Russell

There is much written about the importance of nutrition in blood pressure but the casual reader can come away feeling very confused about what the final messages are. For example, many believe that sodium intake is extremely important as a cause of high blood pressure, that is, the more salt eaten the higher the blood pressure. Yet, there have been many studies that have challenged this conclusion and it appears that there are only some subpopulations that are perhaps sensitive to sodium.

Could you clarify this situation and tell us how a person would know whether or not they are in a salt-sensitive category?

Suter

Salt and blood pressure, or sodium and blood pressure, is a very controversial issue as you said. Most of the epidemiological studies which have shown a strong relationship between salt intake and blood pressure have been done in very heterogeneous populations -- people living in the United States or Europe have been compared with populations living in very remote areas, such as the Yanomani Indians in Brazil or populations in Papua, New Guinea. These studies demonstrate considerable differences in blood pressure as a function of sodium intake as well as other lifestyle factors among the populations. However, if you look at more homogeneous populations, let's say Caucasians living in the United States, you find a much smaller or often no relationship between salt intake and blood pressure.

These contradictory results do not mean that there is no relationship between sodium and blood pressure, for we know that there are, apparently, individuals and/or populations who, upon a higher salt intake, react with an increase in blood pressure. This reactivity corresponds to the concept of "salt sensitivity."

Salt Sensitivity

In salt sensitive people, the blood pressure natriuresis curve is shifted to the right, towards higher blood pressure values, that is, for a certain salt load a higher filtration pressure is needed. This shift results from several hormonal changes, especially alteration in the renin aldosterone system. Indeed, salt sensitive individuals may be identified by biochemical profiling. It is important to remember that it is the salt sensitivity and not the sodium in the diet that leads to an increase in blood pressure.

A variety of tests are used for the assessment of salt sensitivity such as intravenous saline infusion (which is not feasible in daily practice and may even be dangerous) or measuring blood pressure response after a period of low and/or high sodium intake. In daily practice, several clinical signs and constellations are suggestive for salt sensitivity (see Table 1 below).

The most important non-modifiable factor affecting salt sensitivity is age. With increasing age, salt sensitivity increases. African-Americans do have a much higher prevalence of salt sensitivity than Caucasians. Another important, but modifiable factor, is weight. Excess weight and obesity are also associated with an increased salt sensitivity.

Russell

What is the percentage of African-Americans that are salt sensitive and what is the percentage of people, regardless of race, over the age of 70 that are salt sensitive? What percentage of people over a body mass index (BMI) of 27 kg/m(2) would be salt sensitive in the general population? And can you also tell me anything about salt sensitivity in the Asian population?

Suter

Because of the lack of specific studies in the different racial population groups, no exact data about the prevalence of salt sensitivity can be given. As a general rule, about 30% of normotensive subjects are salt sensitive, whereas at least 50% of hypertensive subjects have to be regarded as salt sensitive. In African-Americans, up to 75% of hypertensive subjects are salt sensitive. As a general rule, it is important to remember that the absolute level of blood pressure is an important determinant of salt sensitivity. The higher the blood pressure, the higher the probability of salt sensitivity.

Russell

So, what you are telling me is that you as a doctor will not necessarily do a lot of biochemical testing on a patient for salt sensitivity. Instead, you will rely on the statistics that a person, who is black and overweight, for example, will need advice on restricting sodium.

What is your initial advice for these patients? How much will you restrict the salt? And what is the danger that if you restrict the salt in an older person, for example, that the food would become so much less palatable that they would begin to enter the problem of malnutrition over time.

Suter

In most acculturated populations, the salt intake varies between 8-15 g/d. According to data from NHANES III, the current sodium intake in the U.S. population (including all ages and races) is 3289 mg/d, which corresponds to 8.3 g salt/d. 1 g NaCl (salt) contains 393 mg sodium. The highest salt intake of 12.5 g/d was found in young non-Hispanic African-American men aged 16-19 years.

The sources of sodium in the diet can be divided into discretionary and non-discretionary. Up to 30-50% of the salt may be added at the table and could thus be directly influenced by the consumer. This would imply that a sodium sensitive person could control and reduce as much as possible any addition of sodium during the preparation of food, as well as at the table. Instead of salt, it is advised to use fresh herbs as major condiments or dried herb preparations without added sodium. (The use of potassium containing salt substitutes would also be an option, which is, however, not our favorite alternative, since there may be a risk for hyperkaliemia in certain patients -- for example, hypertensive patients with renal insufficiency or patients on potassium sparing diuretics.) In addition, the reduction of sodium containing processed foods can be advised. An optimal blood pressure lowering effect can be obtained by dietary strategies, as propagated by the so-called DASH diet.

