No evidence for sex biases in milk macronutrients, energy, or breastfeeding frequency in a sample of filipino mothers

Abstract

Maternal reproductive investment includes both the energetic costs of gestation and lactation. For most humans, the metabolic costs of lactation will exceed those of gestation. Mothers must balance reproductive investment in any single offspring against future reproductive potential. Among mammals broadly, mothers may differentially invest in offspring based on sex and maternal condition provided such differences investment influence future offspring reproductive success. For humans, there has been considerable debate if there are physiological differences in maternal investment by offspring sex. Two recent studies have suggested that milk composition differs by infant sex, with male infants receiving milk containing higher fat and energy; prior human studies have not reported sex-based differences in milk composition. This study investigates offspring sex-based differences in milk macronutrients, milk energy, and nursing frequency (per 24 h) in a sample of 103 Filipino mothers nursing infants less than 18 months of age. We found no differences in milk composition by infant sex. There were no significant differences in milk composition of mothers nursing first-born versus later-born sons or daughters or between high- and low-income mothers nursing daughters or sons. Nursing frequency also showed no significant differences by offspring sex, sex by birth order, or sex by maternal economic status. In the Cebu sample, there is no support for sex-based differences in reproductive investment during lactation as indexed by milk composition or nursing frequency. Further investigation in other populations is necessary to evaluate the potential for sex-based differences in milk composition among humans.

Sex of preceding sibling and anthropometrics of subsequent offspring at birth and in young adulthood: A population‐based study in Sweden

“…In addition, Fujita et al () observed that economically sufficient mothers in rural Kenya produced richer milk for sons than daughters while poor mothers produced richer milk for daughters than sons. Although these studies are consistent with the Trivers–Willard hypothesis, which predicts the unequal parental investment between males and females depending on maternal condition and offspring reproductive potential (Trivers and Willard, ), a recent study based on milk composition found no support for this hypothesis (Quinn, ). It must be noted that it is difficult to illustrate that energy savings a mother would gain by constraining the growth in 18 g (or less) would be sufficiently beneficial for this “strategy” to become genetically encoded and physiologically expressed as an adaptation, through evolutionary time.…”

Calorie intake is associated with weight gain during transition phase of nutrition in female extremely low birth weight infants

“…In summary, natural biological processes in animals provide milk with distinctively different compositions to suit the nutritional needs based on the sex of the offspring. On the other hand, the available evidence of sex-specific differences in human milk composition are not well established [16][17][18][19][20][21].…”

Holsteins Favor Heifers, Not Bulls: Biased Milk Production Programmed during Pregnancy as a Function of Fetal Sex

“…Suckling behavior, however, is not useful for estimating milk energy transfer as verified by experimental use of radio-labeled isotopes in Equus caballus [22]. Direct evidence for sex-biased milk synthesis among nondomesticated species has now been reported in ungulates (Cervus elaphus hispanicus, [23]), rodents (Myodes glareolus [24]), primates (Macaca mulatta [25][26]; Homo sapiens [27][28][29], but see also [30] for exception), and marsupials (Macropus eugenii, [31]). Drawing systematic conclusions from the studies to date, however, is challenging in part because most have been limited by relatively small sample sizes or report milk composition without accounting for milk yield.…”