'And if there is a causal link then really the advantages are so small that they are not really worth considering'
Erm, no.
Findings from a very good systematic review on studies conducted in developing countries [[http://www.ncbi.nlm.nih.gov/books/NBK38335/ here]
Results summary
We screened over 9,000 abstracts. Forty-three primary studies on infant health outcomes, 43 primary studies on maternal health outcomes, and 29 systematic reviews or meta-analyses that covered approximately 400 individual studies were included in this review.
The association studies of breastfeeding and health outcomes mostly presented results as odds ratios. To facilitate interpretation of the odds ratio, we chose to present these data as a reduction in relative risk, estimated as ?(1 - odds ratio) × 100%,? along with the corresponding 95% confidence interval (CI).
Full term Infant Outcomes
Acute Otitis Media. Our meta-analysis of five cohort studies of good and moderate methodological quality showed that breastfeeding was associated with a significant reduction in the risk of acute otitis media. Comparing ever breastfeeding with exclusive formula feeding, the risk reduction of acute otitis media was 23 percent (95% CI 9% to 36%). When comparing exclusive breastfeeding with exclusive formula feeding, either for more than 3 or 6 months duration, the reduction was 50 percent (95% CI 30% to 64%). These results were adjusted for potential confounders.
Atopic Dermatitis. One good quality meta-analysis of 18 prospective cohort studies on full term infants reported a reduction in the risk of atopic dermatitis by 42 percent (95% CI 8% to 59%) in children with a family history of atopy and exclusively breastfed for at least 3 months compared with those who were breastfed for less than 3 months. The meta-analysis did not distinguish between atopic dermatitis of infancy (under 2 years of age) and persistent or new atopic dermatitis at older ages. It has been postulated that the diagnosis of atopic dermatitis in patients younger than 2 years of age could be attributed to infectious etiologies, which may be prevented by breastfeeding. However, a stratified analysis by duration of followup found the risk reduction from breastfeeding was similar in subjects with less than 2 years compared with more than 2 years of followup.
Gastrointestinal Infections. For non-specific gastroenteritis, one systematic review identified three primary studies that controlled for potential confounders. These studies reported that there was a reduction in the risk of non-specific gastrointestinal infections during the first year of life in breastfed infants from developed countries. But a summary adjusted estimate taking into account potential confounders could not be determined because the studies did not provide usable quantitative data. However, a recent case-control study from England that took into account the role of potential confounders reported that infants who were breastfeeding had a 64 percent (95% CI 26% to 82%) reduction in the risk of non-specific gastroenteritis compared with infants who were not breastfeeding.
Lower Respiratory Tract Diseases. The summary estimate from a good quality meta-analysis of seven studies reported an overall 72 percent (95% CI 46% to 86%) reduction in the risk of hospitalization due to lower respiratory tract diseases in infants less than 1 year of age who were exclusively breastfed for 4 months or more. The results remained consistent after adjustment for potential confounders.
Asthma. The studies on asthma were equivocal. A previously published good quality meta-analysis reported a moderate protective effect and four recent primary studies reaching mixed conclusions, including two studies finding an increased risk of asthma associated with breastfeeding. We updated the meta-analysis with the new studies. Our analysis showed that breastfeeding for at least 3 months was associated with a 27 percent (95% CI 8% to 41%) reduction in the risk of asthma in those subjects without a family history of asthma compared with those who were not breastfed. For those with a family history of asthma, there was a 40 percent (95% CI 18% to 57%) reduction in the risk of asthma in children less than 10 years of age who were breastfed for at least 3 months compared with those who were not breastfed. However, the relationship between breastfeeding and the risk of asthma in older children and adolescents remains unclear and will need further investigation.
Cognitive Development. One well-performed sibling analysis and three prospective cohort studies of full-term infants, all conducted in developed countries, adjusted their analyses specifically for maternal intelligence. The studies found little or no evidence for an association between breastfeeding in infancy and cognitive performance in childhood. Most of the published studies adjusted their analyses for socioeconomic status and maternal education but not specifically for maternal intelligence. For those studies that reported a significant effect after specific adjustment for maternal intelligence, residual confounding from other factors such as different home environments cannot be ruled out.
Obesity. Three meta-analyses of good and moderate methodological quality reported an association of breastfeeding and a reduction in the risk of obesity in adolescence and adult life compared with those who were not breastfed. One study reported the reduction in the risk of overweight/obesity in breastfeeders compared with non-breastfeeders was 24 percent (95% CI 14% to 33%); another study reported 7 percent (95% CI 1% to 12%). Both of these estimates took into account the role of potential confounders. Furthermore, they also showed that the magnitude of association decreased when more confounders were entered into the analyses. The third study used meta-regression and found a 4 percent reduction in the risk of being overweight in adult life for each additional month of breastfeeding in infancy. Overall, there is an association between a history of breastfeeding and a reduction in the risk of being overweight or obese in adolescence and adult life. One should be cautious in interpreting all these associations because of the possibility of residual confounding.