According to the present guidelines, the implementation of non-pharmacological strategies should be done for a period of 3 to 6 months. From the conceptual viewpoint, this is a basically good strategy. However, there are not many patients who are able to implement and then fully adhere to a non-pharmacological strategy (e.g., sodium restriction) for a period of 6 months. Accordingly, in the case of sodium, we advise patients to restrict intake for a period of 2-3 weeks with "total compliance" and most patients are able to do so. (Sodium intake may be quantified by a 24h-urinary collection or also by a spot urine.) A significant fall of blood pressure after such a short-term sodium restriction -- especially in the presence of risk factors for salt sensitivity (see Table 1) -- is very suggestive for salt sensitivity. In these patients, a lifelong control of sodium intake may be warranted.

Can You Reduce Salt Intake Too Much?

An important question is, How far should sodium intake be reduced? The ideal amount of the sodium reduction in a salt sensitive subject is not known. Those who are salt sensitive react even to a moderate restriction, so I would say than any reduction is better than none. The severity of salt restriction depends on the clinical setting, i.e., the degree of blood pressure control needed.

In view of the loss of palatability of food, the acceptable individual restriction should be tested. If sodium intake is reduced stepwise, patients will become accustomed to the altered taste and often an impressive reduction of sodium intake can be achieved. The concomitant improvement of blood pressure may function as a motivator for the compliance.

In daily practice, it should be remembered that sodium represents only one non-pharmacological strategy for blood pressure lowering. Accordingly, to focus only on sodium would be a bad strategy. A moderate sodium restriction or the avoidance of an excessive sodium intake should be implemented only as one (not necessarily the most important) component of a whole set of non-pharmacological approaches in the therapy. A sodium intake of 6-7 g/d would result in blood pressure lowering effects and seems from the practical point of view also realistic.

The mediation of taste is an important function of sodium in our diet. With increasing age, the normal taste perception declines and the "hunger for salt" may increase to make food more tasty and palatable. Especially in the elderly, who already have a reduced appetite, extreme sodium restriction may be counterproductive by reducing food intake. This supports, once more, the concept of a global non-pharmacological approach with the implementation of different strategies.

Russell

So if I understand, you would not go any further down than 6 grams, even if the blood pressure somewhat responded but remained above normal. What would be your next strategy now that you reduced salt to this level with some response but your patient was still hypertensive?

Potassium and Hypertension

Suter

Although we discussed at first the importance of sodium, evidence suggests strongly that an increase in potassium intake may be even more important than sodium restriction.(1) I think the best strategy is, as in pharmacological therapy of blood pressure, to use combination therapies. Ideally, the different nutritional factors which might have an effect on blood pressure should be combined as much as possible. The central components are increasing potassium and eventually magnesium intake, controlling body weight (especially abdominal obesity), moderate any alcohol intake, increase daily physical activity and, last but not least, stress control.

Non-smoking is a must, as smoking has very strong immediate pressor effects. It is evident that these strategies are implemented stepwise. In clinical practice, it is very important to carefully select the most promising strategy for a certain patient. If you tell a patient to increase his/her potassium intake, lose weight, increase physical activity and, finally, cut back on alcohol, all at the same time, the patient will be completely overwhelmed and unable to follow any advice. In many cases, the patient will not even come back for the next appointment.

Russell

Now that you have mentioned potassium, let's talk about the importance of potassium as a positive agent for lowering blood pressure. How effective is potassium and at what levels?

Suter

Many epidemiological and interventional studies have shown that potassium has a rather strong blood pressure lowering effect, especially in salt sensitive subjects. From interventional studies, it can be said that, as a function of the amount of dietary and/or supplemental potassium, a mean reduction of approximately 6 mm Hg systolic and 4 mm Hg diastolic blood pressure can be achieved.(2),(3) Most studies suggest that the ratio of sodium to potassium is of crucial importance. Potassium may lower blood pressure by "antagonizing" some of the sodium effects.

Potassium, also, has a rather strong natriuretic effect and further inhibits renin release. Accordingly, in the situation of a high sodium diet, it would be best to advise a moderate sodium restriction in combination with an increased potassium intake by raising the intake of fruits and vegetables. In the clinical situation of a low sodium intake, an increased potassium intake has only negligible effects on blood pressure. Skim milk and milk products are additional good sources of potassium. As long as food is not processed, all potassium rich foods are low in sodium. Such a diet leads to a lowering of blood pressure and this concept represents one of the major strategies in the so-called DASH (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 might lead to a dangerous hyperkalemia, especially in hypertensive patients who may have undetected renal insufficiency.

