The dangers of eating too much salt

I love salty food.  Like many people I eat significantly more than the recommended 2300 mg (2.3 g) of sodium per day (about 1 teaspoon of salt); the average American eats 3400 mg/day. There is strong evidence that a high salt diet can raise blood pressure which can lead to cardiovascular disease (CVD). How unhealthy is consuming more than the recommended amount of salt?

A new study published in the New England Journal of Medicine and described by Harvard School of Public Health (HSPH) news reached a stark conclusion:

"Excessive salt consumption may be behind one in 10 deaths from heart disease and stroke worldwide — one in five for people younger than 70. The global total for annual deaths linked to sodium may be as high as 1.65 million."

Woah those are some big numbers: 1 in 10 deaths from heart disease and 1.65 million deaths per year linked to elevated sodium.

Let's start with some background. The physiology behind the relationship between salt and CVD is pretty straightforward. More salt in the diet leads to more sodium in the blood leading to a higher blood osmolarity. The osmotic effect will cause a larger blood volume (because the kidneys will remove less water) which raises blood pressure. Higher blood pressure confers greater risk for heart disease and stroke (link).

What is the evidence? There are two parts to this story. First, there is the overwhelming data supporting the link between eating too much salt and high blood pressure:

"Essential hypertension is also greatly influenced by diet and lifestyle. The link between salt and high blood pressure is especially compelling. People living on the northern islands of Japan eat more salt per capita than anyone else in the world and have the highest incidence of essential hypertension. By contrast, people who add no salt to their food show virtually no traces of essential hypertension. The majority of all people with high blood pressure are "salt sensitive," meaning that anything more than the minimal bodily need for salt is too much for them and increases their blood pressure."

Second, there is the evidence demonstrating a link between high blood pressure and heart disease. High blood pressure damages the inner lining of arteries. This damage facilitates the formation of plaques in the blood vessels leading to atherosclerosis, and raising the risk for coronary heart disease (CHD) and stroke.

As a result, multiple major health organizations have warned of the dangers of excessive salt consumption:

"A high dietary intake of sodium is associated with elevated blood pressure, a major risk factor for cardiovascular disease. The United Nations, World Health Organization, Centers for Disease Control and Prevention, and other organizations have emphasized the relationship between dietary sodium and cardiovascular outcomes."

Here in the United States the dietary guidelines "recommend limiting sodium to less than 2,300 mg a day — or 1,500 mg if you're age 51 or older, or if you are black, or if you have high blood pressure, diabetes or chronic kidney disease."

If you are looking to cut down on salt, the key is to avoid processed food or restaurant food. The typical American obtains 75% of salt from processed or restaurant food and only 25% from home-cooked meals. Some examples of the guilty culprits include the following:

• 2 slices of salami = 362 mg sodium
• Cup of chicken noodle soup = 744 mg sodium
• 1oz potato chips = 136 mg sodium
• 1 Tablespoon of ketchup = 167 mg sodium

The new Harvard study described above builds upon this foundation by examining dietary salt intake and health outcomes in a large number (66) of countries:

"We collected data from surveys on sodium intake as determined by urinary excretion and diet in persons from 66 countries (accounting for 74.1% of adults throughout the world)."

The study consisted of three parts. First they examined the Effects of Reduced Sodium Intake on Blood Pressure:

"In our primary analysis of reduced sodium intake and blood pressure, In our primary analysis of reduced sodium intake and blood pressure, we found strong evidence of a linear dose–response relationship When the data were evaluated with the use of inverse-variance weighted meta-regression, each reduction of 2.30 g of sodium per day was associated with a reduction of 3.82 mm Hg"

Thus, with this huge new dataset they identified a linear relationship between dietary salt and blood pressure with each reduction in 2.3g of sodium per day associated with a reduction of 3.82 units of blood pressure.

Second they assessed the Effects of Blood Pressure on Cardiovascular Mortality:

"The pooled analyses of blood pressure and cardiovascular mortality showed a log-linear (proportional) dose–response relationship, with no evidence of a threshold as low as a systolic blood pressure of at least 115 mm Hg"

In other words, they collected data on how high blood pressure affects cardiovascular mortality and established a log-linear relationship.

Finally, they calculated the Reference Levels of Sodium Consumption; the threshold beyond which one would expect these adverse effects to happen i.e. the maximum amount of salt that would be healthy:

"To estimate cardiovascular mortality attributable to sodium consumption, we used a reference level corresponding to a population mean (±SD) intake of 2.0±0.2 g of sodium per day."

Putting these three results together, the researchers calculated how each incremental increase in dietary salt caused an incremental increase in blood pressure, how each increase in blood pressure caused an increase in mortality, and the reference level of salt beyond which these unhealthy relationships hold:

"On the basis of the correlations between sodium intake and blood pressure and between blood pressure and cardiovascular mortality that are described above, and using a reference level of sodium intake of 2.0±0.2 g per day, we found that 1.65 million deaths from cardiovascular causes (95% uncertainty interval, 1.10 million to 2.22 million) worldwide in 2010 were attributable to sodium consumption above the reference level

Of these deaths, 687,000 (41.7%) were due to coronary heart disease, 685,000 (41.6%) were due to stroke, and 276,000 (16.7%) were due to other cardiovascular disease. Globally, 40.4% of these deaths occurred prematurely (i.e., in persons younger than 70 years of age)."

These conclusions are consistent with the current medical recommendations described above. On the other hand, there are important caveats described by an accompanying NEJM Editorial:

"However, given the numerous assumptions necessitated by the lack of high-quality data, caution should be taken in interpreting the findings of the study."

In summary, one can conclude that "too much" salt is unhealthy but how much is too much? The authors chose a reference level of 2000 mg of sodium/day, but they also calculated the mortality totals for a reference level of 4000 mg (and also 1000 mg). The debate continues. In another study published in the same issue, researchers only observed a salt effect at higher levels above 6000 mg of sodium per day.

Although the question is not completely settled, the weight of the evidence favors reducing salt consumption especially if you have hypertension:

"While the two additional studies raised questions about the relationship between sodium and cardiovascular disease, HSPH’s Walter Willett called the links “very clear.” Willett, Fredrick John Stare Professor of Epidemiology and Nutrition department chair, was not involved in any of the studies. In an interview with the Los Angeles Times, he said that while the optimal level of sodium consumption is not clear, “we should be moving toward lower sodium intakes.”

Figure 1. If you have high blood pressure, you may want to avoid this.