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Foremilk-Hindmilk Imbalance: Revising our Strategies

A breastfeeding mother who produces milk beyond her infant’s needs often develops problems affecting both herself and her baby. The mother experiences discomfort due to overfull breasts and often develops blocked ducts and mastitis. The baby consumes too much foremilk which is more watery, lower in fat, and higher in carbohydrate than the creamier hindmilk obtained as the breast empties. The baby is usually gaining well and appears to be thriving, but is often irritable, cries a lot, shows on-off behaviour or refusal at breast, and appears to be experiencing abdominal cramping and pain.

Milk oversupply is most often seen in the week following the more frequent feeding which babies do to increase their mother’s milk production during a growth spurt. Growth spurts are commonly seen around ten days, three weeks, and six weeks.1 The growth spurts at three and six months do not usually seem to cause oversupply problems.

To manage an oversupply and decrease milk production to the right amount needed by the baby, it is necessary to understand how “foremilk” is created, how it is differentiated from “hindmilk”, and how milk supply is decreased. Babies do not obtain the same amount of foremilk from different sized breasts even if the different sized breasts belong to the same mother. The composition of foremilk is dependent on the volume of milk in the breast and the length of time since milk was last removed from that breast. Foremilk is never the same composition. Foremilk in a smaller breast would generally be more dense than foremilk in a larger breast because of the differences in volume in the breasts at a feeding time.

How is foremilk created? The alveolus, containing secretory cells, is the milk producing unit within the breast. Lactose, fat, and protein synthesis occurs within the alveolar cells. Raw materials for milk production are transported via the bloodstream to the secretory cells. Milk is in osmotic equilibrium with plasma. Its concentration is controlled mainly by lactose, but also by potassium, sodium, and chloride.2 These components draw water into the lactocytes by osmosis. As milk is produced the increase in lactose leads to an increase in water in the milk. Osmotic pressure causes the body to attempt to equalize the concentration of the milk to the concentration of other body fluids such as serum. The longer milk stays in the breast, the more volume there is, and the more dilute the milk becomes. Hence foremilk after 6 hours is much more dilute than foremilk after three hours.

Foremilk-hindmilk imbalance does not occur if a baby is underweight. Every underweight breastfed baby has a mother with an insufficient milk supply unless the mother is expressing milk which is not being consumed by the baby. Insufficient volume means insufficient lactose to draw serum into the milk. So milk does not become overly dilute when there is insufficient supply.

Fullness in a mother’s breast facilitates an inhibitory effect on milk secretion. “ Prolactin uptake from the blood by the lactocytes may be inhibited in full alveoli”.3 Mothers who become severely engorged usually decrease their milk production significantly. “Milk accumulation triggers apoptosis of mammary epithelial cells. These findings suggest a cell death pathway..... which is triggered by an accumulation of apoptosis-inducing factors in the milk, in the lobulo-alveolar structures or by a physical distortion of secretory epithelial cells generated by the engorgement”.4 In other words, pressure in the breast causes the lactocytes to collapse and atrophy. Although this work was done in mice, it is apparent that the same is true for human mothers. Following episodes of engorgement resulting from lack of milk removal due to postponed or missed feeds, poorly feeding infants, blocked ducts, or mastitis, a mother must work very hard to reestablish her milk supply. She often needs to supplement at breast, post-feed double pump, and take galactagogues in order to do so.

Chemical forces can also reduce a mother’s milk production. A mother’s maintenance of milk production is under endocrine, autocrine, and metabolic control. However, it is the autocrine, or local control, which actually regulates the short-term synthesis of human milk. Feedback inhibitor of lactation (FIL) refers to “ active whey protein that inhibits milk secretion as alveoli become distended and milk is not removed. Its concentration increases with longer periods of milk accumulation, down regulating milk production in a chemical feedback loop.”5 Residual milk left in the breast results in a reduction in milk production. “Thus the degree to which the breast is emptied may be the factor driving overall milk synthesis.”6 Removing residual milk prevents apoptosis of the cells and FIL, and helps stimulate increased milk production.

