Objectives: The present study identified multiple proteins in early mature milk and explored the correlation between protein compositions in HM and maternal BMI during lactation. Methods: A total of 70 mothers giving birth to single-term infants from four representative cites were enrolled in this research. Milk samples were collected between 9 and 11 a.m. to avoid the influence of circadian rhythms. The concentration of total protein in the milk samples was determined using the Bradford method, and the concentrations of α-lactalbumin, lactoferrin, osteopontin, α_s−1 casein, β-casein, and κ-casein, butyrophilin, periodic acid Schiff 6/7, fatty acid-binding protein, and xanthine oxidoreductase in the milk samples were measured through a previously published method using ultra-performance liquid chromatography coupled with mass spectrometry. A semi-structured questionnaire investigation and body measurements were carried out by trained investigators to collect the information of subjects. Results: In the univariate models, the concentrations of TP (r = 0.306), α-La (r = 0.260), LF (r = 0.371), OPN (r = 0.286), and α_S1-CN (r = 0.324) were all positively and significantly correlated with maternal BMI. In the models’ adjusted covariates, the concentrations of TP (Lg β = 7.4 × 10⁻³), LF (Lg β = 19.2 × 10⁻³), α_S1-CN (Lg β = 8.2 × 10⁻³) and the proportion of LF (β = 0.20%) were positively correlated with continuous maternal BMI changes. TP concentrations in the HM of obese mothers were higher than in the other three groups (Lg β: 66.7 × 10⁻³~140.5 × 10⁻³), α-La concentrations were higher than in the underweight and normal groups (Lg β: 94.4 × 10⁻³~145.7 × 10⁻³), and OPN concentrations were higher than in the overweight groups (Lg β = 103.6 × 10⁻³). The concentrations of LF (Lg β: −298.2 × 10⁻³~−191.0 × 10⁻³), OPN (Lg β: −248.9 × 10⁻³~−145.3 × 10⁻³), and α_S1-CN (Lg β: −160.7 × 10⁻³~−108.3 × 10⁻³) in the HM of underweight mothers were lower than those in the other three groups. β-CN concentrations were lower than normal (Lg β = −125.1 × 10⁻³) and obese groups (Lg β = −165.7 × 10⁻³), κ-CN concentrations were lower than the overweight (Lg β = −132.5 × 10⁻³) and obese groups (Lg β = −147.9 × 10⁻³), and the proportion of LF was lower than that of the overweight (β = −2.80%) and obese groups (β = −2.52%). The proportion of LF in normal mothers was lower than that in the overweight group (β = −1.15%). No statistically significant associations between four MFGM proteins and maternal BMI were determined as the equation models could not be fitted (p for F-test < 0.05). Conclusions: Obese mothers had higher concentrations of multiple protein components than other groups, while underweight mothers had lower concentrations. The association between BMI and protein compositions may be more pronounced for certain protein types.

Objectives: The present study evaluated anthropometry Z-scores of exclusively breastfed infants aged 0~6 months and examined their associations with various parent–infant factors. Methods: This cross-sectional study included 383 mother–infant dyads from 10 Chinese cities in the final analyses, under strict inclusion and exclusion criteria. Data were collected by trained investigators using questionnaires covering demographic characteristics, perinatal health, maternal and infant factors during lactation. Nutrient intake was assessed and calculated by 24 h recall. Anthropometric measurements of parents and infants were taken using calibrated instruments, with infant growth assessed via Chinese growth standards. Statistical analyses included correlation and linear mixed-effect models accounting for regional clustering, with variable selection guided by backward elimination step regression. Nonlinear relationships were explored using spline and piecewise regression methods. Results: Over 60% of the mothers had inadequate energy and protein intake. Approximately two-thirds of the participants had fat intakes exceeding the upper limit. Inadequate or excessive gestational weight gain, poor maternal sleep quality, lactational mastitis, higher maternal fat intake and infant gastrointestinal symptoms were associated with lower infant anthropometry Z-scores. A threshold effect was detected between maternal fat intake and infant WAZ, BMI Z, and WLZ. Conclusions: This study found that anthropometry Z-scores of exclusively breastfed infants aged 0–6 months were significantly associated with certain maternal–infant factors and maternal fat intake, emphasizing the need for early intervention on adverse factors and balanced maternal diet nutrition during lactation.