The Relationship Between Low Birth Weight Systolic Blood Pressure And Glomerular Number

Hypertension is an independent risk factor for the development of chronic diseases in adulthood such as ischaemic heart disease [45, 46] and its presence prior to the development of diabetes increases the risk of developing diabetic nephropathy [47 - 49]. All of the large epidemiological studies investigating the link between hypertension and LBW have been unable to determine whether this association was due to a reduction in glomerular number. It is likely that other factors such as family history, adulthood obesity and smoking are much more important in the pathogenesis of hypertension.

Very few published animal studies have measured systolic blood pressure and estimated glomerular number in LBW rats. In studies using intra-uterine low protein diet, rats exposed to low protein diet have reduced glomerular number, raised systolic blood pressure and lower birth weights compared to controls [36, 50]. This suggests that the relationship between LBW and systolic blood pressure may only hold true in a LBW individual was exposed to an abnormal intra-uterine environment, which in turn impaired renal development and reduced glomerular number. Even if this is true, interaction with other risk factors may be required to develop hypertension.

A small post-mortem study estimated glomerular number and volume in hypertensive Caucasian men (age range 35 - 59) and compared results to age height and weight matched normotensive controls [51]. This study demonstrated a reduction in glomerular number (median 702,379 Vs 1,429,200) and an increase in glomerular volume (median 6.50 x10-3 mm3 Vs 2.79 x 10-3 mm3) and there did not appear to be an increase in glomerular sclerosis in the hypertensive group. This study did not control for smoking status or family history of hypertension and the methodology used could have led to underestimation of glomerular volume. Birth weight data were not available for the subjects in this study and as such it is difficult to determine whether the reduction in glomerular number was due to adverse intra-uterine environment or as a consequence of hypertension itself.

Studies in populations with high rates of renal disease may provide more insight into the link between glomerular number and hypertension and one such population is the Aboriginals of Australia. An ultrasound study of 174 Aboriginal children and adolescents aged 5 - 18 years showed that the average renal size, corrected for current body size, was reduced in LBW children [52].

Obviously, this study could not estimate glomerular number. The authors suggested that the reduction in renal size with reduced birth weight may be a mechanism to explain the increased incidence of renal disease in this community, but the lack of estimation of renal size in age matched Caucasian controls weakens this augment. Autopsy studies have shown that Aboriginals have increase glomerular volume compared to Caucasians [53] but this population have high rates of post-streptococcal glomerulonephritis and this could have influenced the results. A study using a single albumin: creatinine ratio and self reported birth weights suggested that, amongst the Aboriginal population, increasing body mass index, blood pressure and LBW act together to increase urine albumin excretion with age [54]. This study was limited by incomplete ascertainment of birth weight amongst the older subjects in the study.

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