Femke Hitzert won the NIDI Master Thesis Award in 2014 for her research on the association between maternal body composition and breast milk transfer. This guest post by Femke describes her findings.

Breast milk is the most important source of nutrients for infants during the first six months of their life, which is underlined by the recommendation of the WHO to breastfeed a child for at least six months before complementary feeding is added to the infant’s diet. The antibodies found in breast milk offer protection against infections and lower mortality due to diarrhea. The WHO estimates that if every child would be optimally breastfed, 800,000 deaths could be prevented per year among children under five years of age. Being breastfed is not only beneficial at the very beginning of life, but may also be a ‘protective factor throughout life’. It is found that having been breastfed reduces overweight in adulthood, and reduces the prevalence of cardiovascular diseases. The positive effects of breastfeeding are widely known and reported, and it is therefore worth focusing on the process of breastfeeding itself. The objective of the study was to examine the association between maternal body composition and breast milk transfer. With the nutrition transition becoming more dominant in lower and middle income countries, resulting in a shift from under- to overnutrition in the population, the second objective was to examine whether the association between body composition and breast milk transfer is the same for countries that are in different stages of this transition.

A mother and her baby

One of the main issues faced in breast feeding studies is the fact that dose-response relations are difficult to make. Unlike other food sources, the actual amount of milk that is consumed by the infant is not directly observable. This constraint has been overcome by the introduction of isotope tracer methods, a technique by which deuterium oxide (‘heavy water’) is given to the mother to determine the rates of water flux from mother to baby. With this technique measures of the mother’s body composition can be obtained as well, making it a highly useful technique for studying associations between maternal body composition and breast milk transfer. Earlier studies have been done on this topic, and a database was recently constructed where data collected with the ‘deuterium oxide dose to the mother’ method were pooled together. This gave the opportunity to study variations in breast milk transfer based on a much larger dataset than was done in previous research. On top of that, the pooled data from different countries allowed for the inclusion of a macro-level factor in the analysis, meaning that associations with breast milk transfer could now be studied across contexts!

The first part of my research focused on the biological processes behind breast milk transfer and studied the association between maternal body composition and breast milk output. It was found that fat mass, and not fat free mass was negatively associated with breast milk output, meaning that an increasing fat mass results in a lower breast milk output. This was in line with results from earlier studies and this can be explained by hormonal factors (certain hormones stored in fat mass delay the onset of lactation) or by psychosocial factors (being overweight may result in a lower confidence in successful breastfeeding). The latter explanation is underlined by the finding in the current study that feeding pattern acts as a mediator in the association between fat mass and breast milk output, suggesting that mothers with a higher fat mass are more likely to practice partial breastfeeding instead of exclusive breastfeeding. Smoking was found to be an important confounder in the association between fat mass and breast milk output: for smoking mothers, an increase in weight results in an even lower breast milk output than for non-smoking mothers. For the association between smoking and breast milk output, again hormone levels could be an explanatory factor. Smokers tend to have lower prolactin levels (a hormone necessary to start milk production) during pregnancy than non-smokers. Smoking is also found to be associated with earlier weaning in other studies, suggesting that smokers tend to have a certain lifestyle in which the practice of breastfeeding does not fit.

The nutrition transition theory, proposed by Popkin in 1994 was used in the current study as the overarching concept to study breast milk transfer on the macro level. The theory describes the changes in dietary and physical activity patterns of populations over time, resulting in a shift from undernutrition to overnutrition and a shift in nutrition related health issues. These changes in dietary and activity patterns are paralleled by demographic and socioeconomic changes in society. The concept of the nutrition transition is nowadays mostly used when referring to the rapid shift of low and middle income countries from the stage of receding famines to the stage of increasing obesity rates and nutrition related non-communicable diseases (i.e. from stage 3 to 4). My study examined whether the association between body composition and breast milk output was the same for countries that are in different stages of this transition. Five countries were included: Brazil as a country in transition, Senegal, Kenya and Bangladesh as countries that have not yet undergone the transition, and the United Kingdom as having undergone the transition and currently being in the fourth stage. Data from the third stage countries showed an association between fat mass and breast milk output that was significantly different from the results for Brazil and the United Kingdom; where the latter countries showed a negative association between fat mass and breast milk output, the association was positive for countries not yet in transition towards stage 4. In third stage countries a rise in fat mass results in a higher breast milk output, while the opposite is true for transition and fourth stage countries. It thus might be the case that living circumstances in third stage countries are leading to differences in the metabolic system, resulting in the deviant association between body composition and breast milk output for mothers residing in these countries. However, it is important to note that rural and urban areas within one country might be in different stages of the transition, and that potential diversity within a country is ignored in this study. Of course, more countries should be included in future studies to be able to verify the above results.

The results give valuable insights in the factors that play a role in breast milk transfer from mother to baby. A more close examination of the underlying processes of breast milk transfer on the biological and contextual level could be highly useful; overweight and obesity rates among women in their reproductive ages are increasing nowadays, and given the central role of breastfeeding in infant health, interventions are needed to address this challenge.

Femke Hitzert is now continuing her research into breast milk transfer at the Population Research Centre of the University of Groningen, the Netherlands.