The average American sodium intake is about 3,400 milligrams a day (about 1.5 teaspoons). We are regularly told we need to reduce it. The recent widely-cited Institute of Medicine (IOM) report now tells us that the evidence from studies is insufficient to recommend restricting sodium intake to the previously recommended 1,500 mg or 2,300 milligram levels. Some studies even claim that a low sodium intake can be harmful.
Is the widespread medical belief about salt wrong? Or is this recommendation wrong? Where does the truth lie? I think somewhere in the middle. Here's why.
Let's start by reviewing what studies consistently show, and about which there is no disagreement: (1) on average, a high sodium intake raises blood pressure, (2) high blood pressure increases the risk of cardiovascular events such as stroke and heart attack, and (3) reducing sodium intake lowers blood pressure, increases the effectiveness of medications prescribed to treat hypertension, and reduces the amount of medication needed. And here is where the controversy lies: Does lowering sodium intake over many years translate into fewer strokes, heart attacks and deaths? Or, contrarily, is it harmful? This is where the IOM report came to the conclusion that there is insufficient evidence that it does, and some evidence of harm, and, therefore, recommends against substantial reduction of sodium intake.
There are two major problems with their recommendation:
First, it offers a single recommendation for all. It ignores the fact that we differ from one another. There is considerable evidence that restricting sodium intake would be more likely to help some than others. A high salt intake raises blood pressure in some people and not at all in others. If your blood pressure is salt-sensitive, reducing salt intake will lower your blood pressure; if it isn't, it won't. How can you tell if your hypertension is salt-sensitive? Several ways. First, as discussed in my recent book, by demographics: About 25 percent of whites and 50 percent of blacks with hypertension are salt-sensitive. With aging, or with reduced kidney function, the proportions are considerably higher. Second, a low level of renin on a blood test. Third, you can do your own experiment at home -- cut way down on salt for four days or so and see if your blood pressure is lowered.
Second, many of the studies that are the basis for the IOM report are simply not very good. Here are some of the problems with the evidence:
1. Study results are very inconsistent with each other. Results are all over the place, telling us that restricting sodium intake may increase, decrease or have no effect on the risk of cardiovascular events!
2. The quality of most of the studies was not good. There are very few controlled trials because it is next to impossible to do a study in which subjects restrict their sodium intake for 10 years. Instead, most studies compare the outcome of people with low vs. higher sodium intake. But estimations of sodium intake is quite inaccurate. Also, many who painstakingly adhere to a low sodium diet have medical issues or are at higher risk to begin with. They do worse because of their increased risk, and not because of their lower sodium intake.
3. If a high sodium intake is elevating your blood pressure, your doctor will treat it, neutralizing the harmful effect of sodium on blood pressure and cardiovascular risk. Which is better: a low sodium diet with less medication, or a high sodium diet with more medication? If the blood pressure ends up the same, probably little difference, although ideally the less medication the better -- fewer adverse effects and less cost.
So what common sense conclusions can we reach?
1. If you have salt-sensitive hypertension, restricting salt can lower your blood pressure, reduce your need for medication, and increase the effectiveness of the medication you take. If you do not have salt-sensitive hypertension, or do not have hypertension, there is less evidence that a low sodium intake will provide benefit, but little reliable evidence that it will harm you.
2. What level of sodium intake should you aim for? We don't have enough good data to answer this, and perhaps never will. An extremely low sodium intake might be better for some, but could be harmful for others. As in so many areas, moderation provides most of the benefit. Reducing your sodium intake from the average 3,400 or higher to below 2,300 milligrams would seem unlikely to harm you.
3. Most of our sodium comes from restaurant and processed food, baked goods, and very salty foods such as chips and fries. This dwarfs the amount of salt we add at the table. Reducing the unnecessarily high sodium content of many processed foods and restaurant food cannot be harmful.
4. There are uncommon exceptions, people who do need the extra sodium in their diet. Examples are people whose kidneys excrete excessive amounts of sodium, or whose blood pressure drops excessively when standing. And yes, after hours of intense work or exercise with vigorous sweating, we do need a bit of a sodium load. Otherwise, we should not be dissuaded from seeking a diet with moderate rather than high sodium content.
5. It is virtually impossible to carry out studies in young people because very few will suffer an event in the next 10 to 20 years. But moderation makes sense as diets lower in sodium are more natural and healthier.
Rigid exclusion of all sodium from our diet is difficult to do and could conceivably be harmful. Moderation can only help and will not hurt us. The goal of reducing salt intake should not be cast aside.
For more by Samuel J. Mann, M.D., click here.
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