By how much does dietary salt reduction lower blood pressure? I--Analysis of observational data among populations

Diet and blood nutrient correlations with ischemic heart, hypertensive heart, and stroke mortality in China

Though major differences exist in subcategory mortality levels, cardiovascular disease remains a leading cause of death among both Asian Chinese and Westerners. This paper examines the possible relationship between cardiovascular mortality and biochemical, diet and lifestyle factors based on two surveys in China. Statistically significant associations indicate five variables negatively correlated: molybdenum, oleic acid, liquor consumption (males), legumes, and age at first pregnancy with ischemic heart disease; molybdenum, oleic acid (females) and age at first pregnancy with hypertensive heart disease; and legumes and age at first pregnancy with stroke. Five variables were positively correlated: triglycerides and herpes antibodies with ischemic heart disease; salt and phosphorus (females) with hypertensive heart disease; and only albumin (males) with stroke. Some findings confirm those observed in the West (salt, triglycerides, herpes, legumes, oleic acid, and liquor), but molybdenum and age at first pregnancy have not been emphasized previously. Still others significant in the West have not been observed here, such as cholesterol and smoking.

How much does the kidney participate in the origin of primary hypertension?

The existence of a relationship between the kidney and arterial hypertension has long been known. Renal participation in the development of arterial hypertension has been clearly shown in different animal models that mimic human essential hypertension. Different theories have tried to explain the mechanism(s) underlying the renal participation in human hypertension. According to the data contained in the literature renal vasoconstriction is present in the kidney since the very early stages of the hypertensive disease and could constitute the mechanism facilitating the development of arterial hypertension.

Increased activity of the Mg2+/Na+ exchanger in red blood cells from essential hypertensive patients

Epidemiological, clinical, and experimental evidence suggests a relation between Mg2+ metabolism and essential hypertension. The aim of the present study was the detection of abnormalities of the erythrocyte Mg2+/Na+ exchanger in essential hypertensive patients. We studied 66 untreated essential hypertensive patients and 36 normotensive control subjects. Maximal efflux rates of total Mg2+ efflux and the Na(+)-dependent and Na(+)-independent components of Mg2+ efflux were determined in Mg(2+)-loaded red blood cells. Mg2+/Na+ exchanger was calculated as the Na(+)-dependent component of the Mg2+ efflux. Mean values of Mg2+/Na+ exchanger were clearly elevated in hypertensive subjects with respect to normotensive control subjects [184.7 +/- 15.7 versus 84.4 +/- 6 mumol(L.cell.h)-1; P < .001]. This elevation was due primarily to the increased total Mg2+ efflux [324.2 +/- 21.9 versus 257.9 +/- 17.3 mumol(L.cell.h)-1; P < .05], whereas the Na(+)-independent component was not significantly different between the groups [154.5 +/- 11.8 versus 173.4 +/- 15.5 mumol(L.cell.h)-1; P = NS]. Moreover, total erythrocyte Mg2+ content was slightly reduced in hypertensive patients with respect to normotensive control subjects (1.84 +/- 0.04 versus 2.07 +/- 0.04 mmol/L.cell; P < .001). Using the 99% confidence limits of the normotensive population as the normal range, 30 (45.5%) hypertensive subjects showed values of Mg2+/Na+ exchanger higher than 160 mumol(L.cell.h)-1. The Mg2+/Na+ exchanger was inversely correlated with basal intraerythrocyte Mg2+ content (r = -.323; P = .001). From a clinical point of view, we found a positive correlation between diastolic blood pressure values and Mg2+/Na+ exchanger (r = .246; P < .05) in the sample of essential hypertensive patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Improvement of cardiovascular effects of metoprolol by replacement of common salt with a potassium- and magnesium-enriched salt alternative

