The objective of this study was to determine whether maternal SCM status was associated with infant milk intake. Our results showed that infants whose mothers were diagnosed with SCM using the two diagnostic criteria (CMT score ≥1 and Na/K ratio >1.0) consumed significantly less milk (p
0.034). The observed association, however, disappeared when the model accounted for infant weight and breastfeeding frequency. Part of the effect of SCM on intake may be captured by these two factors. Both low feeding frequency and poor infant nutritional status could contribute to suboptimal breastfeeding practice, thereby increasing the risk of SCM. In addition, the small number of SCM cases (n
11) limited the power of the regression analysis to detect an effect.
To our knowledge, this is the first study to investigate the relationship between SCM and infant intake of breastmilk beyond early lactation. Manganaro et al.29
have recently reported an inverse relationship between breastmilk sodium and infant milk intake during the first week postpartum. Their results were consistent with findings in dairy cattle in which SCM is known to reduce milk output and permanently impair lactational performance.17,30
The outcome of the current study suggests that for children beyond early infancy (children older than 3 months), maternal SCM may not influence breastmilk intake.
SCM typically occurs unilaterally, and therefore it is possible for milk output from a healthy breast to compensate for the adverse effect of SCM on an affected breast.9
As reported by Connor,19
infants may be capable of differentiating between normal breastmilk and that with elevated sodium and thus exhibit a preference for the latter. An ideal design for a study of the effect of SCM on lactational performance, therefore, would be to measure milk secretion from one breast independently of the other. The test weighing methodology that was used in the present research would impose considerable interference on the “on-demand” feeding relationship between mother and infant because it would require a new weighing each time the child changed breasts during a feeding session.
An elevated Na/K ratio was more prevalent among younger and primiparous mothers in this study. A study in Zambia has also reported that primiparous mothers had significantly higher Na/K ratios from week 1 through 16 postpartum.11
These findings demonstrate a need to focus interventions on supporting young and first-time mothers to maintain optimal breast health during lactation.
In this study, we measured infant milk intake using test weighing as a proxy for milk output because it is a simple procedure and its effect on the maternal–infant feeding relationship is only minimal.25
Also, in this study insensitive water loss was not estimated. Arthur et al.25
have demonstrated significant underestimation of breastmilk intake using an infant test weighing without consideration for insensitive water loss. The comparison of breastmilk intake between groups in this study is, however, not affected by not controlling for insensitive water loss.
The CMT was used in this study as a screening test during recruitment because it is a simple and inexpensive diagnostic procedure that gives immediate results.27
Dorosko et al.28
had reported that CMT could serve as a screening tool for SCM based on the high correlation with somatic cell counts. We have observed in our studies in this community that the CMT overestimates the prevalence of SCM.24
The Na/K ratio, which is commonly used to diagnose SCM,8,10
was therefore used to confirm CMT scores. Either CMT or Na/K ratio alone failed to demonstrate significant differences in breastmilk intake between infants based on their mothers' SCM status. The ability of the combined SCM diagnostic criteria to distinguish breastmilk intake differences may be related to the ability of the combined diagnoses to detect more severe mammary inflammation.
The Na/K ratio diagnosis is typically made using a threshold of 1.0, which is considered to be equivalent to a sodium concentration of 18 mmol/L.31
This level of milk sodium is observed in breastmilk during mammary tight junction opening as well as during weaning. However, it is not known whether the fluctuations in milk electrolytes observed during onset of lactation or weaning follow the same pattern as the acute changes occurring in the milk of non-weaning lactating women.20
Improvement in the sensitivity and specificity of diagnostic tests for SCM would be useful.
The answer to our research question “does mammary inflammation reduce infant breastmilk intake?” could have useful implications for clinical decision-making. Although the study outcome is limited by a small sample size, it appears that as far as breastmilk intake is concerned, SCM may not be an important predictor among children beyond 3 months old. We are currently preparing a manuscript for publication of a longitudinal study to test the relationship between SCM and infant growth that was carried out in the same district but with a different sample of women. In this study, however, breastmilk intake data were not measured. A longitudinal study design with adequate sample size will be needed to adequately establish the relationship between infant feeding and breastmilk composition.