Chrononutrition: 24-hour variation in macronutrients in very preterm human milk

Background: Human milk composition varies in time to meet the evolving growth and developmental requirements of infants. While 24-hour (diurnal) variations in human milk composition for term-born infants have been reported, evidence concerning diurnal variation of preterm human milk composition remains inconclusive. Research aim: To examine the presence of diurnal variations in macronutrient content in very preterm human milk. Method: Milk samples were donated by mothers of very preterm infants (<30 weeks gestation). Milk series (defined as sequential milk samples collected from one mother) consisted of 13 to 17 (median 15) samples, collected over three consecutive days. Milk macronutrient (fat, true protein and carbohydrates) content were measured using mid-infrared spectrophotometry (Miris HMA™). Rhythmicity analyses and logistic regression analyses were performed to explore associations between macronutrient rhythmicity and pregnancy duration (weeks), infants' sex, and lactational stage). Results: Of the 22 milk series studied, 12 (55%) showed 24-hour rhythmicity in one or more macronutrients. Fat content showed rhythmicity in 41% of the series, with varying peak times. For carbohydrates, protein and calculated energy content, rhythmicity was only identified in 18%, 14% and 27% of the milk series, respectively. Longer pregnancy duration was associated with increased odds of rhythmicity (odds ratio 2.1 (95% confidence interval 1.02–4.21), p = 0.04), but not with sex or stage of lactation. Conclusions: Over half of the preterm human milk samples exhibited diurnal macronutrient rhythmicity, mainly in fat levels. The observed rhythms had varying peak times, suggesting phase differences of the maternal rhythms.