Absence of siblings--a risk factor for hypertension?

Is only-child status associated with a higher blood pressure in adolescence? An observational study

Growing up with siblings may affect quality of life as well as hemodynamic parameters including blood pressure. Using weighted data from the nationwide and representative German KiGGs study, we assessed the relationship between only-child status, birth order, and arterial blood pressure in a cohort of 7311 adolescents aged between 11 and 17 years. Our data showed that only-children had the highest mean arterial blood pressure (87.3 ± 8.4 mmHg) as compared to first-born (86.3 ± 8.0), middle-born (86.4 ± 8.7), and youngest-born siblings (86.6 ± 8.2; p = 0.012). The two groups of only-children and first-borns differed significantly with respect to their age- and sex-specific, z-scored data for systolic (p = 0.047), diastolic (p = 0.012), and mean arterial blood pressure (p = 0.005). Linear regression models with blood pressure recordings as dependent variable adjusted to age, migration background, and age- and sex-specific z-scores of body-mass index confirmed that only-child status was an independent predictor of a higher diastolic blood pressure (p = 0.037). A similar result was observed for mean arterial blood pressure (p = 0.033), whereas systolic blood pressure was not associated with only-children status (p = 0.258).   Conclusion: In summary, we found a significant and positive relationship between only-child status and blood pressure, with the highest recordings in only-children and the lowest in first-borns. Models adjusted for relevant clinical confounders demonstrated slightly higher blood pressure in only-children compared to first-borns, who are both in an alpha birth order. Although these blood pressure differences were statistically significant, they have limited, if any, clinical meaning in this age group. What is Known: • In children and adults, blood pressure is significantly higher in only-children compared to children with siblings. However, it is unclear whether there are also similar blood pressure differences in adolescents and between only-children and first-borns. What is New: • Among adolescents in the alpha birth order, growing up as an only-child is associated with significantly higher mean blood pressure than among first-borns living with younger siblings.

Evaluating indirect genetic effects of siblings using singletons

Estimating effects of parental and sibling genotypes (indirect genetic effects) can provide insight into how the family environment influences phenotypic variation. There is growing molecular genetic evidence for effects of parental phenotypes on their offspring (e.g. parental educational attainment), but the extent to which siblings affect each other is currently unclear. Here we used data from samples of unrelated individuals, without (singletons) and with biological full-siblings (non-singletons), to investigate and estimate sibling effects. Indirect genetic effects of siblings increase (or decrease) the covariance between genetic variation and a phenotype. It follows that differences in genetic association estimates between singletons and non-singletons could indicate indirect genetic effects of siblings if there is no heterogeneity in other sources of genetic association between singletons and non-singletons. We used UK Biobank data to estimate polygenic score (PGS) associations for height, BMI and educational attainment in self-reported singletons (N = 50,143) and non-singletons (N = 328,549). The educational attainment PGS association estimate was 12% larger (95% C.I. 3%, 21%) in the non-singleton sample than in the singleton sample, but the height and BMI PGS associations were consistent. Birth order data suggested that the difference in educational attainment PGS associations was driven by individuals with older siblings rather than firstborns. The relationship between number of siblings and educational attainment PGS associations was non-linear; PGS associations were 24% smaller in individuals with 6 or more siblings compared to the rest of the sample (95% C.I. 11%, 38%). We estimate that a 1 SD increase in sibling educational attainment PGS corresponds to a 0.025 year increase in the index individual's years in schooling (95% C.I. 0.013, 0.036). Our results suggest that older siblings may influence the educational attainment of younger siblings, adding to the growing evidence that effects of the environment on phenotypic variation partially reflect social effects of germline genetic variation in relatives.

Relation between number of siblings and adult mortality and stroke risk: 25 year follow up of men in the Collaborative study

STUDY OBJECTIVE

To investigate the relation between number of siblings, mortality risk, and stroke risk.

DESIGN

Prospective cohort study.

SETTING

27 workplaces in Scotland.

PARTICIPANTS

5765 employed men aged 35-64 from a variety of different workplaces, screened between 1970 and 1973.

MAIN RESULTS

There were strong relationships between number of siblings and socioeconomic variables and also with adult behavioural measures. Men with greater numbers of siblings had an increased risk of dying of all causes, coronary heart disease, lung cancer, stomach cancer, and respiratory disease over a 25 year follow up period. Adjustment for risk factors could explain these associations, excepting stomach cancer mortality. With the definition of stroke as either a hospital admission for stroke or death from stroke, there was a strong relation between number of siblings and haemorrhagic stroke, but not ischaemic stroke.

CONCLUSIONS

Number of siblings is strongly related to mortality risk, but as it is also related to many risk factors, adjustment for these can generally explain the relation with mortality. The exceptions are stomach cancer mortality and haemorrhagic stroke, which are known to be related to deprivation in childhood, and, in the case of stomach cancer to childhood infection.

Do maternal and intrauterine factors influence blood pressure in childhood?

It has been proposed that maternal health and nutrition may be important in the development of adult cardiovascular risk, and that blood pressure may be an important intermediate step in this process. To examine the relevance of this hypothesis in contemporary British children, the relationships of several maternal factors to blood pressure were studied in 3360 children of European origin aged 5-7 years. Maternal age, height, and body mass index were all positively related to blood pressure in childhood but these relationships were abolished once the child's body build was taken into account. Maternal social class, educational attainment, and history of smoking in pregnancy showed no relationship with blood pressure in childhood. Parity showed an inverse association with blood pressure, but this appeared to be due to an association between total sibship size and blood pressure, suggesting a postnatal rather than a prenatal origin. Blood pressure was higher in children whose mothers had a history of high blood pressure but this association was no stronger than that for paternal history. Both birth weight and gestational age were inversely related to blood pressure at 5-7 years. The association between birth weight and blood pressure was attenuated by standardisation for gestational age, and the relationships between birth weight and blood pressure were similar in preterm and full term infants. No specific association between blood pressure and the maternal factors studied have been observed in this population. Hypotheses relating maternal factors to cardiovascular risk need to specify the timing and nature of their effects more precisely. Although the relationship between birth weight and blood pressure is not fully understood, it appears to reflect size at birth rather than fetal growth rate.