Our systematic review found that publication bias may partly explain the lower mean systolic blood pressure observed in participants that had been breast fed in infancy, with large studies showing little difference. Studies with more than 1000 participants, which are less subject to publication bias, reliably excluded a mean difference greater than 0.6 mm Hg, suggesting an effect of little clinical or public health importance. Feeding in infancy was not significantly associated with diastolic blood pressure.
Even if publication bias is discounted, the overall difference in systolic blood pressure of 1.1 mm Hg is modest. The relatively weak association between infant feeding and systolic blood pressure is not likely to reflect imprecise ascertainment of early feeding practices, as most were documented either during infancy from health records or from questionnaires administered to parents, the accuracy of which has shown to be valid up to 20 years after birth.24
In addition, the difference in blood pressure between feeding groups was similar in studies that recorded infant feeding status in infancy to those based on a parental questionnaire later in life.
Random allocation of infants to breast feeding or bottle feeding has been regarded as inappropriate, except in the special circumstances of preterm birth, in which one randomised trial has been carried out.18
That trial showed no marked difference in blood pressure between 8 year old children who were breast fed or bottle fed as infants.18
A follow up examination showing noticeably higher mean blood pressures among bottle fed infants was based on only a quarter of the study population; at such low follow up rates the validity of the comparisons cannot be assumed, and the degree of control for the original study centre has been questioned.25
The other studies in this review were observational, so that there is a possibility of confounding, particularly by social factors, current body size, and diet in later life. However, since bottle feeding tends to be related to lower social class, a greater tendency to obesity, and a less healthy diet in later life, all of which are likely to be related to higher mean blood pressure, any confounding effects are likely to have exaggerated, rather than reduced, the extent to which mean blood pressure levels are higher among participants that were bottle fed.26,27
Although our review effectively excludes any important overall lowering effect of breast feeding on blood pressure, it is possible that prolonged breast feeding has a protective effect.15
This issue requires further systematic examination.
We were unable to examine whether infants fed formula milk supplemented with long chain polyunsaturated fatty acids had lower blood pressure in childhood compared with infants fed standard formulas.12
The results do not, however, suggest that naturally occurring long chain polyunsaturated fatty acids in breast milk have an appreciable effect on blood pressure. The results from large studies effectively exclude the effect size observed in the recently reported small trial of supplementation with long chain polyunsaturated fatty acids, although the confidence intervals around that estimate were wide.12
The absence of marked differences in blood pressure between infant feeding groups (either before or after 1980) are also of interest because of the higher sodium content of formula milk up to 1980.5,6
In the 1970s, the average sodium intake of formula fed infants at one month of age in most of the populations studied here would have been around 20 mmol a day higher than that of infants who were breast fed.5
However, the interpretation of the results depends on the strength of association between sodium intake and blood pressure assumed. Using the large effect estimates derived from a randomised controlled trial of salt restriction in neonates, a 20 mmol a day difference might be expected to produce a difference in systolic blood pressure in the order of 5-6 mm Hg between infants that were breast fed or bottle fed—a difference that is easy to exclude among the infants in four studies before 1980 (data not presented).7
However, the more conservative effect estimates from systematic reviews of sodium reduction trials in adults would be consistent with differences between bottle fed and breastfed infants of 1-2 mm Hg, which would be considerably more difficult to exclude with confidence.28,29
The absence of any appreciable change in the size of this difference between the 1970s and 1990s, particularly in the largest studies, suggests that the effects of changing sodium intake in infancy on subsequent blood pressure have been modest.
The lower levels of systolic blood pressure associated with breast feeding in infancy identified in our review were observed mainly in small studies; with little difference shown in studies of 1000 participants or more. The association of breast feeding and lower blood pressure may well be partly explained by publication bias. Hence the results of small studies showing large differences in blood pressure should be treated cautiously. Our analysis suggests that any effect of breast feeding on blood pressure is modest and of limited clinical or public health importance. However, blood pressure is not the only relevant outcome; the case for breast feeding rests on a combination of short and long term benefits, including improved neural and psychosocial development, potential protection against obesity and allergic disease, and lower blood cholesterol levels in later life.19,26,30-33
What is already known on this topic
Early studies suggested that breast feeding in infancy may protect against high blood pressure in later life
Many studies examining this effect have been small, raising the possibility of publication bias
What this study adds
Selective publication of small studies shows that breast feeding reduces blood pressure in later life
The effect in larger studies seems to be small and of limited clinical or public health importance