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Blood Pressure Levels in and Across Populations

Jeremiah Stamler, M.D.
Northwestern University Medical School

Based on extensive cross-sectional data accumulated over decades, the common blood pressure (BP) pattern prevailing within populations of most countries is: for systolic pressure (SBP), steadily and substantially higher population average SBP from youth-young adulthood through middle age and into older age; for diastolic pressure (DBP), higher average DBP from youth-young adulthood into middle age and plateau from latter middle age into older age. Whereas at ages 18-24 or 20-29, average SBP/DBP of populations are generally at optimal levels (=<120/=<80 mmHg) at ages =>35 or =>40--due to upward slope with age--only a minority have optimal SBP/DBP, and substantial proportions have high-normal levels or frank high BP.

With the upward sloping of SBP/DBP with age, inter-individual differences in levels become greater with age, as reflected in higher standard deviations and coefficients of variation; the BP distributions are progressively skewed to the right due to steady increases with age in prevalence of people with frank high BP Stages 1, 2, 3.

Prospective data from long-term follow-up of cohorts, with multiple repeat BP measurements over time, yield findings similar to those from cross sectional data. They further show that the upward slopes reflect a mix of individual BP patterns over the decades of adulthood, i.e., most persons experience BP rise with age of varying degrees, only a small percent have little or no rise with age.

Across populations, there are quantitative variations in the degree of upward slope of SBP/DBP with age. As repeated studies over decades have shown, there are also extant in the world remote isolated populations with little or no upward slope of SBP/DBP during the decades of adulthood, average SBP/DBP at optimal levels throughout adulthood, and little or no high BP.

In the United States, three well-established subgroup patterns are: 1. by gender: for men, compared to women, higher average SBP/DBP at all ages up to 40-49 or 50-59, and at older ages a trend to the reverse (a crossover); 2. by ethnicity: for African-Americans, higher average levels than for non Hispanic whites, with differences modest in youth-young adulthood, greater at older ages; 3. by socioeconomic level (SES): for population strata of lower SES, higher SBP/DBP than for those of higher SES. Populations of other countries tend to have gender and SES patterns of BP similar to those for the U.S.

Research initiated in the 1980s and 1990s has greatly increased knowledge on factors related to within- and across-population BP patterns. Within populations, BP levels of individuals are known to be related to lifestyle--especially dietary--factors, as well as genetic factors (almost certainly polygenic, as yet largely unidentified). Thus, evidence is abundant showing that dietary NaCl, body mass, alcohol intake (particularly >2 drinks per day) are directly and independently related to BP, and potassium intake is inversely related. These and other dietary factors account significantly for the higher blood pressure levels of people from lower SES strata.

Cross-population analyses show that population sample average NaCl intake relates strongly to slope of SBP and DBP with age, as well as to sample average SBP, DBP, and prevalence of high BP.

All these data formed the basis for the 1993 landmark recommendations from NHLBI on the primary prevention of high BP, along with evidence indicating that exercise can also have favorable effects on BP. In the last years, epidemiologic studies obtained further evidence indicating direct relations to BP of dietary saturated fats and cholesterol, and inverse relations to BP of protein, beta-carotene and vitamin C, and to intake of minimally processed foods of vegetable origin.

The two DASH feeding trials have made a critical research contribution on the impact of multiple dietary factors on BP. In DASH-1, the DASH combination diet was shown substantially to lower SBP/DBP compared to usual American fare. The combination diet was higher in fruits and vegetables, higher in fat-free and low-fat dairy products--hence higher in K, Mg, fiber, protein, Ca; it was also lower in total fat, saturated fat, cholesterol, and sweets. By design, diets were isocaloric, NaCl was held constant at an average of about 7.5 g/d (slightly less than U.S. average), and participants consumed little or no alcohol. Favorable effects of the combination diet were recorded overall, and for nonhypertensives and hypertensives, men and women, African Americans and whites. In the second DASH feeding trial--DASH-NaCl--participants were randomized to combination or usual U.S. diets, and crossed over to ingest NaCl at three levels, 150 mmol Na/day (8.7 g NaCl--close to usual U.S. average intake), 100 mmol, and 50 mmol. Effects on SBP/DBP of the combination diet and of lower NaCl were partially additive: largest BP reductions were with the combination diet and 50 mmol Na/day, compared to usual U.S. diet and 150 mmol Na/day--for all participants, SBP/DBP lower by 9/4.5 mmHg; for nonhypertensives, lower by 7/4 mmHg; for hypertensives, by 11.5/6 mmHg. Corresponding favorable BP reductions were recorded with the intervention for men and women, African Americans and whites. Effects of NaCl reduction were graded--greater at 50 vs. 100 mmol/d than at 100 vs. 150 mmol/d.

Conclusions: As for serum cholesterol, so for SBP/DBP, multiple dietary factors have substantial influences on levels. Knowledge is now in hand to prevent much of the SBP/DBP rise with age during adulthood, markedly increase the proportion of the population with optimal SBP/DBP, and markedly reduce the proportion of the population with high-normal and high BP. That is, the science is now extant to put an end to the epidemic of adverse BP levels (as well as adverse serum cholesterol levels) by safe nutritional means, thereby contributing importantly to ending the epidemic of CHD-CVD.

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