Russell

So the general advice would be to increase fruit and vegetable intake, perhaps also including skim milk in the diet. For both Europe and the United States, you would also add a salt substitute to replace sodium with potassium. What role, if any, do these substitutes have in lowering blood pressure?

Suter

In view of the physiological effects of the replacement of sodium from table salt by potassium chloride, a blood pressure lowering effect can be achieved. In some "special" patients, however, we do not favor salt substitutes for several reasons. First, salt substitutes are not usually associated with an overall awareness about changing dietary habits. Second, salt substitutes do not necessarily increase palatability since they have a somewhat bitter aftertaste. Third, and very important, salt substitutes may represent an important source of large amounts of potassium. In subjects with renal insufficiency, the risk of hyperkalemia should not be neglected. 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 sodium.

What About Calcium and Magnesium?

Russell

I realize that there are a number of conflicting studies on calcium but I wonder if you can give us your personal opinion on how important you think calcium is in lowering blood pressure? Can you also review for us the largest studies that have looked at calcium as a blood pressure lowering agent?

Suter

Observational and interventional studies have reported blood pressure lowering effects of calcium. As compared to other nutrients, the blood pressure lowering effects of calcium were rather low and inconsistent. By itself, calcium seems to be of minor importance in this regard. In daily practice, a single nutrient focus should be avoided. However, calcium in combination with potassium, magnesium and moderate sodium restriction may have greater importance. This is consistent with the results from the DASH study where low fat milk products represented one important dietary modification (see below). Targeting a global risk management for blood pressure control includes the advice to increase calcium. So we recommend to patients, a regular daily consumption of low fat milk products such as 1 to 2 glasses of skim milk per day or a skim milk yogurt per day.

Russell

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

Suter

The importance of this cation in the non-pharmacological treatment of hypertension is similarly as for calcium. In view of the difficulties in assessing magnesium status and nutrition, it is not surprising that the importance of this cation as a therapeutic agent has not been as extensively studied as other nutrients. An adequate magnesium intake, according to the present guidelines, is essential. However, any increase of magnesium by dietary means or supplements alone has not produced an effect on blood pressure.

Alcohol and Coffee

Russell

Two commonly used substances in the diet are alcohol and coffee. It is pretty clear that alcohol can raise blood pressure after a period of drinking. Can you tell us about the mechanism and how important is a prohibition of alcohol in managing a patient's blood pressure?

Suter

According to our experience, alcohol is probably the most important pressor agent in daily practice. Most of our hypertensive patients, especially the ones who are difficult to treat, often report an excessive alcohol intake. The blood pressure increasing effect of alcohol has been reported in many studies.(4)

There is no single mechanism by which alcohol increases blood pressure. Suggested mechanisms include impaired baroreceptor function, altered hormone metabolism (catecholamines, renin, insulin), altered electrolyte metabolism and altered hypothalamic adrenal hormones. There seems to be a dose dependency -- low to moderate (up to 2 drinks/day) alcohol intakes are not associated with increased blood pressure.

Alcohol may elicit other unfavorable effects in hypertensive patients: antihypertensive drug compliance is often worse in subjects with heavier alcohol intake. Additionally, because of alcohol induced liver pathology, antihypertensive drugs are differently metabolized, so that they are less effective, requiring larger dosages that are more likely to produce side effects.

There is a strong relationship between the frequency of alcohol intake and the absolute amount of alcohol consumed. Nevertheless, it seems that for a given amount of alcohol, the frequency of consumption is also important for the blood pressure increasing effects. Accordingly we try not to forbid alcohol completely, which would be a unrealistic strategy; however, we try to convince our patients not to drink daily. This is usually associated with a reduction of the amount of alcohol ingested.

Russell

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, then, could you tell us the story about coffee and blood pressure?

Suter

Women do indeed have a lower so-called first pass metabolism of alcohol in the gastric mucosa. And the volume of distribution for alcohol is lower in women.(5),(6) Both factors lead, for a given dose of alcohol, to higher blood alcohol levels in women. Although these metabolic characteristics have never been tested in relation to blood pressure, it is conceivable that the alcohol-blood pressure relationship for a certain dose may be stronger in women. This would be of importance at low to moderate levels of intake; however, in heavier consumers the gender difference in alcohol metabolism is not seen.

