Duration of exclusive breastfeeding and risk of anaemia
There's been a lot of stuff in the news today about a paper that's just been published in the BMJ by Mary Fewtrell and colleagues, which questions the current recommendation that infants should be exclusively breastfed for 6 months. There are many issues here, and I don't have time to look at all of them, but one thing that I found interesting is that the paper raises the possibility that exclusive breastfeeding for 6 months may increase the risk of iron-deficiency anaemia in the infant, compared with exclusive breastfeeding for only 4 months.
So what does the evidence show?
The short answer is: not much. A systematic review, last updated in 2006 (bizarrely described as a 2002 paper in Fewtrell et al's paper) found conflicting evidence. A randomised trial from Honduras found some evidence that hinted in the direction that breastfeeding for 6 months might increase the risk of anaemia, but the evidence was weak. There was a statistically significant, but small reduction in haemoglobin levels, but no significant increase in the risk of haemoglobin levels below 110 g/L in infants exclusively breastfed for 6 months, compared with those weaned at 4 months.
In contrast, however, an Italian study found that exclusively breastfed infants had higher haemoglobin concentrations. So the evidence from that review that exclusive breastfeeding increases the risk of anaemia is very weak, particularly in developed countries. It is quite possible that the different nutritional statuses in developed and developing countries mean that trials in one setting are not generalisable to the other.
Anyway, back to Fewtrell et al's paper. They discuss the risk of iron deficiency anaemia further, and and point to a study published since the systematic review (in 2007) by Chantry and colleagues. Fewtrell et al's paper is not a systematic review, and so it's possible that there are other relevant papers that have been published in recent years. I don't know whether they have been or not. I couldn't find any in a 5 minute literature search, but it's quite possible that a more careful search would have revealed something.
Chantry et al's study is described by Fewtrell et al as showing that "US infants exclusively breast fed for six months, versus four to five months, were more likely to develop anaemia and low serum ferritin". They also point out that iron deficiency has "irreversible long term adverse effects on motor, mental, and social development".
Let's take the second point first. The evidence cited for the risk of adverse consequences resulting from iron deficiency was not specific to breastfed infants. To claim that 6 months exclusive breastfeeding causes iron deficiency and iron deficiency causes irreversible developmental problems and therefore that 6 months exclusive breastfeeding causes irreversible developmental problems is a rather weak argument. It would be much more convincing if it were shown that 6 months exclusive breastfeeding causes irreversible developmental problems, but that has not been shown. It is quite possible that infants with iron deficiency as a result of delayed weaning could be very different from infants with iron deficiency due to other causes.
But anyway, how sure are we that 6 months exclusive breastfeeding really does increase the risk of iron deficiency?
Well, after reading Chantry et al's paper, I have to say the answer is "not sure at all". Chantry et al looked at 2 separate cohorts of children, and measured iron deficiency by 3 different measures: low serum ferritin, low haemoglobin, and a history of anaemia. A major weakness of the study is that history of anaemia was based mainly on reports by the parents, with no medical verification. I'm not at all sure how reliable such self-reporting is.
If I were to be convinced that 6 months exclusive breastfeeding increases the risk of iron deficiency, then I would like to see some sort of consistency in the results. However, the results were highly inconsistent. The risk of low haemoglobin was not significantly increased in either cohort after 6 months exclusive breastfeeding. The risk of low serum ferritin was increased in one cohort, and the risk of a history of anaemia was increased in the other cohort. Only 2 significant results out of a possible 6 results does not strike me as convincing. It's also worth noting that although the risk of low haemoglobin was not significantly different between those exclusively breastfed for 6 months and those exclusively breastfed for 4-5 months, the risk of low haemoglobin was significantly higher in those exclusively breastfed for less than 4 months in one cohort.
To cite that paper as evidence that exclusive breastfeeding for 6 months increases the risk of iron deficiency anaemia therefore strikes me as misleading. Now of course it's possible that there may be an increased risk of anaemia, as the evidence is inconclusive, and although we don't have good evidence that there is an increased risk, we also don't have enough evidence to say with certainty that there is no increased risk.
But, of course, there is much good evidence that 6 months exclusive breastfeeding is beneficial on other outcomes. It's probably still wise to recommend breastfeeding for 6 months given that we know there are benefits, and the possibility of harm is at this stage unproven. As Chantry et al concluded in their paper "Full or exclusive breastfeeding for 6 months provides greater protection against both respiratory and gastrointestinal infections than does shorter durations, and should continue to be recommended."
Not surprisingly, the paper has been poorly presented in the media. And if you are a regular reader of this blog, you will know by now that it's important to read the research paper before making up your mind about any health stories reported in the media. But there is a further lesson here. Even the journal article may not give the full story. I found out how weak the evidence was here by going back a stage further and looking at the papers cited in the first paper. Just because something is cited in a journal article as being evidence of something doesn't mean that it's good evidence.