Greater Diurnal Variation in Human Milk Cortisol Is Associated with Larger Infant Height-for-Age: A Take on the Maternal Capital Evolutionary Framework

Maternal capital (e.g., anthropometrics, hormones, bioactives) enables investment in infant somatic growth during lactation. We draw from the maternal capital hypothesis to contextualize the associations between human milk cortisol variation and infant height-for-age z-score (HAZ) and weight-for-height z-score (WHZ). We hypothesize stable conditions (e.g., low perceived stress, high socioeconomic status/SES including income and education attained) will be associated with greater milk cortisol declines (i.e., steeper slopes), signaling increased capital to invest in larger infant anthropometrics, while stressful conditions (e.g., illness within the dyad, low SES, poor maternal sleep quality) may be associated with flatter slopes and subsequent reduced investment. We measured milk cortisol (N = 110) in U.S. mother–infant dyads using enzyme immunoassay. We collected milk 30–45 min after waking and at the last evening infant feeding, and provided questionnaires (e.g., demographics, perceived stress, Pittsburgh sleep quality index) for mothers to complete. We calculated milk cortisol slope (%Δ) (AM-PM/AM) to assess variation between collections. Larger %Δ indicated steeper slopes; smaller %Δ indicated flatter slopes. We employed an information-theoretic approach to estimate associations between maternal stress and %Δ, and %Δ and infant anthropometrics using multivariable linear regressions. The %Δ best-fit model suggested maternal postgraduate education and poor sleep quality was associated with steep %Δ. The HAZ best-fit model suggested the combined effect of steeper %Δ and milk intake was associated with increased HAZ. Additionally, the combined effect of steeper %Δ and infant milk intake was associated with increased HAZ among mothers with postgraduate degrees and who woke up three or more times to night-feed. %Δ was not associated with WHZ. We speculate %Δ exposure calibrates infant development to their shared environment, with implications for lifelong fitness.