December 1, 2001

Good Health to Diet For: Blood Pressure and Nutrition

Robert M. Russell, M.D. and Paolo M. Suter, M.D.
Some people are "salt sensitive." If they increase their salt, they increase their blood pressure.
Dr. Suter is Chief of the Hypertension Clinic at the Medical Policlinic, Department of Internal Medicine, University Hospital, Zurich, Switzerland.

Dr. Rob Russell
First 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 Suter
The 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.

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?

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 Sensitivity
Salt 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).

Table 1.
Factors Associated with Salt Sensitivity.
  • Female gender
  • Age
  • Obesity (abdominal obesity)
  • Alcoholism
  • African-American origin
  • Level of blood pressure
  • Isolated systolic hypertension (ISH)
  • Low renin hypertension (hypertensive patients with a low plasma renin activity)
  • Impaired glucose tolerance
  • Diabetes
  • Renal insufficiency
  • Positive family history of hypertension
  • (Higher) microalbuminuria

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.

Do 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?

At 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.

So, 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?

For 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.

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.

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.

View sample menus and tips for getting started on the DASH diet here.

Can You Overdo Reducing the Salt?
An important question is how far should salt intake be reduced. The short answer is that we are not sure. Those who are salt sensitive react even to a moderate restriction, so I would say than any reduction is better than none. Those with more serious blood pressure problems should try an even stricter program of salt reduction.

Every person is different as to how much salt they need for taste. Some people, however, become used to a low salt diet and continue to enjoy eating. Others may have to give up some of the pleasure of eating in order to lower their blood pressure and enjoy better health.

It is true that reducing salt intake is particularly difficult for the elderly. With increasing age, our ability to taste food declines and we develop a greater 'hunger for salt.'

It should be remembered that salt reduction is only one of many weapons in the fight against high blood pressure. A realistic goal for most people is a sodium intake of 6-7 g/d. For most people, getting their salt intake down to that level will lead to a significant drop in blood pressure.

It is true that reducing salt intake is particularly difficult for the elderly. With increasing age, our ability to taste food declines and we develop a greater "hunger for salt." In the elderly, the combination of reduced appetite and extreme sodium restriction may lead to malnutrition and other problems far worse than high blood pressure.

So if I understand, you don't want a patient to go any lower than 6 grams, even if their blood pressure remained above normal. What would be your next strategy for patients who have reduced their saltintake to this level and still have high blood pressure?

Potassium and High Blood Pressure

Although we have been focusing on the importance of salt, a lot of evidence suggests that increasing potassium intake may be even more important in fighting high blood pressure than reducing salt. I think the best strategy is to combine all the nutritional factors that might have an effect on blood pressure. These include increasing potassium and magnesium intake, controlling body weight (especially abdominal obesity) and alcohol intake, increasing daily exercise and, last but not least, controlling stress.

Quitting smoking is also a must. All of these things are important, but it is not realistic to do them at the same time. It is important to carefully select the most promising first step. If you tell someone to eat more fruits and vegetables, lose weight, get more exercise and stop drinking and smoking, all at the same time, the patient will be shellshocked. And, chances are, the doctor will never see them again.

Now that you have mentioned potassium, let's talk about the importance of potassium in lowering blood pressure. How effective is it?

Many studies have shown that potassium has a rather strong blood pressure lowering effect, especially in salt sensitive subjects. These same studies also suggest that the proportion of salt to potassium is of crucial importance. Potassium may lower blood pressure by counteracting some of the bad effects of salt.

The healthiest way to get more potassium is to eat more fruits and vegetables. Skim milk and milk products are also good sources of potassium. With the exception of processed foods, all potassium rich foods tend to be low in salt as well. This is at the heart of the so-called DASH diet (Dietary Approaches to Stop Hypertension) diet.

Potassium intake should be increased by the consumption of foods rich in potassium and not by supplements. Potassium supplements can lead to serious kidney and other health problems.

So the general advice would be to increase fruit and vegetable intake, perhaps also including skim milk in the diet. You would also recommend adding a salt substitute to replace salt with potassium.

Salt substitutes certainly help lower blood pressure. In some "special" patients, however, we do not favor salt substitutes for several reasons. First, salt substitutes are not a good substitute for a healthier overall diet. Second, salt substitutes do not taste good to everyone. Third, and very important, salt substitutes may contain large amounts of potassium. This can be unhealthy for those with kidney problems. Whenever possible, potassium should be obtained from the diet and natural food sources, since a diet rich in potassium is in most instances lower in salt.

What About Calcium and Magnesium?

There are a number of conflicting studies on calcium. I wonder if you can give us your personal opinion on how important you think calcium is in lowering blood pressure?

Some studies have found that calcium may help lower blood pressure. Compared to potassium and other nutrients, however, these effects were rather small and inconsistent. By itself, calcium seems to be of minor importance for blood pressure. However, calcium in combination with potassium, magnesium and moderate salt restriction seems to be a lot more effective. We recommend to patients that they drink 1 to 2 glasses of skim milk per day or eat one low-fat yogurt per day.

