The salt of the earth

Increased sodium consumption raises blood pressure, and high blood pressure is strongly correlated with (and perhaps causes) heart disease. Thus, a low salt diet reduces the risk of heart disease.

Sounds reasonable.  But apparently wrong.  A committee set up by the National Institute of Medicine (part of the CDC) just released their review of the research: no benefits from low salt consumption.  The leader author on the report says, according to the NY Times:

Although the advice to restrict sodium to 1,500 milligrams a day has been enshrined in dietary guidelines, it never came from research on health outcomes… Instead, it is the lowest sodium consumption can go if a person eats enough food to get sufficient calories and nutrients to live on. As for the 2,300-milligram level, that was the highest sodium levels could go before blood pressure began inching up.

In its 2005 report, the Institute of Medicine’s committee said that sodium consumption between 1,500 and 2,300 milligrams a day would not raise blood pressure.

That range, Dr. Strom said, “was taken by other groups and set in stone.” Those other groups included the Department of Agriculture and the Department of Health and Human Services, which formulated dietary guidelines in 2005.

This reveals a lot about how government guidelines are made:

  • Lack of a research foundation was not a significant obstacle to creating the guideline.
  • The upper end of the “safe” range was dropped in favor of the lower end.
  • The lower end that became the guideline is that minimum level necessary to maintain life; thus it is not surprising that some studies have shown that as salt intake approaches that 1500 milligram level that other bad things start happening.
  • There are a lot of warnings (for good reason) about the political power of the food industry in setting dietary guidelines.   Manufacturers of high sodium foods make lots of money, yet this low guideline still persisted. Lesson: the political power of those who oppose processed foods should not be ignored.  [Interesting political query:  how did this diffuse interest conquer the concentrated interest?]
  • Lots of people think that sodium is bad.  How many of them know that a sodium deficiency will kill you? Really fast.

Before you start adding more salt to your potatoes, however, it is good to remember that this new body of research is not based on the carefully controlled experimental framework that we would like to see.  Indeed, much of what we “know” about nutrition is similarly suspect because it is very, very hard to do those types of studies on human beings.

Dietary guidelines are largely based on pieces of empirical observation connected by theories–not on RCTs of real diets.  The same could be said of a lot of social science work, including economics, so I’m not picking on nutritionists.  And if you think nutrition science is suspect and politicized, just take a little trip through psychology.  Those folks have been fighting about revisions to the DSM for a long time, and the casualty count is very high.

Because people are much harder to study than laboratory mice, we know much, much less than we think we do.  Even about mice, I presume.

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