In a caffeine naive person, that is, a person who never consumes caffeine, blood pressure increases upon the ingestion of a cup of coffee or a cup of espresso. However, in a person who is used to caffeine consumption, you don't see a blood pressure increasing effect of coffee consumption. In the MRFIT study, the regular coffee consumers had a lower blood pressure than the non-coffee consumers because caffeine leads to an increased diuresis and natriuresis. So, in daily practice, it is not necessary to advise coffee-drinking hypertensive patients to cut down their coffee consumption. In a difficult to treat patient -- after having ruled out all other causes of therapy resistance -- a reduction in coffee consumption may be warranted.

The Skinny on Exercise and Body Weight

Russell

One thing we haven't discussed is the issue of exercise and body weight. High blood pressure is known to correlate 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?

Suter

Losing weight is a difficult issue for most people. If you have a really obese patient, it probably makes the most sense to control blood pressure pharmacologically (and thus avoid any potential complications of the high blood pressure), while at the same time trying to reduce body weight by individually tailored strategies. If somebody loses only 10% of their initial body weight, this loss can have a considerable and positive impact on blood pressure and the other cardiovascular risk factors.

What is important in daily practice is that you should not tell your patients that weight loss automatically produces a reduction in blood pressure. As with other strategies, there are responders and non-responders. So you have to tell the patient that there might be no measurable blood pressure effect because of weight loss; however, in any case, weight loss is associated with an improved overall cardiovascular risk profile.

We all know how frustrating weight loss is, since most patients regain their body weight within a short timeframe. Accordingly, the most important initial strategy represents the stabilization of body weight. Most patients show a weight history over time of continuing increase. Therefore, any interruption of the upward trend in should be regarded as an initial success. After a successful stabilization of body weight, the steps towards weight loss are much smaller and also more realistic. In addition, weight maintenance at a lower level is much easier if the patient has first experienced a successful weight maintenance period.

One central component in any weight loss and weight maintenance life strategy is physical activity. In the setting of the obese hypertensive patient, increased levels of physical activity in daily life at work and at home lead to an increased energy expenditure and thus weight loss or at least weight stabilization. In addition, physical activity is associated with many favorable effects on most cardiovascular risk factors (e.g., insulin resistance, dyslipidemia) including hypertension. We recommend to our patients -- independent of their body weight -- an individually tailored amount of increased physical activity 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 schedule of daily life and add sport activities later.

Every person -- independent of their age, body weight and blood pressure status -- should pursue at least 30 minutes of daily physical activity that raises the pulse rate. The physical activity should be pursued regularly, at least every other day and should be an aerobic and endurance activity. In hypertensive patients, isometric exercise training, such as weight lifting or rowing, is not a recommended physical activity because of irregular repetitive bouts of heavy physical activity, which are associated with an increase in blood pressure. The rise in blood pressure occurs because there is an increased pressure in the abdominal area that increases resistance.

The DASH Diet

Russell

There is a recently published diet that has caused much interest in the United State. The DASH diet ("Dietary Approaches to Stop Hypertension") seems particularly effective in lowering blood pressure. Could you talk about the DASH diet for a few minutes and does the DASH diet incorporate all the factors we have been discussing?

Suter

The DASH diet is a type of global summary of the important dietary strategies to control hypertension. During the last few years two DASH studies have been published.(7),(8)

The DASH-I study tested the effect of a so-called combination diet rich in fruits and vegetables and low fat dairy products on blood pressure as compared to the usual U.S. diet and a diet rich in fruits and vegetables. The DASH diet resulted in a significant reduction of blood pressure, especially in hypertensive subjects. Among these people, the combination diet reduced the systolic and diastolic blood pressure by 11.4 mm Hg and 5.5 mm Hg, respectively. It is important to note that the DASH diet had favorable effects not only in hypertensive subjects but also in normotensives. In view of the colinearity of nutrients in food, it cannot be said which nutrient was responsible for the reduction in blood pressure. Potassium is probably of major importance.(9)

DASH-II used the same dietary intervention except that, additionally, sodium intake was controlled at three different levels (150 mmol/d, 100 mmol/d and 50 mmol/d). The greatest effect of the DASH diet was seen at the lowest level of sodium intake. As already discussed, sodium may be of great importance in salt sensitive subjects where a reduced intake may be indicated. However, for daily practice as well as at the population level, the DASH-I findings are more important. First, the blood pressure effects of the DASH-I diet are comparatively very large as compared to the additional blood pressure reduction by the DASH-I + sodium restriction. Second, sodium restriction is mainly operating in salt sensitive subjects. Third, the difficulties in maintaining a strict control of sodium over longer periods of life are well known (both DASH studies lasted only 30 days).

For daily practice, we recommend starting with the DASH-I diet and, then, if there is still insufficient blood pressure control and the presence of salt sensitivity is suspected, adding an additional salt restriction.