1. The influence of sodium chloride (NaCl)-enrichment of the diet (6% of the dry weight) and that of a novel sodium-reduced, potassium-, magnesium-, and L-lysine-enriched salt alternative on the cardiovascular effects of the beta 1-adrenoceptor blocking drug, metoprolol, was studied in stroke-prone spontaneously hypertensive rats. 2. Increased dietary sodium chloride intake produced a marked rise in blood pressure and induced left ventricular and renal hypertrophy. By contrast, the salt alternative did not increase blood pressure and caused remarkably less cardiac and renal hypertrophy than did sodium chloride. 3. Metoprolol treatment at a daily dose of 250 mg kg-1 lowered blood pressure and decreased left ventricular hypertrophy index during the control diet. Sodium chloride-enrichment blocked the antihypertensive effect of metoprolol, while a partial protective effect on left ventricular and renal hypertrophy persisted. In the presence of the salt alternative-enrichment both at the level of 6% and 10.5% (corresponding to a NaCl level of 6%), metoprolol was fully able to exert its beneficial cardiovascular and renal effects. 4. Both salt supplementations, irrespective of metoprolol treatment, induced a 3 to 4 fold increase in the urinary excretion of calcium. There was a linear correlation between the urinary excretions of sodium and calcium. The urinary excretion of magnesium rose by 90% and that of potassium by 110% in the salt alternative group. 6. Our findings suggest that replacement of common salt by a potassium-, and magnesium-enriched salt alternative in the diet produces beneficial cardiovascular effects and improves the antihypertensive efficacy of metoprolol in stroke-prone spontaneously hypertensive rats. Increased intake of potassium and/or magnesium and L-lysine from the salt alternative is involved in the beneficial effects of the salt alternative. The NaCl-induced myocardial and renal hypertrophies appear to be partially mediated by Beta-adrenoceptor activation.

Relation between birth weight at term and systolic blood pressure in adolescence

OBJECTIVE

To examine whether birth weight is related to systolic blood pressure during adolescence.

DESIGN

Retrospective (comparative) cohort study. The observers who traced and studied the subjects were unaware of their case-control status.

SUBJECTS

330 subjects were born in Cardiff in 1975-7. Cases who were low birth weight at term (< 2500 g) were matched with controls of normal birth weight (3000-3800 g) at term.

MAIN OUTCOME MEASURES

Systolic blood pressure measured by random zero sphygmomanometry in the subject's right arm with the subject supine, corrected for size and age.

RESULTS

The mean age at examination was 15.7 years. The mean systolic blood pressure of the cases was 105.8 mm Hg and of the controls 107.5 mm Hg. The corrected difference (95% confidence interval) in systolic blood pressure between the cases and controls was 1 mm Hg (-3 to +1 mm Hg; two tailed probability 0.33).

CONCLUSION

Systolic blood pressure in adolescents of low birth weight is not significantly different from that of adolescents of normal birth weights.

Replacement of salt by a novel potassium- and magnesium-enriched salt alternative improves the cardiovascular effects of ramipril

1. The influence of salt (sodium chloride; NaCl) (an additional 6% in the diet) and that of a novel sodium-reduced, potassium-, magnesium-, and L-lysine-enriched salt alternative on the cardiovascular effects of ramipril was studied in stroke-prone spontaneously hypertensive rats in a 6-week study. The intake of sodium chloride was adjusted to the same level by adding the salt alternative at a 1.75 times higher amount than regular salt. 2. Salt produced a marked rise in blood pressure and induced cardiac hypertrophy and significant mortality, while the salt alternative neither increased blood pressure nor caused any mortality and produced less cardiac hypertrophy than salt. 3. Ramipril treatment at a daily dose of 3 mg kg-1 normalized blood pressure and prevented the development of cardiac hypertrophy of rats on control diet. These effects of ramipril were blocked by the addition of salt but were only slightly attenuated by the addition of the salt alternative. The mortality in the salt group was prevented by ramipril. 4. Responses of mesenteric arterial rings in vitro were examined at the end of the study. Salt, but not the salt alternative, increased vascular contractile responses to noradrenaline. Ramipril treatment improved the arterial relaxation responses to acetylcholine and to sodium nitroprusside. The vascular relaxation enhancing effect of ramipril was blocked by salt but only slightly attenuated by the salt alternative. 5. Ramipril treatment did not significantly increase plasma renin activity in the presence or in the absence of salt supplementation. The salt alternative did not cause hyperkalaemia, either alone or in combination with ramipril treatment. 6. Both salt supplementations, irrespective of ramipril treatment, induced a six to eight fold increase in the urinary excretion of calcium. There was an expected 90 to 140% rise in the urinary excretion of magnesium and 200% rise in the urinary excretion of potassium in the salt alternative group. Salt also produced an approximately 50% increase in magnesuria.7. Our findings suggest that replacement of salt by the potassium-, magnesium- and L-lysine-enriched salt alternative improves the cardiovascular effects of ramipril. In the present study the beneficial effect was related to the increased intakes of potassium and/or magnesium and L-lysine from the salt alternative because the amount of sodium chloride was the same.