Risk of Cardiovascular Diseases. Results from two moderate quality meta-analyses concluded that there was a small reduction of less than 1.5 mm Hg in systolic blood pressures and no more than 0.5 mm Hg in diastolic blood pressures among adults who were breastfed in their infancy compared with those who were formula-fed. The association weakened after stratification by study size, suggesting the possibility of bias in the smaller studies.
One meta-analysis of cohort and case-control studies reported that there was a reduction in total and LDL cholesterol levels by 7.0 mg/dL and 7.7 mg/dL, respectively, in adults who were breastfed during infancy compared with those who were not. However, these findings were based on data from adults with a wide age range. The analysis did not segregate the data according to gender and potential confounders were not explicitly analyzed. Detailed information (e.g., fasting or non-fasting) on the collection of specimen for cholesterol testing was not included. Because of these deficiencies, the correct characterization of a relationship between breastfeeding and adult cholesterol levels cannot be determined at this time.
One meta-analysis found little or no difference in all-cause and cardiovascular mortality between adults who were breastfed during infancy and those who were not. There were possible biases and limitations in the studies reviewed, however. Presence of statistical heterogeneity across studies suggests that it may not have been appropriate to combine estimates from individual studies into one summary estimate. Because of these reasons, no definitive conclusion could be drawn regarding the relationship between a history of breastfeeding and cardiovascular mortality.
In summary, the relationship between breastfeeding in infancy and the risk of cardiovascular diseases cannot be confidently characterized at this time and will need further investigation.
Type 1 Diabetes. Two moderate quality meta-analyses suggest that breastfeeding for at least 3 months reduced the risk of childhood type 1 diabetes compared with breastfeeding for less than 3 months. One reported a 19 percent (95% CI 11% to 26%) reduction; the other reported a 27 percent (95% CI 18% to 35%) reduction. In addition, findings from five of six studies published since the meta-analyses reported similar results. However, these results must be interpreted with caution because of the possibility of recall biases and suboptimal adjustments for potential confounders in the studies.
Type 2 Diabetes. In one well-performed meta-analysis of seven studies of various designs, breastfeeding in infancy was associated with a 39 percent (95% CI 15% to 56%) reduced risk of type 2 diabetes in later life compared with those who were not. However, only three of seven studies adjusted for all the important confounders such as birth weight, parental diabetes, socioeconomic status, and individual or maternal body size. Though the crude and adjusted estimates did not differ in these three studies, the lack of adjustments for potential confounders such as birth weight and maternal factors by all studies could exaggerate the magnitude of an association.
Childhood Leukemia. The published studies on childhood acute lymphocytic leukemia (ALL) were equivocal; a good quality meta-analysis reported a moderate protective effect from breastfeeding and the other good quality systematic review reached the opposite conclusion. We conducted a meta-analysis including only good and fair quality case-control studies identified in the systematic review, since the meta-analysis did not provide methodological quality grading of primary studies. We found breastfeeding of at least 6 months duration was associated with a 19 percent (95% CI 9% to 29%) reduction in the risk of childhood ALL. The previous meta-analysis also reported an association between breastfeeding of at least 6 months duration and a 15 percent reduction (95% CI 2% to 27%) in the risk of acute myelogenous leukemia (AML). Overall there is an association between a history of breastfeeding for at least 6 months duration and a reduction in the risk of both leukemias (ALL and AML).
Infant Mortality. One study of moderate methodological quality evaluated the relationship between breastfeeding and infant mortality. The study reported a protective effect of breastfeeding in reducing infant mortality after controlling for some of the potential confounders. However, in subgroup analyses of the study, the only statistically significant association reported was between ?never breastfed? and Sudden Infant Death Syndrome (SIDS) or the risk of injury-related deaths. Because of the limited data in this area, the relationship between breastfeeding and infant mortality in developed countries remains unclear. Further investigation is needed.
Sudden Infant Death Syndrome (SIDS). We conducted a meta-analysis by including only studies that reported clear definitions of exposure, outcomes, and results adjusted for well-known confounders or risk factors for SIDS. Our meta-analysis of seven case-control studies found that a history of breastfeeding was associated with a 36 percent (95% CI 19% to 49%) reduction in the risk of SIDS compared to those without a history of breastfeeding.
Preterm Infant Outcomes
Cognitive Development. No definitive conclusion can be made regarding the relationship between breast milk feeding and cognitive development in preterm infants. One meta-analysis reported a five points advantage in standardized mean score and one systematic review identified one primary study that reported an eight points advantage in IQ in preterm or low birth weight infants who received breast milk feeding. In three of four primary studies of moderate quality that controlled for either maternal education or maternal intelligence, the advantage from breastfeeding was reduced to a statistically non-significant level after adjustment. The roles of maternal intelligence and home environment should be accounted for in future studies on breastfeeding and cognitive development. Keeping in mind that cognitive function measured at an early age is not necessarily predictive of later cognitive ability, one should also consider carefully the timing and the selection of appropriate testing instrument in future studies.