Another mineral which has been much discussed for a possible blood pressure lowering effect is magnesium. Can you tell us the latest on its role?

An adequate magnesium intake is essential for a healthy diet. However, increasing magnesium intake alone through diet or by supplements has not produced an effect on blood pressure.

Alcohol and Coffee

It is pretty clear that alcohol can raise blood pressure. Can you tell us about the importance of avoiding alcohol in managing blood pressure?

In our experience, most of the people we treat for high blood pressure, especially the ones who are difficult to treat, drink a lot of alcohol. This is no surprise; the blood pressure increasing effect of alcohol has been reported in many studies. Alcohol's bad effects go beyond simply driving up blood pressure. Those who drink tend to be erratic about taking prescribed blood pressure-lowering drugs. Alcohol also damages the liver, making these drugs less effective and requiring larger dosages that are more likely to produce side effects.

In our experience, most of the people we treat for high blood pressure, especially the ones who are difficult to treat, drink a lot of alcohol. This is no surprise...

Women appear to have a more difficult time metabolizing alcohol than men. Paolo, are there any sex differences with regard to sensitivity of alcohol? And could you tell us about coffee and blood pressure?

Women do indeed metabolize alcohol more slowly. As a result, they will have higher blood alcohol levels than men would have after drinking the same amount. Although no one has studied this, it is conceivable that the effect of alcohol on blood pressure may be stronger in women.

In a person who never consumes caffeine, blood pressure increases after a cup of coffee or espresso. However, in a person who is used to caffeine consumption, you do not see this effect. In fact, studies have shown that regular coffee drinkers have lower blood pressure than the non-drinkers because of the dehydrating effects of caffeine. Reducing the amount of fluid in the body tends to lower blood pressure. So it is not necessary to advise coffee-drinkers with high blood pressure to cut down their coffee consumption. However, in a difficult to treat patient — after having ruled out all other causes — a reduction in coffee consumption may be worth a try.

The Skinny on Exercise and Body Weight

One thing we have not discussed is the issue of exercise and body weight. High blood pressure is known to be associated with excessive body weight. Do you tell patients to lose pounds as part of your blood pressure control strategy? And, is it realistic to expect that hypertensive patients will actually follow the advice?

Losing weight is a difficult issue for most people. In fact, one recent study showed that in the long term, overweight people are statistically more likely to become rich than to become thin. For a really obese person it probably makes the most sense to treat their blood pressure using drugs while at the same time trying to reduce body weight by individually tailored strategies. A loss as small as 10% of total body weight can have a very positive impact on blood pressure, as well as on other cardiovascular risk factors.

It is important to remember that weight loss does not automatically cause a reduction in blood pressure. As with other strategies, there are some who respond and others who do not. Of course, weight loss does lower your risk for a wide range of cardiovascular problems.

For most people, weight increases gradually over time. Therefore, any interruption in the normal upward trend should be regarded as a success.

We all know how frustrating weight loss is, since most people diet and then regain their body weight within a short time. Accordingly, the most important goal is to stabilize body weight. For most people, weight increases gradually over time. Therefore, any interruption in the normal upward trend should be regarded as a success. After a successful stabilization of body weight, the steps towards weight loss should be small and realistic.

The key component in any weight loss and weight maintenance life strategy is exercise. Increased activity takes some of the pressure off reducing calorie intake. In addition, it is associated with many favorable effects on most cardiovascular risk factors, including high blood pressure. We recommend to our patients, whatever their body weight, an individually tailored amount of increased exercise such as daily walking, power walking, jogging and biking sessions, using stairs instead of the elevator or walking to the restaurant at noon time. Usually, we start with advice on integrating physical activity into the daily routine and add sport activities later.

Every person — independent of age, body weight and blood pressure — should pursue at least 30 minutes of daily physical activity that raises the pulse rate. For those with high blood pressure, isometric exercise training, such as weight lifting or rowing, is not recommended, as irregular bouts of heavy physical activity can actually increase blood pressure.

The DASH Diet

There is a recently published diet that has caused much interest in the United States. The DASH diet seems particularly effective in lowering blood pressure. Could you talk about the DASH diet for a few minutes?

The DASH diet is basically a summary of all the important dietary strategies to control high blood pressure. During the last few years, two DASH studies have been published, DASH-I and DASH-II.

The DASH-I study tested the effect of a diet rich in fruits, vegetables and low fat dairy products on blood pressure. It resulted in a significant reduction of blood pressure. It is important to note that the DASH diet had favorable effects not only on those with high blood pressure but also on those with normal blood pressure. Although we cannot be sure which nutrient was primarily responsible for the reduction in blood pressure, potassium is a likely candidate.

DASH-II used the same dietary intervention except that salt intake was monitored at three different levels The greatest effect of the DASH diet was seen at the lowest level of salt intake.

For a number of reasons, including the difficulty of sticking to a low salt diet, we recommend that most people start with the DASH-I diet. Then, if they are still having trouble with high blood pressure and salt sensitivity is suspected, we attempt to lower salt intake.

NOTE: We regret that we cannot answer personal medical questions.
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