Salt and blood pressure in Belgium

Blood pressure and cardiovascular risk in relation to birth weight and urinary sodium: an individual-participant meta-analysis of European family-based population studies

BACKGROUND

Although the relation of salt intake with blood pressure (BP) is linear, it is U-shaped for mortality and cardiovascular disease (CVD). This individual-participant meta-analysis explored whether the relation of hypertension, death or CVD with 24-h urinary sodium excretion (UVNA) or sodium-to-potassium (UNAK) ratio was modified by birth weight.

METHODS

Families were randomly enrolled in the Flemish Study on Genes, Environment and Health Outcomes (1985-2004) and the European Project on Genes in Hypertension (1999-2001). Categories of birth weight, UVNA and UNAK (≤2500, >2500-4000, >4000 g; <2.3, 2.3-4.6 and >4.6 g; and <1, 1-2, >2, respectively) were coded using deviation-from-mean coding and analyzed by Kaplan-Meier survival functions and linear and Cox regression.

RESULTS

The study population was subdivided into the Outcome ( n  = 1945), Hypertension ( n  = 1460) and Blood Pressure cohorts ( n  = 1039) to analyze the incidence of mortality and cardiovascular endpoints, hypertension and BP changes as function of UVNA changes. The prevalence of low/medium/high birth weight in the Outcome cohort was 5.8/84.5/9.7%. Over 16.7 years (median), rates were 4.9, 8 and 27.1% for mortality, CVD and hypertension, respectively, but were not associated with birth weight. Multivariable-adjusted hazard ratios were not significant for any endpoint in any of the birth weight, UVNA and UNAK strata. Adult body weight tracked with birth weight ( P  < 0.0001). The partial r in the low-birth-weight group associating changes from baseline to follow-up in UVNA and SBP was 0.68 ( P  = 0.023) but not significant in other birth weight groups.

CONCLUSION

This study did not substantiate its prior hypothesis but showed tracking of adult with birth weight and suggest that low birth weight increases salt sensitivity.

Sodium Intake and Cardiac Sympatho-Vagal Balance in Young Men with High Blood Pressure

We have previously reported that a high sodium intake increases sleep-time blood pressure (BP) in young men. However, there are cases in which this relation does not apply. To account for them, we investigated the relation between sodium intake and cardiac sympatho-vagal balance (SVB) in young men with high BP. Sodium intake was estimated from the amount of urinary sodium excretion over 1 week. Twenty-four-hour (24-h) urinary sodium excretion (Salt24), 24-h ambulatory BP and ECG were obtained on the last day of the observation period. As an index of sodium intake, the expression In(Salt24/Cr24) (Cr24, 24-h urinary creatinine excretion) was used. From power-spectral analysis of ECG-RR intervals during sleep, we obtained the LF/HF ratio between the low-frequency component (LF) and the high frequency component (HF) and used it as an index of SVB. The subjects were male medical students divided into a normal BP group (N-group; n=103) and a high BP group (H-group; n=26, 24-h BP>125/75 mmHg). Mean In(Salt24/Cr24) and LF/HF in the H-group were significantly higher than those in the N-group (LF/HF: 1.86+/-0.44 [SD] vs. 1.37+/-0.30, p<0.001). The calculated discriminant function (D) for the H-group and N-group was D=1.6x + 5y - 11, where x is In(Salt24/Cr24) and y is LF/HF. This formula (D) resulted in high discriminant predictive accuracy (82%) between the groups. If D=0 (the value of the cut-off line determining separation of the groups), the relation y=-0.32x + 2.2 (negative relation between y and x) was obtained. These results suggest that excessive sodium intake in combination with accentuated SVB (LF/HF) increases BP in young men.

Life style as a blood pressure determinant

In Belgium, an affluent Western European country, participation in sports, alcohol intake, and living in a working class area were identified as the life style factors with the closest associations with the blood pressure level. Obesity was another important blood pressure correlate. Sodium intake, determined from the 24 h urinary output, and smoking were not associated with blood pressure. Controlled intervention studies have proven that weight reduction, endurance training and alcohol abstinence effectively reduce blood pressure. In the light of these intervention studies, the Belgian findings and the published work highlight the potential of preventive strategies aimed at these major life style factors.

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

OBJECTIVE

To estimate the quantitative relation between blood pressure and sodium intake.

DESIGN

Data were analysed from published reports of blood pressure and sodium intake for 24 different communities (47 000 people) throughout the world.

MAIN OUTCOME MEASURE

Difference in blood pressure for a 100 mmol/24 h difference in sodium intake. Allowance was made for differences in blood pressure between economically developed and undeveloped communities to minimise overestimation of the association through confounding with other determinants of blood pressure.

