Circadian Variation in Human Milk Composition, a Systematic Review

Study answer

What this study found

Strong evidence of circadian variation in tryptophan, fats, triacylglycerol, cholesterol, iron, melatonin, cortisol, and cortisone in human milk; carbohydrate and total protein show no consistent circadian variation. Some amino acids (e.g., tryptophan) may vary; fat-related lipids tend to peak in the evening; iron peaks in the evening/night; melatonin peaks at night; cortisol and cortisone peak in the morning. Evidence for many other components is inconclusive due to limited data and heterogeneity. These rhythms may provide circadian cues to the infant and support circadian development;…

Study & population

Observational studies; cross-sectional and longitudinal designs; 83 studies included; participants were lactating mothers; age range 13-53 years; mostly term deliveries; lactational stage ranged from colostrum to mature milk; sampling frequency ranged from 2 to 12 times per 24 h; observation duration 1-14 days.

Key limitation

High heterogeneity across studies; small and varied sample sizes; short observation windows (often one day); variable sampling frequencies; limited data for many components; potential confounders (maternal factors, lactation stage, mode of delivery) not consistently accounted for; limited generalizability.

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Original abstract