Bread as a source of salt: an international comparison

OBJECTIVE

The salt (NaCl) content of bread, obtained from Belgium and 25 other regions or countries worldwide, was analyzed to evaluate the importance of bread as a source of dietary salt in a population.

METHODS

A total of 1166 dried bread samples were analyzed using a hot HCl extraction technique followed by flame photometry and performed by the same laboratory technician in Leuven (Belgium); 534 samples from Leuven were analyzed over 10 time periods from 1967-73 to 1992 together with 562 samples from 11 other European countries, 44 from the United States and Peru, and 26 from Asia.

RESULTS

The average mean salt content of fresh bread (= dried bread times 0.6) in European countries was 12.4 +/- 1.7 g/kg for white and 13 +/- 1.0 g/kg for brown bread. In Japan and Korea it was about 8.5 g/kg and in Pakistan, Thailand, Nepal, and Laos about 5 g/kg. The mean sodium/potassium ratio with Na and K in mmol was 8.0 for white and 4.5 for brown bread, much higher than the recommended value of 1.0. Bread with a very low salt content was found in certain areas of Peru, Spain, and Italy.

CONCLUSION

The overall salt content of bread in Western countries is excessive, and a gradual reduction is highly desirable.

Feasibility and efficacy of sodium reduction in the Trials of Hypertension Prevention, phase I. Trials of Hypertension Prevention Collaborative Research Group

Phase I of the Trials of Hypertension Prevention was a multicenter, randomized trial of the feasibility and efficacy of seven nonpharmacologic interventions, including sodium reduction, in lowering blood pressure in 30- to 54-year-old individuals with a diastolic blood pressure of 80 to 89 mm Hg. Six centers tested an intervention designed to reduce dietary sodium to 80 mmol (1800 mg)/24 h with a total of 327 active intervention and 417 control subjects. The intervention consisted of eight group and two one-to-one meetings during the first 3 months, followed by less-intensive counseling and support for the duration of the study. The mean net decrease in sodium excretion was 43.9 mmol/24 h at 18 months. Women had lower sodium intake at baseline and were therefore more likely to decrease to less than 80 mmol/24 h. Black subjects were less likely to decrease to less than 80 mmol/d, independent of sex or baseline sodium excretion. The mean (95% confidence interval) net decrease associated with treatment was -2.1 (-3.3, -0.8) mm Hg for systolic blood pressure and -1.2 (-2.0, -0.3) mm Hg for diastolic blood pressure at 18 months (both P < .01). Multivariate analyses indicated a larger systolic blood pressure effect in women (-4.44 versus -1.23 mm Hg in men), adjusted for age, race, baseline blood pressure, and baseline 24-hour urinary sodium excretion (P = .02). Dose-response analyses indicated an adjusted decrease of -1.4 mm Hg for systolic blood pressure and -0.9 mm Hg for diastolic blood pressure for a decrease of 100 mmol/24 h in 18-month sodium excretion. These results support the utility of sodium reduction as a population strategy for hypertension prevention and raise questions about possible differences in dose response associated with gender and initial level of sodium intake.