RESULTS

Blood pressure was higher on average in the developed communities, but the association with sodium intake was similar in both types of community. A difference in sodium intake of 100 mmol/24 h was associated with an average difference in systolic blood pressure that ranged from 5 mm Hg at age 15-19 years to 10 mm Hg at age 60-69. The differences in diastolic blood pressure were about half as great. The standard deviation of blood pressure increased with sodium intake implying that the association of blood pressure with sodium intake in individuals was related to the initial blood pressure--the higher the blood pressure the greater the expected reduction in blood pressure for the same reduction in sodium intake. For example, at age 60-69 the estimated systolic blood pressure reduction in response to a 100 mmol/24 h reduction in sodium intake was on average 10 mm Hg but varied from 6 mm Hg for those on the fifth blood pressure centile to 15 mm Hg for those on the 95th centile.

CONCLUSIONS

The association of blood pressure with sodium intake is substantially larger than is generally appreciated and increases with age and initial blood pressure.

Relation of electrolytes to blood pressure in men. The Yi people study

The relations of sodium, potassium, calcium, and magnesium to blood pressure were investigated in four groups of men (119 high-mountain Yi farmers, 114 mountainside Yi farmers, 89 Yi migrants, and 97 Han people) with a wide range of electrolyte intake in Puge County, Sichuan Province, People's Republic of China. Electrolytes were measured in diet, serum, and urine. Sodium excretion was 73.9 mmol/24 hr in high-mountain Yi farmers, 117.9 mmol/24 hr in mountainside Yi farmers, 159.4 mmol/24 hr in Yi migrants, and 186.0 mmol/24 hr in the Han people. In ecological correlation analysis, dietary and urinary sodium were significantly and positively correlated with both systolic and diastolic pressure, whereas serum sodium showed no relation with blood pressure. In diet, serum, and urine, potassium was negatively related to systolic and diastolic pressure, whereas the sodium/potassium ratio showed a positive association. With regard to calcium, only urinary excretion was significantly and positively related to blood pressure. No relation was found between magnesium and blood pressure. Analyses at the individual level confirmed the results for sodium and potassium seen at the ecological level, but in addition, dietary calcium and magnesium were significantly and negatively correlated to both systolic and diastolic pressure, and urinary magnesium was inversely related to diastolic pressure. These relations persisted after controlling for age, body mass index (kg/m2), heart rate, alcohol, and total energy intake in multiple regression analysis performed separately for electrolytes in diet, serum, and urine. In multiple regression analysis, an increase in sodium intake of 100 mmol/day corresponded to an increase of 2.3 mm Hg systolic blood pressure and 1/8 mm Hg diastolic pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

[Blood pressure and relative body weight, alcohol consumption and electrolyte excretion in the FRG and the GDR: the Intersalt Study. The Intersalt Study Group form the FRG and the GDR]

The relationships between body mass index (BMI) and age, alcohol consumption, 24-hr urinary electrolyte excretion, and BP were studied in 588 subjects from three German centers participating in Intersalt, a highly standardized, previously reported protocol. Men and women aged 20-59 were sampled in Bernried, FRG; Cottbus, GDR; and Heidelberg, FRG. The subjects from the three centers did not differ in BMI, level of education, physical activity, cigarette- or alcohol-consumption patterns, or urinary Cl excretion. Mean Na excretion was 167, 147, and 172 mmol/24 hr in Bernried, Cottbus, and Heidelberg, while mean K excretion was 72, 55, and 73 mmol/24 hr, respectively. The excretion of these electrolytes was significantly lower in Cottbus than in Bernried or Heidelberg. BMI increased progressively in men with age; in women BMI plateaued until the 5th decade, after which it increased to equal that of men. In individual centers, the excretion of electrolytes was correlated with BMI. Sodium and chloride excretion were highly correlated. The data from each individual center were fitted to a multiple regression model. Age, BMI, sex, and alcohol consumption entered the model.

Urinary electrolyte excretion, alcohol consumption, and blood pressure in the Scottish heart health study

As part of a study of risk factors for coronary heart disease 24 hour urine collections were obtained from 7354 men and women aged 40-59 selected at random from 22 districts throughout Scotland (Scottish heart health study). The mean of two standardised measurements of blood pressure was related to the reported consumption of alcohol and measurements of height, weight, pulse rate, and electrolyte excretion. Several significant correlations were found with both systolic and diastolic pressure, but only the coefficients for age, body mass index, and pulse rate were greater than 0.1. Alcohol consumption showed a weak positive correlation with blood pressure in men. Sodium excretion showed a weak positive correlation with blood pressure in both sexes, and potassium excretion showed weak negative correlations. In multiple regression analysis age, pulse rate, body mass index, alcohol consumption, and potassium excretion had significant independent effects but sodium excretion did not. Although measuring blood pressure twice on one occasion and 24 hour urinary sodium excretion only once may have weakened any potential correlation, the most likely explantation of these results is that the relation between sodium and blood pressure in the population is weak and that potassium and alcohol are of greater importance.

