Maternal smoking and reduced duration of breastfeeding: a review of possible mechanisms

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Abstract

Aim: To systematically review the possible mechanisms by which maternal smoking affects lactation.

Methods: Databases (Medline, CINAHL, Current Contents, Psychinfo, Sociological Abstracts and the Cochrane Library) were searched for smoking and breastfeeding or infant feeding. The Journal of Human Lactation and Birth were hand-searched, searches were also conducted at NMAA's Lactation Resource Centre and references cited were located where appropriate. Articles were divided into the various ways that smoking could affect breastfeeding and were tabulated.

Results: Most studies were conducted on small samples of animals or humans; the majority were prior to 1985. Most animal studies exposed the animals to much greater levels of nicotine than those to which humans are exposed. Most studies did not examine if breastfeeding behaviour was similar in smokers and non-smokers, and did not consider that any physiological difference found could be the result of poor lactation practices, rather than the cause of poor lactation in smokers. The definition of breastfeeding infants was also problematic in many studies. The effect of smoking on oxytocin in women was only examined in one study, and no effect was found, yet a negative effect of smoking on oxytocin release is reported in the breastfeeding literature.

Conclusion: Although there is consistent evidence that women who smoke breastfeed their infants for a shorter duration than non-smokers, the evidence for a physiological mechanism is not strong.

Introduction

It is well recognised that women who smoke are less likely to breastfeed than non-smokers [1], [2]. The mechanism for this may be physiological and/or social, but critical review in this area is lacking.

It has been generally accepted that nicotine in the mother's blood stream reduces the level of prolactin, the hormone closely associated with lactation [3], [4]. However, recent studies have thrown doubt over the importance of prolactin in the ongoing production of breast milk [5].

Another widely accepted belief is that smoking interferes with the release of oxytocin, the hormone responsible for the milk ejection reflex (or let-down) [6]. The evidence for this will be reviewed, and an alternative understanding of the mechanism of how smoking may affect lactation will be presented.

The aim of this paper is to systematically review the possible mechanisms by which cigarette smoking may affect lactation. Publications in the areas of biochemistry, physiology, pathology, psychology and clinical practice have all been included in order to expand the understanding of the relationship between maternal smoking, lactation and behaviour. Studies in animal models have been included because, firstly, their findings have been extrapolated to humans and, secondly, research in women is sparse. Directions for future research in the area of smoking and breastfeeding will be considered. A second paper has reviewed epidemiological studies to examine the effect of maternal smoking on initiation and duration of breastfeeding (Amir and Donath, in preparation).

Section snippets

Background

It is thought that the first report of an adverse effect of maternal smoking on breastfeeding was published by Mills in 1950 [7]. Although many studies have not accounted for the socio-demographic differences between women who smoke and non-smokers [2], large studies have found an association between maternal smoking and artificial feeding, which remains after adjusting for confounding factors [3], [8], [9], [10], [11].

Tobacco smoke contains at least 3800 components, of which nicotine is the

Method

The following data sources were used to identify articles or book chapters reporting maternal smoking and the effect on lactation:

  • The Cochrane Library

  • Medline, CINAHL, Current Contents, Psychinfo and Sociological Abstracts

  • Lactation Resource Centre, Nursing Mothers' Association of Australia

Key words were “smoking” and “breastfeeding” or “infant feeding”, and searches were conducted from the start of the database to Week 4 2001 (Medline, Current Contents), Nov. 00 (Psychinfo), Oct. 00 (CINAHL),

Hormonal effects of smoking on the mother

Table 1 is a collection of studies that have investigated the relationship between smoking and maternal hormone levels. The rat studies found that animals exposed to nicotine had lower levels of prolactin or blockage of prolactin release and impaired lactation compared to controls. Blake and Sawyer state that “Although the smoker's consumption of nicotine would certainly be less than our rat's intake on a per kilogram basis, species differences can make weight-basis comparisons meaningless” (p.

Physiological factors

Hormonal studies of animal models are included in Table 1 because these are widely cited as evidence for the reduced duration of breastfeeding in human smokers [4], [35], [36]. The studies of hormones and nicotine were conducted prior to 1986, and it appears that investigations have not continued in this area. The observations of rats and mice have been made on small samples and the conditions of the studies are contrived. The validity of extrapolating from animal studies to human populations

Conclusion

There are a number of possible explanations for an adverse effect of maternal smoking on breastfeeding. These include a physiological effect of nicotine on the mother's hormonal system or on her breasts directly, or the infant may be directly affected and have difficulty feeding or show irritable behaviour. The mechanism is generally thought to be lowering of prolactin by nicotine, however, the long-term levels of prolactin do not correlate with milk production. The possibility that nicotine

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