in sodium excretion was published in which a single urine sample (“spot” urine sample) was used to extrapolate to a 24-hour sodium excretion using adjustment equations set about by an international research consortium called INTERSALT. In this study, they used random samples from the US National Dietary Surveys[2].  Overall, they also found no change in sodium excretion over the period 1988 to 2010 (3,160 mg/d for 1988-1994, 3,290mg/d for 2003-2006 and 3,290 for 2010). This equated to a daily sodium intake of 3,317 milligrams per day, which is very close to what the Harvard researchers found.

What do we take from these findings of a complete resistance to change in sodium intakes? Reducing sodium in the human food chain is quite different to reducing the levels of fat or sugar (the other two so-called  “evils” of modern food). There are techniques and technological solutions, which allow sugar intake to be reduced without a loss of sweetness and also for fat reduction without the loss of the mouth feel of fat. That is not the case for salt. If the level of salt intake is reduced in breads it needs to be done over a long period so that consumers slowly adapt their palate to lower salt levels.  There is no adequate sodium alternative.

There is a second way to look at these data. According to the Darth Vaders of public health nutrition who would protect us all from the inferior aspects of the  modern diet, the food industry has dramatically manipulated salt, fat and sugar levels in foods over the last 40 years to manipulate our palate and their sales. As Michael Pollan put it in his book “In Defense of Food”:
“Today foods are processed in ways specifically designed to sell us more food by pushing our evolutionary buttons-our inborn preferences for sweetness and fat and salt”.

If salt intake hasn’t changed between the early 70’s and today, a 40-year period, then, has the food industry failed in their attempts to push our evolutionary button for salt preference? To me, the most likely explanation is very simple. Those who promulgate a food conspiracy, do so without addressing the available evidence on food and nutrient intake patterns, which should guide their thinking. But regrettably, when individuals set our their stance on some issue of science, it is rarely for changing, whatever the data might say.

[1] Bernstein AM & Willett WC (2010) AJCN 92, 1172
[2] Pfeiffer CM et al (2014) J Nutr 144, 698

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