Necrotizing Enterocolitis (NEC). Our meta-analysis of four randomized controlled trials of breast milk versus formula in comparing the outcome of NEC demonstrated that there was a marginally statistically significant association between a history of breast milk feeding and a reduction in the risk of NEC (P = 0.04). The estimate of the reduction in relative risk ranged from 4 percent to 82 percent. The absolute risk difference between the two groups was 5 percent. Because of the high case-fatality rate of NEC, this difference is a meaningful clinical outcome. The wide range of the estimate reflects the relatively small number of total subjects in the studies and the small number of events. One must also be cognizant of the heterogeneity underlying these trials in interpreting the findings of the meta-analysis. Examples of which included gestational age that ranged from 23 to more than 33 weeks; birth weight ranged from less than 1,000 g to more than 1,600 g; and some trials included only ?healthy? infants, while others included both ?healthy? and ?ill? infants.
Maternal Outcomes
Return to Pre-pregnancy Weight. Three moderate quality prospective cohort studies reported less than 1 kg weight change from pre-pregnancy or first trimester to 1 to 2 year postpartum period in mothers who breastfed. Results from four moderate quality prospective cohort studies showed that the effects of breastfeeding on postpartum weight loss were unclear. Results from all seven studies consistently showed that many factors other than breastfeeding had larger effects on weight retention or postpartum weight loss. Methodological challenges in these studies included the accurate measurement of weight change, adequate control for numerous covariables including the amount of pregnancy weight gain, and quantifying accurately the exclusivity and the duration of breastfeeding.
Maternal Type 2 Diabetes. Two large cohorts from a high quality longitudinal study of 150,000 parous women in the United States examined the relationship between breastfeeding and the risk of maternal type 2 diabetes. In parous women without a history of gestational diabetes, each additional year of breastfeeding was associated with a 4 percent (95% CI 1% to 9%) reduced risk of developing type 2 diabetes in the first cohort and a 12 percent (95% CI 6% to 18%) reduced risk in the second cohort. In women with a history of gestational diabetes, breastfeeding had no significant effect on the already increased risk of diabetes. Because only nurses were included in the cohorts, generalization of findings to the rest of the population must be done with care.
Osteoporosis. There is little or no evidence from six moderate quality case-control studies for an association between lifetime breastfeeding duration and the risk of fractures due to osteoporosis. In two of three moderate or good quality prospective cohort studies using bone mineral density as a surrogate for osteoporosis, lactation does not appear to have an effect on long-term changes in bone mineral densities. The third study found a small decrease in the bone mineral contents in the distal radius with increased duration of breastfeeding, but no significant changes in bone mineral contents in the femoral neck or the trochanter.
Postpartum Depression. Four prospective cohort studies of moderate methodological quality reported on the relationship between a history of breastfeeding and postpartum depression. None of the studies explicitly screened for depression at baseline before the initiation of breastfeeding and none of them provided detailed data on breastfeeding. Three of the four studies found an association between a history of short duration of breastfeeding or not breastfeeding with postpartum depression. The results were adjusted for socio-demographic and obstetric variables. More investigation will be needed to determine the nature of this association. It is plausible that postpartum depression led to early cessation of breastfeeding, as opposed to breastfeeding altering the risk of depression. Both effects might occur concurrently.
Breast Cancer. Two meta-analyses of moderate methodological quality concluded that there was a reduction of breast cancer risk in women who breastfed their infants. The reduction in breast cancer risk was 4.3 percent for each year of breastfeeding in one meta-analysis and 28 percent for 12 or more months of breastfeeding in the other. In addition, one of the two meta-analyses and another systematic review reported decreased risk of breast cancer primarily in premenopausal women. Findings from primary studies published after the meta-analyses concurred with the findings from the earlier meta-analyses. In summary, consistent evidence from these studies suggests that there is an association between breastfeeding and a reduced risk of breast cancer.
Ovarian Cancer. We reviewed 15 case-control studies that examined the relationship between breastfeeding and the risk of ovarian cancer, and performed meta-analyses in nine studies that adjusted for potential confounders. The overall result from the nine studies showed an association between breastfeeding and a 21 percent (95% CI 9% to 32%) reduction in the risk of ovarian cancer, compared to never breastfeeding. Because not all the studies reported similar comparisons of breastfeeding durations, we had to estimate the comparable risks in five studies. Excluding these five studies from the meta-analysis results in loss of statistical significance for this association.
There was indirect evidence for a dose-response relationship between breastfeeding and a reduced risk of ovarian cancer. Breastfeeding of more than 12 months (cumulative duration) was associated with a reduced risk of ovarian cancer, compared to never breastfeeding. The 12-month cutoff was arbitrary, and the odds ratios were estimated in half of these studies.
Overall, there is evidence to suggest an association between breastfeeding and a reduction in the risk of maternal ovarian cancer. Because of the aforementioned limitations, one must be cautious in interpreting this association.