The relationship between both sodium and potassium intake and blood pressure in London Civil Servants. A report from the Whitehall Department of Environment Study

The relationship was examined in 459 male London Civil Servants between the casual lying systolic and diastolic (phase V) blood pressures measured in the clinic, and 24-hr urinary sodium and potassium excretion collected an average of 6 weeks later. Systolic and diastolic pressures, after adjusting for age and weight, were both negatively associated with the 24 hr potassium excretion (r = -0.17 [p less than 0.001, 95% confidence limits -0.07, -0.27) and r = -0.11 (p less than 0.05, 95% CL 0.00, -0.21) for systolic and diastolic pressure respectively]. Adjusted systolic but not diastolic pressure was also negatively associated with plasma potassium estimated at the time of blood pressure measurement (r = -0.15, p less than 0.001, 95% CL -0.06, -0.24). In 159 female London Civil Servants both systolic and diastolic pressures were negatively correlated with plasma potassium. However, there was no evidence for a negative relationship between urinary potassium excretion and blood pressure in women. Urinary sodium excretion was not related to blood pressure either in men or women. Blood pressure was also measured by the subjects at home over a one week period during which time the urine collection was made. There was no evidence for an association between blood pressure measured at home 6 weeks after the time of drawing blood, and plasma sodium. On the other hand, in men, the negative relationship between blood pressure and urinary potassium excretion was consistent and independent of the time and place of measuring blood pressure. This is consistent with a true effect of diet, a low potassium intake being associated with an increased blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium is more important than calcium in essential hypertension

There is much circumstantial and some direct evidence in humans to suggest that a high consumption of salt predisposes communities and individuals to the development of essential hypertension. Recent work has suggested possible mechanisms whereby a high salt intake could cause a rise in blood pressure in genetically susceptible persons. Restriction of salt intake in the diet lowers blood pressure in many subjects with high blood pressure and this fall in blood pressure is mediated in part by a diminished renin response to sodium restriction as hypertension develops. The effect of sodium restriction, like diuretics, is additive to most blood pressure lowering drugs, particularly those that inhibit the renin system such as beta-blockers and angiotensin converting enzyme inhibitors. Claims that a slight reduction in calcium intake may be important in the development of high blood pressure are disputed. Furthermore, no satisfactory hypothesis has been put forward to explain how a small reduction in dietary calcium intake could cause high blood pressure. Large increases in calcium intake have been reported to lower blood pressure in both normotensive and hypertensive humans. The three published studies, however, are not in agreement.

Monovalent and divalent cations in hypertension

The relationship of four cations (sodium, potassium, calcium, magnesium) to hypertension is reviewed. It seems reasonable to advise some reduction in sodium intake, and an initial goal of a mean intake of 120 mmole per day for men and 90-100 mmole per day for women is suggested. Some increase in potassium intake may well be justifiable but is better achieved through potassium-containing foods than by any artificial supplements. The data for calcium and magnesium are not sufficiently strong to warrant any recommendation for a change in intake at present.

Familial aggregation of blood pressure, anthropometric characteristics and urinary excretion of sodium and potassium--a population study in two Belgian towns

Blood pressure and anthropometric characteristics were measured on two separate occasions and the urinary sodium and potassium excretion determined from one 24-hr urine collection in a random population sample (n = 612) of two Belgian towns. After adjusting for confounding factors familial aggregation of these measurements was studied by computing correlation coefficients for the various intrafamilial relationships. Spouses are genetically dissimilar and share a common home environment only from adulthood on. The spouse-spouse correlations for blood pressure, body weight, body mass index and urinary sodium were therefore not significant, whereas the close spouse-spouse correlation for height (r = 0.29) may be explained by assortive marriage. Sibs not only share a common home environment, but also part of their genetic material. Significant sib-sib correlations could therefore be demonstrated for diastolic blood pressure (r = 0.21), height (r = 0.32), body mass index (r = 0.24) and urinary sodium (r = 0.34). For systolic pressure the father-son (r = 0.33) and mother-daughter (r = 0.24) correlations were significant, whereas the other parent-offspring correlations for systolic and diastolic pressure were not statistically different from zero. The tendency of the correlation coefficients for body weight and body mass index to be higher in mother-offspring than in father-offspring pairs suggests a closer maternal role in the determination of calorie intake in offspring. The parent-offspring correlations for urinary sodium were only significant in father-daughter (r = 0.24) and mother-son (r = 0.21) pairs. The fact that potassium, in contrast to sodium, is rarely used as an artificial additive in food preparation may explain why the correlations for urinary potassium were relatively high in all relationship pairs, varying from 0.20 to 0.34.

