Masako Fujita

Grant Type

Post PhD Research Grant

Institutional Affiliation

Michigan State U.

Grant number

Gr. 9278

Approve Date

April 18, 2016

Project Title

Fujita, Dr. Masako, Michigan State U., East Lansing, MI - To aid research on 'The Impact of Maternal Nutrition and Infant Sex on Breast Milk Quality in Polygynous Ariaal Agro-pastoralists of Northern Kenya'

Preliminary abstract: The Trivers-Willard hypothesis (TWH) predicts unequal parental investment between daughters and sons in polygynous populations where somatic or economic conditions of males determine their marriageability. Specifically, it predicts that mothers in good condition will invest more in sons while mothers in poor condition will invest more in daughters because these strategies may enhance their reproductive success. A small but growing number of studies investigate sex bias in human milk quality, particularly milk nutrient levels with mixed results. This literature focuses on energy-yielding nutrients (e.g. fat, sugar) to test the hypothesis, and misses other nutritive and non-nutritive components of milk indispensable for infants to survive and thrive. The proposed study contributes by examining TWH, using breast milk vitamin (B9 or folate) and antibody (secretory immunoglobulin A or sIgA) as indicators of maternal investment. A cross-sectional random sample of breast milk specimens from 220 mothers of the Ariaal tribe, a polygynous population of rural Kenya, originally collected in 2006, cryogenically archived (-80°C), will be analyzed for the levels of folate-binding protein and sIgA. The resulting data will be used to statistically test the hypotheses: 1) maternal nutrition (e.g. adiposity, micronutrient status) will positively predict folate/sIgA, 2) this effect will be moderated by infant sex, and 3) there will be a TW effect, i.e. milk of under-nourished mothers will exhibit preference toward daughters and milk of other mothers toward sons. Findings will facilitate a multi-faceted view on possible sex bias in human milk components in a specific sociocultural and ecological context. This will contribute to the refined understanding of the conditional variation in human parental investment which has important implications for sex differences in infant growth and health in relation to different marriage rules.