Childhood prevention of essential hypertension

Childhood prevention of essential hypertension requires knowledge of alterable determinants of blood pressure in children; these include obesity and sodium intake and perhaps physical activity and intake of potassium and calcium. Altering these determinants may involve two general preventive strategies. The first is a population strategy, which attempts to lower blood pressure (or keep it from rising) among all children. Population strategies may require educating children to active participants in changing their behaviors (active approach) or may merely change their environment (passive approach). The second general strategy aims to focus on children at high risk of developing hypertension as adults. To determine the usefulness of this high-risk strategy, more information is needed about prediction of adult blood pressure from childhood values and about the efficacy of interventions to control blood pressure levels in high-risk children.

The effect of antihypertensive treatment on the quality of later years

There is now a considerable body of evidence demonstrating the benefit of pharmacological therapy in reducing cardiovascular risk in elderly hypertensive patients. With the high prevalence of hypertension that exists in the elderly, the impact of the widespread use of antihypertensive treatment on the quality of life of older individuals requires consideration. Quality of life is a difficult concept to define and thus to measure scientifically. It is most commonly measured in clinical trials using questionnaire-based instruments designed to assess the subjects' perception of their own health and the degree of well-being and satisfaction in various health-related areas of their lives. Nonpharmacological methods of blood pressure reduction are generally regarded as being neutral or beneficial in their effects on quality of life, and deserve prolonged evaluation in elderly patients before introducing pharmacological therapy. The many antihypertensive drug treatments now available have varying impact on quality of life, which should be taken into account when developing a rational approach to antihypertensive intervention in the elderly patient.

The kidney and arterial hypertension

It has been known for some time that a relationship exists between the kidney and blood pressure. The renal origin of arterial hypertension has been demonstrated in different animal models resembling human hypertension, with data from humans seeming to confirm this hypothesis. On the other hand, the renal vasculature also suffers the consequences of arterial hypertension, and renal insufficiency can develop as a result of elevated blood pressure levels. Antihypertensive therapy can prevent the development of renal damage secondary to hypertension. For example, calcium antagonists possess specific renal effects that not only facilitate their antihypertensive capacity but also protect the kidney from the development of renal failure.

The epidemiology of diet and blood pressure

Epidemiological studies of diet and blood pressure support the concept that some dietary nutrients are strongly related to blood pressure and are amenable to modifications, with a likely impact on the incidence of hypertension and of related vascular events. For other potential nutrients, the evidence is more difficult to obtain. Further studies are required to establish 1) the practical value of assessment of individual nutrients versus dietary patterns in risk prediction, 2) the effects of changes of these individual nutrients versus dietary patterns on blood pressure, and 3) whether long-term changes in these individual nutrients or dietary patterns reduce the incidence of hypertension and can be implemented at a population level.

The Salt Step Test: its usage in the diagnosis of salt-sensitive hypertension and in the detection of the salt hypertension threshold

The Salt Step Test was devised to characterize the response of the hypertensive patient to dietary salt. The test has three phases: unrestricted salt, to document hypertension and customary salt intake; restricted salt (2 g/day), to identify the salt-sensitive patient; and stepwise increased salt (each step = 1 g/day), to find the level that precipitates hypertension. The Salt Step Test identified that out of 30 well-established adult hypertensives, 13 were salt-sensitive. It also revealed that in each salt-sensitive patient, a distinct level of salt (range 3-16 g/day) precipitated hypertension, i.e., a Salt Hypertension Threshold. Definition of the Salt Hypertension Threshold should be useful in providing specific, individualized guidelines for dietary salt restriction.

