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Objective: To demonstrate the magnitude, timing, and cause of changes in blood pressure that occur in migrants from a low blood pressure population on moving to an urban area., Design: A controlled longitudinal observational study of migrants as soon after migration as possible and follow up at three, six, 12, 18, and 24 months after migration. A cohort of controls living in a rural area who were matched for age, sex, and locality were also observed at the same periods., Setting: 35 Villages on the northern shores of Lake Victoria in western Kenya and Nairobi., Participants: 325 Members of the Luo tribe aged 15 to 34 years who had migrated to Nairobi and 267 controls living in villages. The numbers of both groups reduced during follow up such that only 63 migrants and 143 controls were followed up for two years., Main Outcome Measures: A medical questionnaire and three 24 hour diet histories were completed by migrants and controls. Height, weight, pulse, and blood pressure were measured. Three 12 hour overnight urine samples were collected from all participants and analysed for sodium, potassium, and creatinine concentrations., Results: The mean systolic blood pressure of migrants was significantly higher than that of controls throughout the study, and the distribution of blood pressure was shifted to the right compared with controls. The mean diastolic blood pressure of the two groups diverged over time. Blood pressure differences were not due to selective migration. The migrants' mean urinary sodium:potassium ratio was higher than that of controls (p less than 0.001) throughout, and weight and pulse rate were also higher among migrants, although differences diminished with time., Conclusions: Urinary sodium:potassium ratio, pulse rate, and weight are important predictors of increased blood pressure among migrants from a low blood pressure community and may also be implicated in the initiation of essential hypertension.

A longitudinal study of the effects of migration on blood pressure and related factors is being carried out in members of a black Kenyan population who migrate from a traditional rural community to an urban environment. Data on the first 139 migrants (78 male, 61 female) and 204 control non-migrants (126 male, 78 female) who have been followed up for a period of 6 months are presented. Blood pressure changes rapidly on migration (within the first 2 months); thereafter trends between migrants and controls differ. Significant differences in systolic pressure between migrants and controls are found at all examinations during the 6 month follow-up in both sexes. Diastolic pressure falls in controls but rises in migrants, the greatest difference being seen at the 6 month examination. Migration is associated with a marked increase in dietary sodium and a fall in potassium demonstrated by measurements of urinary electrolyte excretion in 3 X 12 h or 3 X 24 h urine collections. Analysis of covariance shows that the blood pressure differences between migrants and controls are partly explained by urinary sodium/potassium ratios and in some instances by body weight.

A relationship between salt intake and blood pressure has been described in cross-sectional studies of several populations. However, serious methodological problems in many studies could have resulted in erroneous results. In the present studies, the blood pressure profiles of a Kenyan tribe have been shown to differ according to environment, being lower in rural and higher in urban communities. Although several factors could explain the observed blood pressure differences, dietary factors, including sodium and potassium intake, could be important determinants of the observed differences. A within-population study of 1,737 rural male subjects was carried out in which it was found that systolic and diastolic pressures correlated positively with urinary sodium/potassium ratios. In this rural group, within subject day-to-day variations in dietary electrolytes are small, as indicated from multiple urinary collections, and are less than those observed in Western societies. We have demonstrated in this rural population that causal samples of urine correlated well with average sodium and potassium excretion over a 7-day period. Preliminary results are reported of a longitudinal study of migrants from a low to a high blood pressure environment and a nonmigrant population. The blood pressure rise consequent to migration was related independently to changes in body weight and urinary electrolytes. The implications of these observations for active intervention studies are discussed.

Blood pressure (BP) and associated factors were determined in 1737 men in a remote Kenyan agricultural community. Systolic BP showed no significant rise with age until after 54 years; diastolic BP showed a small rise with age. Both systolic and diastolic BP correlated with weight independent of age. Systolic and diastolic BP correlated positively with casual urinary sodium/potassium and negatively with potassium/creatinine ratios. Both systolic and diastolic BP correlated significantly with the number of years of education, as did urinary sodium/potassium and sodium/creatinine ratios. Potassium/creatinine ratios were negatively correlated with the number of years of education. Blood pressure and urinary sodium/creatinine ratios were significantly lower in subsistence farmers compared with those in other occupations, and potassium/creatinine ratios were significantly higher. Two pilot studies of Luo tribesmen showed a strong correlation between casual urinary electrolyte ratios and those derived from 24-hour urine samples and a greater variance of sodium excretion between these people than that found within individuals. These results suggest that a relationship between BP and casual urine electrolyte estimations may be identifiable in communities where there is less day-to-day dietary variation. They also suggest that some of the changes in BP associated with urbanization could be mediated by changes in dietary electrolytes.

As part of a longitudinal study of migrants who move from a subsistence farming rural society to Nairobi, blood pressures and associated factors were measured in cross sectional studies of members of the Luo tribe in their traditional rural environment and in the urban environment of Nairobi. Blood pressures in Nairobi correlated with duration of urban residence. In the rural area men showed a negligible rise in blood pressure with age, and both sexes showed a significantly smaller rise than in the urban area. Although mean weights of the rural group were smaller, this did not account for all the urban/rural differences in blood pressures. Nevertheless, mean urinary sodium concentration and sodium ratios (sodium/potassium and sodium/creatinine) were significantly higher in the urban group whereas mean urinary potassium concentration and potassium/creatinine ratio were significantly lower. Perhaps the ratio of sodium to potassium in the diet contributes to the different blood pressure profiles of these two populations.

The blood pressure patterns of three Kenyan communities have been studied: 861 members of the Luo tribe in a rural community, 281 members of the Kamba tribe in a rural community and 310 "urban" Luo living in Nairobi. The slope of linear regression of blood pressure with age was significantly different in each population: rural Luo had the smallest rise with age and urban Luo had the largest rise with age, while the Kamba tribe were in between. The differences could not be accounted for by weight. Urinary electrolyte data showed rural Luo had the lowest sodium ratios (sodium/creatinine, sodium/potassium) and the highest potassium/creatinine ratio: urban Luo had the highest sodium and lowest potassium ratios and rural Kamba had values in between. The differences in blood pressures between the two groups of the same tribe in different environments (rural and urban Luo) were far greater than those between different genetic groups (Kamba and Luo) underlining the importance of environmental factor(s) in the determination of arterial pressure. Furthermore, these data support the theory that the dietary intake of sodium and potassium are the major influential environmental factors affecting blood pressure.