Interrelationships between blood pressure, sodium, potassium, serum cholesterol, and protein intake in Japanese

Interrelationships among blood pressure (BP), sodium (Na), potassium (K), dietary protein, and serum cholesterol level (Chol) were examined in 62% (1120) of 1818 Japanese inhabitants of both sexes aged over 30 years who lived in a rural village in Japan. Fasting single-spot urine specimens were collected in the morning to measure Na, K, urea nitrogen (UN), inorganic sulfate (SO4), and creatinine (Cr). The Cr ratios of Na, K, UN, SO4, Na/K, and SO4/UN were analyzed by multiple regression analysis to determine independent associations with BP together with age, obesity index, hematocrit (Hct), Chol, triglyceride (TG), and fasting serum glucose level (Glu). Except for Na/Cr in men, Na/Cr and Na/K were found to be independently and positively related to BP, particularly to systolic BP (SBP). In contrast, K/Cr and SO4/UN (an index related to the dietary score of sulphur-containing amino acids derived mainly from animal protein) were both negatively associated with SBP, and UN/Cr (an index of total protein intake) was positively associated with SBP in men. Chol was linked to BP negatively in men but positively in women. Age, obesity index, TG, and Hct were generally positively and significantly related to BP in both sexes. The results confirmed on epidemiological grounds the positive link of Na and the negative link of K to BP within a single population in Japan. They further suggest, although only in men, that there is a negative relationship of Chol and dietary animal protein with BP.

Water sodium, urinary electrolytes, and blood pressure of adolescents

Blood pressure measurements were made on children in their fourth year at secondary schools in parts of Scunthorpe Health District supplied with drinking water of varying sodium content. Of the 3131 children, 2740 were examined (1394 boys and 1346 girls). Boys had slightly higher systolic pressures and slightly lower diastolic pressures than the girls. There was no difference between the blood pressure distributions of children in areas supplied with water containing 105 mg/l sodium, 50 mg/l sodium, or less than 15 mg/l sodium. Small differences were found in the weights of children in these areas, and slightly more of the children in the area supplied with water containing more than 105 mg/l sodium had relatives who had been treated for hypertension. Standardisation for these factors did not show any relation between water salinity and either systolic or diastolic blood pressure. Studies of the urinary sodium, potassium, and the ratios of these to creatinine on a sample of 769 boys showed no correlation with assessments of usual sodium intake, but urinary sodium correlated well with salt and fluid intake at the meal immediately preceding examination. The relation between median blood pressure and urinary sodium concentration and lack of a clear relation with sodium creatinine ratio supports the hypothesis that it may be the ratio of salt to fluid intake rather than total dietary sodium that is relevant to the regulation of blood pressure.

Urinary cadmium and lead concentrations and their relation to blood pressure in a population with low exposure

The 24 hour urinary excretion of cadmium (U-Cd) and lead (U-Pb), and the excretion of beta-2- microglobulins and retinol binding protein concentration in spot urines, were determined in a random 4% sample of the population of a small Belgian town. Blood pressure and body weight were measured on two separate occasions. U-Cd averaged 2.4 nmol/24 h in 46 youths, increased with age, and was significantly higher in 57 adult men as compared with 59 women (9.3 v 7.2 nmol/24 h; p less than 0.01). U-Pb averaged 28 nmol/24 h in youths and similarly increased with age: adult men excreted more lead than women (64 v 40.0 nmol/24 h; p less than 0.001). Among men, manual workers excreted more cadmium (12.6 v 7.5 nmol/24 h; p less than 0.05) but a similar amount of lead (62 v 61 nmol/24 h) compared with office workers. After adjusting for sex and age, U-Cd and U-Pb were not related to body weight and cigarette consumption. In simple regression analysis, U-Cd was positively correlated with both systolic (r = +0.30; p less than 0.05) and diastolic (r = +0.38; p less than 0.01) blood pressure in women. After adjusting for other contributing variables, however, a weak but negative relation became apparent between systolic pressure and U-Cd in women (t = -2.21; p = 0.033) and between diastolic pressure and U-Cd in men (t = -2.04; p = 0.047). In women urinary beta-2-microglobulin was related to diastolic pressure (r-0.44; p<0.01) and after adjusting for age to both systolic (t=2.75; p=0.009) and diastolic (t=-3.07; p=0.004) pressure. In none of the sex-age groups did U-Pb and retinol binding protein contribute to the blood pressure variability.