Clustering of cardiovascular risk factors in confirmed prehypertensive individuals

Numerous studies have indicated that hypertensive subjects have an atherogenic lipoprotein pattern, hyperinsulinemia, and impaired glucose tolerance relative to normotensive individuals. These abnormalities could be due to adverse effects of certain antihypertensive agents, to pathophysiological concomitants of the hypertensive state itself, or to both. In this report, we describe the cardiovascular risk factor profile of 1,440 subjects who were normotensive and were not taking any antihypertensive medications when first examined and who subsequently participated in the 8-year follow-up of the San Antonio Heart Study. Hypertension developed in 130 subjects during the follow-up period. At baseline these prehypertensive individuals had significantly higher levels of blood pressure, fasting total and low density lipoprotein cholesterol, triglyceride, glucose, and insulin, and 2-hour glucose than those who remained free of hypertension. In addition, they had higher body mass indexes, a less favorable body fat distribution, and lower levels of high density lipoprotein cholesterol. In multiple linear regression analyses, baseline levels of triglyceride and blood pressure remained significantly higher and high density lipoprotein cholesterol remained significantly lower in the subjects who later converted to hypertension than in those who remained normotensive. Although baseline insulin levels were also higher in the prehypertensive subjects, this difference was not statistically significant. In nonobese subjects, however, those with high baseline insulin concentrations had an increased incidence of hypertension compared with those with low insulin concentrations. The present results suggest that the cluster of atherogenic changes associated with hypertension actually precede the development of the hypertensive state.

Variance imputation for overviews of clinical trials with continuous response

Overviews of clinical trials are an efficient and important means of summarizing information about a particular scientific area. When the outcome is a continuous variable, both treatment effect and variance estimates are required to construct a confidence interval for the overall treatment effect. Often, only partial information about the variance is provided in the publication of the clinical trial. This paper provides heuristic suggestions for variance imputation based on partial variance information. Both pretest-posttest (parallel groups) and crossover designs are considered. A key idea is to use separate sources of incomplete information to help choose a better variance estimate. The imputation suggestions are illustrated with a data set.

Beneficial effects of a potassium- and magnesium-enriched salt alternative

The effects on blood pressure and the development of cardiac hypertrophy of sodium chloride (regular salt) and a novel potassium-, magnesium-, and l-lysine-enriched salt alternative, which in a previous study prolonged the life span of hypertensive rats nearly threefold as compared with the animals receiving regular salt, were compared both in spontaneously hypertensive rats and their hypertension-resistant genetic controls. In particular, the possible protective effect of increased intakes of potassium, magnesium, and l-lysine during a high intake of sodium chloride was examined. Therefore, the salt alternative was added at 1.75 times higher levels to produce the same dietary levels of sodium chloride in the regular salt and the salt alternative groups. Regular salt produced a remarkable left ventricular hypertrophy in both rat strains, but as compared with the respective control groups, it induced an increase of blood pressure only in the spontaneously hypertensive rats. The salt alternative did not induce a rise in blood pressure in either of the rat strains, nor did it produce left ventricular hypertrophy in the hypertension-resistant rats and, in the spontaneously hypertensive animals, significantly less hypertrophy than regular salt. The salt alternative appeared to prevent the sodium chloride-induced volume load since plasma levels of atrial natriuretic peptide were increased in the regular salt groups but remained normal in the salt alternative groups. Therefore, potassium, magnesium, and/or l-lysine of the salt alternative produced a powerful protection against the harmful effects of sodium chloride.

Statistics of chronic disease control

Control of the chronic diseases of middle age is increasingly important, in developed and in developing countries, as other causes of premature death decrease. It requires bigger clinical trials, and better epidemiology.

