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Good Health to Diet For: Blood Pressure and Nutrition
Dr. Suter is Chief of the Hypertension Clinic at the Medical Policlinic, Department of Internal Medicine, University Hospital, Zurich, Switzerland.
Dr. Rob RussellFirst we're told to avoid salt. Then, it's suggested that we eat all we want. Now, the experts are not sure what to recommend!
So many different articles have been written over the past decade or two about the connection between nutrition and blood pressure that it's no wonder the ordinary person is probably very confused about what to believe.
For example, based on a few studies some authorities claimed that salt intake is an extremely important cause of high blood pressure; that is, the more salt a person eats, the higher their blood pressure will become. But other more recent studies, however, seem to point to the opposite conclusion.
It now appears that the blood pressure-salt connection is true only for certain "salt sensitive" people or groups.
Could you tell us what is really going on? And how is an individual supposed to know whether or not they need to worry about how much salt they put on their french fries?
Dr. Paolo SuterThe relationship between salt and blood pressure is obviously very controversial. Most of the epidemiological studies that have shown a strong relationship compared large categories of people that are physically and culturally quite distant. For example, people living in the United States or Europe who eat a lot of salt have been compared with the Yanomani Indians in Brazil or people in Papua, New Guinea who have very little salt in their diets. These studies have found considerable differences in blood pressure that initially appeared to be related to sodium intake.
The problem with these kinds of studies is that there are so many other differences between the two groups in diet, lifestyle, stress level, medical treatment and other areas that it is nearly impossible to isolate one factor and say that it is responsible for any differences in blood pressure. In other words, one group may have lower blood pressure, but how do we know for sure that the reason is salt, and not how many yams they eat or how often they do the samba — or a combination of all three? Complicating the picture is the fact that studies that have compared people within more homogeneous populations, say different groups of Caucasians living in the United States, have found little or no relationship between salt intake and blood pressure.
These contradictory results do not prove that there is no relationship between sodium and blood pressure, just that the relationship may not be as simple as more salt = higher blood pressure. We do know that some individuals and groups are "salt sensitive" — that is, if they increase their salt intake, they will have an increase in blood pressure. The bottom line may be that some people will need to watch their salt intake carefully in order to help control blood pressure, but that most people will not. Unfortunately it is not always easy to tell whether a particular person is salt sensitive or why.
Salt SensitivitySalt sensitivity is linked to a number of hormonal changes within the body. These changes can be detected by doctors using a variety of tests.
These tests include intravenous saline infusion (a dangerous procedure which is only performed in measuring blood pressure extraordinary circumstances) and taking a person's blood pressure after a period of unusually high or low sodium intake. We also know that there are a number of factors that increase the likelihood that a particular person may be salt sensitive (see Table 1 below).
Factors Associated with Salt Sensitivity.
The most important of these is age. With increasing age, salt sensitivity increases. African-Americans also have a much higher prevalence of salt sensitivity than Caucasians. Another important factor is weight. Excess weight and obesity are clearly associated with increased salt sensitivity.
RussellDo we know what percentage of African-Americans are salt sensitive and what is the percentage for all people over the age of 70? What percentage of people who are obese are salt sensitive?
SuterAt this point, we have no exact data about the prevalence of salt sensitivity in the general population. We believe about 30% of those with high blood pressure are salt sensitive. Among African-Americans, this percentage is much higher — up to 75% of African-Americans with high blood pressure are salt sensitive. As a general rule, the higher a person's blood pressure, the more likely that a high salt diet will make it worse.
RussellSo, what you are telling me is that you as a doctor will not necessarily test every patient who walks in for salt sensitivity. Instead, you will rely on the statistics and tell a person with risk factors, who is black and overweight, for example, that they should watch their salt intake.
What exactly do you tell people who are at risk for salt sensitivity? How much should they restrict their salt? You brought up age as a risk factor. What is the danger that if you restrict the salt intake of an older person, their food would become so bland that they might eat less and become at risk for malnutrition, which is a common problem among the elderly?
SuterFor most people in western nations, average salt intake varies between 8-15 g/d (grams per day). What we call "salt" (or "table salt") is chemically sodium chloride (NaCl). Roughly 40% of salt is sodium (Na+) and 60% is chloride (Cl-). Accordingly, 1 g of salt contains about 400 mg of sodium. On food labels usually the mg of sodium are listed. To transform the amount of sodium (Na+) in food into grams of salt the mg of sodium (Na+) multiplied by 2.5 will give approximately the grams of salt (NaCl). According to the best studies, overall salt intake in the U.S. population today is about 8.3 g/d, but this can vary quite a bit from one group to another, depending on ethnic or regional cooking styles, how much processed or fast food people tend to eat, and other factors. Within the American population, for instance, the category with the highest salt intake, is non-Hispanic African-American men aged 16-19. They average a whopping 12.5 g/d.
Salt in the diet comes in two forms: discretionary (i.e., added at the table) and non-discretionary (or already present in food). Because as much as 30-50% of the average person's total salt is added at the table, it is clear that significant reductions could be achieved simply by educating people to put down the salt cellar. The next step would be to use less salt while preparing food in the kitchen. A good way to do this is by using salt substitutes such as fresh or dried herbs such as oregano, thyme and rosemary that do a good job of bringing out the flavor of food. You can buy salt substitutes that contain potassium, although these carry health risks for people with high blood pressure and kidney problems, or people taking diuretic medications.
Another strategy would be simply to avoid "junk" food, fast food and other processed foods that tend to contain a lot of salt. Those worried about their blood pressure may wish to use the so-called DASH diet.
These are the approaches doctors can offer to those who are at risk for salt sensitivity. The idea is to try to lower blood pressure by following a low salt diet for 3 to 6 months before trying to do the job with drugs. Unfortunately, there are not many patients who are able to stick to a low salt diet for long. For that reason, we advise our patients to drastically reduce the amount of salt they eat for period of only 2-3 weeks. If this leads to a significant drop in blood pressure, the person probably is salt sensitive. We then work with these people to create a workable, life-long dietary program to reduce their salt intake.
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