Infant sex predicts breast milk energy content

Abstract

During human evolutionary history, and for many around the world, breast milk is the primary source of nutritional energy for infants. Variation in breast milk quality might logically have important effects on infant health, growth, and development, yet the sources of this variation remain largely unelucidated. We quantified nutrient and energy content of breast milk from 25 healthy, well-nourished Massachusetts mothers with infants aged 2-5 months. We examined several potential sources of variation in milk quality, particularly feeding patterns, infant sex, and maternal breast growth during pregnancy. After controlling for time since last feeding, a known correlate of milk composition, we found that mothers of male infants produced milk that had 25% greater energy content than mothers of female infants (P < 0.001). Change in maternal bra cup size during pregnancy was associated with 16.17 kcal/100 ml greater energy content of milk (P = 0.009), but was not significant after taking infant sex into account. Greater nutritional investment in sons may account for the greater observed growth rates in male compared to female infants.

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

“…Although bST was able to overwhelm sex-biased milk synthesis among multiparous cows, significant sex-bias remained among primiparous cows whose mammary glands had functionally developed for the first time in the context of the fetal sex of the first gestation. The magnitude of sex bias is strongest among first parity rhesus monkeys [25][26] and possibly humans [28][29] and Tamar wallabies [31] in which primiparous females have been disproportionately represented in published studies. The effect of fetal sex may diminish to some extent among multiparous females due to the aggregate effects on mammary gland architecture of sequential gestations of different fetal sexes.…”

“…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.…”

High levels of DDT in breast milk: Intake, risk, lactation duration, and involvement of gender

“…The finding by Powe et al (2010) that male infants receive breast milk with a 25% greater energy content than female infants, prompted us to investigate the possible involvement of gender with ΣDDT levels. Since milk fat makes up a significant proportion of the energy component of breast milk (Nommsen et al, 1991), and organochlorine pesticides associate with the triglycerides in the fat globules of the milk (Hugunin and Bradley, 1971), we hypothesised that DDT levels in milk of mothers breastfeeding baby boys might also be greater.…”

“…The underlying mechanisms of how infant gender would affect energy content of breast milk (Powe et al, 2010) needs further investigation of the possibility that the gender of the breastfeeding infant may affect the content of DDT or any other pollutants in breast milk for that matter. If so, infant gender should be included in future studies as a classification variable as it may affect risk assessment.…”

Development: Mother's milk: A rich opportunity

“…Several recent studies support this view by identifying clear differences in the breast milk consumed by males and females. In humans, for example, baby boys receive milk that has substantially more fat and protein than the milk girls get 2 metabolism, even though their mothers have no more cortisol circulating in their blood than when nursing a daughter. It is unclear whether this cortisol-related sex difference has a function.…”