Human breast milk contains an array of biologically active compounds that can modulate the immune system of an infant (1). Studies of human breast milk cytokines and their biological activity were undertaken quite recently (2,3). Their results suggest that breast milk cytokines can modulate the immune response of breast-fed infants (4), and their concentration decreases throughout lactation (5,6).
Smoking can modulate both the cellular and humoral immune response through a variety of mechanisms. An increase in blood concentration of proinflammatory cytokines: interleukin (IL)-1, IL-6, IL-8, tumor necrosis factor (TNF)-α, and granulocyte-macrophage colony-stimulating factor (GM-CSF) was reported in smokers (7,8).
The composition of breast milk is modulated by a variety of factors, including tobacco smoke (9–13); however, little is known on the effect of smoking on the cytokine profile of human breast milk. Consequently, the aim of the study was to verify whether smoking during lactation influences breast milk cytokine levels.
The study included 24 healthy postpartum women older than 18 years, who planned to exclusively breast-feed their children for >1 month, and declared smoking >5 cigarettes per day during pregnancy and lactation (group I). Other inclusion criteria were as follows: normal spontaneous full-term vaginal delivery, uncomplicated gestation, and neonates in good general status with normal birth weight and length. The exclusion criteria included passive smoking, acute and chronic disorders, and pharmacotherapy other than vitamin supplementation. The control group (group II) comprised 45 postpartum women who met all of these aforementioned inclusion criteria with the exception of smoking during pregnancy and lactation; moreover, neither they nor their spouses had history of prepregnancy smoking (Table 1).
All the procedures were approved by the local ethics committee of the Medical University in Gdansk. The subjects gave their written informed consent before the start of any procedure.
Breast milk samples (10 mL) were collected between the 30th and the 32nd day after delivery, 2 hours after the first morning feeding, and immediately frozen at −80°C.
IL-1α, IL-1β, IL-6, IL-8, IL-10, and TNF-α concentrations were determined with the use of commercial enzyme-like immunosorbent system (R&D Systems Inc, Minneapolis, MN).
Concentrations of cytokines in groups I and II were compared with the Student t test or the Mann-Whitney test. The distribution of detectable cytokine concentrations was compared with the Pearson χ2 with Yates correction. Associations between continuous variables were tested with Spearman coefficient of correlation (R). Calculations were performed using Statistica 10 software (StatSoft, Tulsa, OK) (P ≤ 0.05).
Smoking and nonsmoking women did not differ significantly in terms of parity, gestational age, anthropometric characteristics of neonates, and their Apgar scores. The only significant difference between the groups pertained to the significantly lower mean age of smokers (27.29 ± 5.66 vs 30.04 ± 4.09; P = 0.02). Nonetheless, we did not observe significant associations between maternal age and breast milk concentrations of studied cytokines (IL-1α: R = −0.05, P = 0.65; IL-1β: R = −0.06, P = 0.56; IL-8: R = −0.09, P = 0.43; IL-10: R = −0.13, P = 0.27; IL-6: R = −0.09, P = 0,45; TNF-α: R = −0.02, P = 0.82).
Compared with nonsmoking women, smokers were characterized by significantly higher breast milk concentrations of IL-1α, whereas no significant intergroup differences were observed in terms of remaining analyzed cytokines (Table 2). Moreover, both groups were characterized by a similar fraction of women with detectable cytokine levels (not shown).
To the best of our knowledge, this is the first study on the effects of maternal smoking on the cytokine profile of mature breast milk. Previous research on this subject matter in question dealt with the cytokine content of colostrum and transitory milk (14,15). Our study revealed that, compared with nonsmokers, women who smoked during lactation were characterized by a significantly higher breast milk concentration of IL-1α. The results of previous studies on the relation between the exposure to tobacco smoke and the synthesis of IL-1 are inconclusive. Although some in vitro experiments revealed that the components of tobacco smoke can stimulate the IL-1 release by human mononuclear cells (16,17), in other in vitro studies the exposure to tobacco smoke was reflected by a decreased synthesis of IL-1β, IL-2, interferon-γ, and TNF-α by peripheral blood mononuclear cells (18).
Regarding in vivo experiments, Zanardo et al (15) claimed that colostrum concentration of IL-1α in smokers is significantly lower than that of nonsmokers, whereas no significant intergroup differences of this parameter can be observed in transitory milk. Conferring these results to opposite findings of our study, one can consider this discrepancy a result of different phases of lactation analyzed. Nevertheless, taken together, these results suggest a lactation phase–dependent growth tendency of breast milk IL-1α concentration in smokers and/or a relative decrease of this cytokine in nonsmokers.
In contrast to IL-1α, we did not observe significant intergroup differences in breast milk concentrations of other analyzed cytokines. One previous study revealed that a relative decrease in TNF-α concentration was observed on the 7th day of lactation in colostrum samples from smoking mothers (14). The same study did not document significant effects of maternal smoking on colostrum levels of such compounds as IL-1β and soluble vascular adhesion molecule-1 (14). According to other authors, the transforming growth factor-β concentration in mature breast milk is not influenced by maternal smoking (19).
Smoking women who participated in our study were significantly younger than nonsmokers. Other authors have similar observations with regard to the age of women who smoke in pregnancy and during lactation (20,21). Age is an established factor modulating blood cytokine concentrations, for example, a tendency to higher levels of IL-6 was reported from older individuals (22); however, significant age-related differences in cytokine concentrations in women were observed only on comparative analysis of pre- and postmenopausal subjects (23), as well as in elderly adults (24). Therefore, it is unlikely that maternal age was a confounder in our study, particularly taking into account the lack of significant correlation between the age and concentrations of analyzed cytokines; however, it should be remembered that the maternal concentration of cytokines can also be modulated by other psychosocial factors such as allergy, use of probiotics in pregnancy, drinking alcohol, and postpartum depression (19), none of these analyzed in our study.
Analysis of previous findings on the elevated serum levels of proinflammatory cytokines in smokers (7,8), and our observations on the lack of smoking effects on breast milk concentrations of those molecules (other than IL-1α), suggest that a potential biological mechanism can exist, downregulating the cytokine content of milk in smoking mothers and thus protecting their offspring against the enhanced inflammatory response. Nevertheless, the results of our study provided another argument against maternal smoking, not only in pregnancy but also during lactation.
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