Breastmilk Changes During Prolonged Lactation
by Anne Eglash MD, IBCLC, FABM
We know that breastmilk changes are dynamic, with several factors playing a role in composition. Most obviously, human milk starts as colostrum, which is high in protein, immune factors, and beta carotene, while low in water, fat, and overall volume. The milk transitions from colostrum to mature milk over the first 3 weeks of lactation, gradually increasing in fat, carbohydrates, and volume. Breastmilk from a mother with a premature infant is higher in protein during the first month as compared to breastmilk for a term infant. Breastmilk also changes during the day, with higher volumes after mom has gone to sleep at night. During a breastfeeding, milk towards the end of the feeding is higher in fat as compared to the milk ingested at the beginning of the feeding.
Given that we have little information on human milk composition beyond 1-2 years postpartum, a group of researchers sought to analyze total fat, protein, and carbohydrates in breastmilk from mothers nursing for more than 1 year. They collected daytime samples of milk from mothers in the following groups: 1) Under 12 months postpartum, 2) 12-18 months postpartum, 3) 18-24 months postpartum, and 4) above 24 months postpartum. Each group included a range of 26-41 mothers.
- The concentration of carbohydrates, from lactose, decreased significantly after 18 months postpartum.
- Mothers with the highest fat and protein in the milk were past 2 years postpartum.
- Breastmilk from mothers who were over 2 years postpartum had approximately 60% more calories per ounce as compared to breastmilk from mothers who were under 12 months postpartum.
- Women who were nursing their second child had higher protein levels in their breastmilk after 1 year postpartum as compared to women who were nursing their first child.
- If a 2 year old toddler increases his nursing frequency, the protein and fat decrease in his mother’s milk, while the carbohydrates increase.
See the Answer
Human milk (HM) is the first dietary exposure in infancy and the best nutritional option for growth and healthy development of the newborn and infant. The concentrations of macronutrients, namely proteins, carbohydrates and lipids, change during lactation stages. HM has been studied extensively in the first 6 months of lactation, but there are limited data regarding milk composition beyond 1 or 2 years postpartum. The aim of our study was to describe longitudinal changes in HM macronutrient concentrations during the prolonged lactation of healthy mothers from the 1st to the 48th month. For the macronutrient content of milk of mothers breastfeeding for longer than 18 months, fat and protein increased and carbohydrates decreased significantly, compared with milk expressed by women breastfeeding up to 12 months. Moreover, the concentration of fat, protein and carbohydrates in HM over 2 years of lactation from the 24th to the 48th month remained at a stable level. However, analyzed macronutrients were positively (carbohydrate, r = 0.51) or negatively (fat, r = ?0.36 and protein, r = ?0.58) correlated with the amount of feeding. Our results create a greater potential for understanding the nutritional contribution of HM over 2 years of lactation and showed that the source of calories in breast milk for older children is mainly fat, while carbohydrates play a greater role in infant nutrition in the early stage. The observed changes of macronutrient concentrations from the 1st to the 48th month of lactation are probably related to the adaptation of milk composition to the increased energy demand of the intensively growing child.
This study is fascinating. We could create many theories as to why breastmilk calories rise over time. Since children over age 1 consume water separately, there is less need for mothers to have an abundance of free water in her breasts. Less free water in breastmilk leads to higher calorie, more concentrated breastmilk. This is because carbohydrates such as lactose draw more water into the breast. If a breastfeeding mother has less volume of breastmilk, she may be at less risk for mastitis, engorgement, and loss of milk supply, allowing her to nurse her toddler/preschooler longer.
Also, toddlers tend to be picky intermittent eaters, as compared to infants, and nurse for short times. Having a source of highly concentrated calories that can be consumed in 1-2 minutes flat, which usually describes a toddler feeding session, makes sense.
One question is whether consumption of higher fat, higher calorie breastmilk beyond 2 years of age has any influence on the child’s cholesterol and risk of heart disease over time.