Epidemiologic aspects of isolated systolic hypertension and implications for future research

Isolated systolic hypertension represents an important public health issue in the 1990s because of its prevalence in the elderly and its importance as a risk factor for cardiovascular morbidity and mortality. Methodologic differences may account for the wide variation between prevalence rates in studies reported. With the advent of newer methods of blood pressure (BP) assessment, such as noninvasive ambulatory BP monitoring, it may be possible to define more accurately the true population at risk. Recent data from the Systolic Hypertension in the Elderly Program has indicated a clear benefit of treatment with a reduction in total stroke of 36%, and a reduction of 25 and 32% in the combined end points of coronary heart disease and cardiovascular disease, respectively. Further studies are now required to elucidate what treatment regimens are most effective in preferentially reducing both systolic BP, without producing undesirable effects such as diastolic hypotension, and fatal and nonfatal events. One such trial is underway in Europe using a drug regimen different from that in the Systolic Hypertension in the Elderly Program. The cost implications associated with treating the population at risk are potentially large but these are now based on firm scientific evidence.

By how much does dietary salt reduction lower blood pressure? III--Analysis of data from trials of salt reduction

OBJECTIVE

To determine whether the reduction in blood pressure achieved in trials of dietary salt reduction is quantitatively consistent with estimates derived from blood pressure and sodium intake in different populations, and, if so, to estimate the impact of reducing dietary salt on mortality from stroke and ischaemic heart disease.

DESIGN

Analysis of the results of 68 crossover trials and 10 randomised controlled trials of dietary salt reduction.

MAIN OUTCOME MEASURE

Comparison of observed reductions in systolic blood pressure for each trial with predicted values calculated from between population analysis.

RESULTS

In the 45 trials in which salt reduction lasted four weeks or less the observed reductions in blood pressure were less than those predicted, with the difference between observed and predicted reductions being greatest in the trials of shortest duration. In the 33 trials lasting five weeks or longer the predicted reductions in individual trials closely matched a wide range of observed reductions. This applied for all age groups and for people with both high and normal levels of blood pressure. In people aged 50-59 years a reduction in daily sodium intake of 50 mmol (about 3 g of salt), attainable by moderate dietary salt reduction would, after a few weeks, lower systolic blood pressure by an average of 5 mm Hg, and by 7 mm Hg in those with high blood pressure (170 mm Hg); diastolic blood pressure would be lowered by about half as much. It is estimated that such a reduction in salt intake by a whole Western population would reduce the incidence of stroke by 22% and of ischaemic heart disease by 16% [corrected].

CONCLUSIONS

The results from the trials support the estimates from the observational data in the accompanying two papers. The effect of universal moderate dietary salt reduction on mortality from stroke and ischaemic heart disease would be substantial--larger, indeed, than could be achieved by fully implementing recommended policy for treating high blood pressure with drugs. However, reduction also in the amount of salt added to processed foods would lower blood pressure by at least twice as much and prevent some 75,000 [corrected] deaths a year in Britain as well as much disability.

By how much does dietary salt reduction lower blood pressure? II--Analysis of observational data within populations

OBJECTIVE

To determine whether the estimates of the size of the association between blood pressure and sodium intake derived from studies of individuals within populations can be quantitatively reconciled with our estimates derived from comparisons of the average blood pressure and sodium intake between different populations.

DESIGN

Examination of data from 14 published studies that correlated blood pressure recordings in individuals against measurements of their 24 hour sodium intake (within population studies).

MAIN OUTCOME MEASURE

Comparison of observed differences in blood pressure per 100 mmol/24 h difference in sodium intake in each within population study with predicted differences calculated from the between population data, after allowing for the underestimation of the true association of blood pressure with sodium intake caused by the large day to day variation in 24 hour sodium intake within individuals.

RESULTS

The underestimation bias inherent in the within populations studies reduced the regression slope of blood pressure on single measures of 24 hour sodium intake to between a half and a quarter of the true value (for example, in one study from 6.0 to 2.4 mm Hg/100 mmol/24 h). Estimates from between population comparisons of the regression slope of blood pressure on sodium intake, after adjustment to take this underestimation bias into account, were similar to the values actually observed in the within population studies.

CONCLUSION

The within population studies confirm our estimates from between population comparisons of the magnitude of the association between blood pressure and sodium intake.