As a mother, I am a passionate advocate of breastfeeding and I breastfed my four children. As a clinician, though, I need to be mindful not to counsel patients based on my personal preferences, but rather based on the scientific evidence. While breastfeeding has indisputable advantages, the medical advantages are quite small. Many current efforts to promote breastfeeding, while well meaning, overstate the benefits of breastfeeding and distorts the risks of not breastfeeding, particularly in regard to longterm benefits.

As Joan Wolf explains in an article entitled Is Breast Really Best? Risk and Total Motherhood in the National Breastfeeding Awareness Campaign:

… Medical journals are replete with contradictory conclusions about the impact of breast-feeding: for every study linking it to better health, another finds it to be irrelevant, weakly significant, or inextricably tied to other unmeasured or unmeasurable factors. While many of these investigations describe a correlation between breast-feeding and more desirable outcomes, the notion that breast-feeding itself contributes to better health is far less certain, and this is a crucial distinction that breast-feeding proponents have consistently elided. If current research is a weak justification for public health recommendations, it is all the more so for a risk-based message that generates and then profits from the anxieties of soon-to-be and new mothers…

Wolf describes the problems with many studies of breastfeeding, particularly those that focus on long term outcomes:

In breast-feeding studies, potential confounding makes it difficult to isolate the protective powers of breast milk itself or to rule out the possibility that something associated with breast-feeding is responsible for the benefits attributed to breast milk. As the number of years between breastfeeding and the measured health outcome grows, so too does the list of possibly influential factors, which means that the challenge is magnifiedwhen trying to evaluate long-term benefits of breastfeeding… Breast-feeding, in other words, cannot be distinguished from the decision to breast-feed, which, irrespective of socioeconomic status or education,could represent an orientation toward parenting that is itself likely to have a positive impact on children’s health. In instances such as this, in which the exposure (breast-feeding) and confounder (behavior) are likely to be very highly correlated, confounding is especially difficult to detect. When behavior associated with breast-feeding has the potential to explain much of the statistical advantage attributed to breast milk, the scientific claim that breast-feeding confers health benefits … needs to be reexamined.

But even studies that may be biased show limited, if any, long term benefits of breastfeeding. The World Health Organization published a comprehensive review of the evidence  in 2007, Evidence on the long-term effects of breastfeeding, by Horta et al. According to the authors:

…[T]here is some controversy on the long-term consequences of breastfeeding. Whereas some studies reported that breastfed subjects present a higher level of school achievement and performance in intelligence tests, as well as lower blood pressure, lower total cholesterol and a lower prevalence of overweight and obesity, others have failed to detect such associations.

Objectives: The primary objective of this series of systematic reviews was to assess the effects of breastfeeding on blood pressure, diabetes and related indicators, serum cholesterol, overweight and obesity, and intellectual performance.

The authors reviewed the existing scientific literature on 5 specific claims.

1. Does breastfeeding leader to lower blood pressure?

The authors reviewed two meta analyses and three studies:

According to Owen et al, the association between breastfeeding and lower blood pressure was mainly due to publication bias, and any effect of breastfeeding was modest and of limited clinical or public health relevance. In spite of not being able to exclude residual confounding and publication bias, Martin et al concluded that breastfeeding was negatively associated with blood pressure. They argued that even a small protective effect of breastfeeding would be important from a public health perspective… Three large studies were published since the last review, two of which found no association and one found a protective effect of breastfeeding.

Both meta-analyses may have been affected by publication bias… Lack of control for confounding is another methodological issue, as pointed out by Martin et al…

In summary, the present updated meta-analyses show that there are small but significant protective effects of breastfeeding on systolic and diastolic blood pressure. Publication bias is unlikely to explain this finding because a significant protective effect was observed even among the larger studies. However, residual confounding cannot be excluded because of the marked reduction in effect size after adjustment for known confounders.

2. Does breastfeeding lead to lower cholesterol levels?

[N]o significant effect was observed in children or adolescents, mean cholesterol levels among adults who were breastfed were 0.18 mmol/L (6.9 mg/dl) lower than among non-breastfed subjects… [T]he observed reduction associated with breastfeeding corresponds to about 3.2% of [the] median.

3. Does breastfeeding reduce the risk of overweight and obesity?

The evidence suggests that breastfeeding may have a small protective effect on the prevalence of obesity. In spite of the evidence of publication bias, a protective effect of breastfeeding was still observed among the larger studies (>1500 participants),.. This effect seems to be more important against obesity than against overweight.

Because the great majority of the published studies were conducted in Western Europe and North America, we are not able to assess whether this association is present in low and middle-income settings.

4. Does breastfeeding lower the risk of type 2 diabetes?

Evidence on a possible programming effect of breastfeeding on glucose metabolism is sparse. Studies assessing the risk of type-2 diabetes reported a protective effect of breastfeeding, with a pooled odds ratio of 0.63 (95% CI: 0.45–0.89) in breastfed compared to non-breastfed subjects. On the other hand, two other studies failed to report an association between HOMA index, a measure of insulin resistance, and breastfeeding duration, and a study on fasting blood glucose levels was also negative. At this stage, it is not possible to draw firm conclusions about the longterm effect of breastfeeding on the risk of type-2 diabetes and related outcomes…

5. Does breastfeeding raise the level of school achievement or intelligence?

This meta-analysis suggests that breastfeeding is associated with increased cognitive development in childhood, in studies that controlled for confounding by socioeconomic status and stimulation at home. The practical implications of a relatively small increase in the performance in developmental tests in childhood may be open to debate. However, evidence from the only three studies on school performance in late adolescence or young adulthood suggests that breastfeeding is also positively associated with educational attainment.

The issue remains of whether the association is related to the properties of breastmilk itself, or whether breastfeeding enhances the bonding between mother and child, and thus contributes to intellectual development. Although in observational studies it is not possible to disentangle these two effects, the positive results from the randomized trial carried out by Lucas et al suggest that the nutritional properties of breastmilk alone seem to have an effect.

In the case of these five longterm outcomes, the existing scientific evidence shows that breastfeeding has either no benefit or a small benefit.

Adriano Canttaneo, an pediatric epidemiologist and enthusiastic supporter of breastfeeding, writing in the Journal of Pediatrics and Child Health in 2008 in The benefits of breastfeeding or the harm of formula feeding? cautions against making sweeping and unsupported claims about breastfeeding:

… We do not need to use weak and shaky arguments to convince mammals to breastfeed. What we need is effective care to let them breastfeed as much and as long as they wish.

Breastfeeding is desirable and beneficial, and we should promote breastfeeding as much as possible. However, breastfeeding advocates should not overstate the benefits of breastfeeding or overstate the risks of formula feeding. Rather, we should do whatever we can to allow women who wish to breastfeed to start and maintain breastfeeding for as long as they would like.