Since the advent of smaller, more portable double electric breast pumps in the early 1990's, many mothers have maintained full milk production for babies unable to feed at breast because of physical or mental handicaps, or anatomical incompatibility between the mother-baby dyad. These mothers have taught us much about milk composition. When pumping is synchronized to the baby’s feeding schedule and intake needs, milk composition is optimal. The mother is only removing at each pumping the amount of milk the baby would have consumed from the breasts at that time. The span between milk removals is the same as if the baby was feeding at breast. True, the baby misses the changes in fat content which occur as a baby feeds at breast, plus the physiological and psychological benefits of being at breast. However, foremilk-hindmilk imbalances do not occur. The milk production can be manipulated by changing the pumping schedule. For example, a mother who pumps more milk per session, but less times per day as the baby feeds, can maintain a full milk supply. However, the milk will have a different composition. This can change the baby’s behaviour, stools, and abdominal comfort.

In summary: 1) foremilk-hindmilk imbalance occurs with overproduction of milk; 2) FIL and apoptosis of cells from pressure result in reduction in milk production when milk is left in the breast; 3) the degree of emptiness of the breast stimulates milk synthesis; and 4) when baby can not breastfeed, double pumping according to the baby’s intake needs and feeding schedule results in optimal foremilk-hindmilk balance.

When considering strategies to manage an oversupply and decrease milk production to the amount needed by her baby, current recommendations that mothers receive must be considered. Mothers are told to nurse on the same breast for 2 or 3 feedings in a row. 7,8 Similar suggestions are found in professional journals “I work with women who experience moderate to severe oversupply syndrome. In most cases the syndrome can be successfully treated with full removal of milk followed by unilateral breastfeeding ad lib with the same breast offered at every breastfeed in a certain time block ("block feeding").9

This advice does utilize the concepts of FIL and pressure which will cause a reduction in milk production, but the baby will certainly experience a foremilk-hindmilk imbalance from over-dilution of milk when the milk in the unused breast is finally consumed. When a mother restricts feedings to one breast per feed or for a specified time period, the unused breast accumulates an increasingly dilute milk over time. The osmotic pressure exerted by lactose draws more and more serum into the milk. The more time between milk removals, the more dilute the milk becomes. Three hour “foremilk” is more creamy than six hour “foremilk”. When a mother uses both breasts at a feed, milk is being diluted in each breast for two to four hours on average. When a mother uses only one breast at a feeding time, milk is being diluted for an average of four to eight hours. The “foremilk” after eight hours would be very “watery”. The baby would be faced with a forceful milk ejection because of an overfull breast. The overly dilute milk would lead to rapid gut transit with increased peristalsis, more watery stools, painful abdominal cramping, and transient lactose intolerance. The milk increases in “creaminess” as the baby feeds because of the milk ejection reflex, but the baby still consumes an inordinate amount of very dilute milk.

Generally speaking, babies know best when the milk flow is slowing. When the baby slows his swallowing rate, he has already gotten the bulk of the foremilk and hindmilk in that breast. If the baby slows his swallowing rate or comes off the breast, it is probably time to switch the baby to the opposite breast. This ensures that some milk is removed from each breast at each feeding and reduces the dilution of milk in the second breast.

Some mothers truly do produce an overabundance of milk. In these cases, if the baby is left on the first breast until regular swallowing slows, the baby would rarely take the second breast because of satiation. This scenario is the rare occasion when timed feedings are helpful. If the baby usually feeds in a total time of five to ten minutes, then the mother should interrupt the feed on the first breast after three to five minutes and offer the second breast for three to five minutes. If the baby is still hungry, he should again be offered each breast for a limited time. The goal is to have the baby remove as equal as possible amounts of milk from each breast at each feeding. Residual milk will be left in both breasts. This will decrease the milk synthesis rate by inhibiting prolactin uptake, increasing FIL, and causing apoptosis of lactocytes due to pressure. The milk supply will be down-regulated in a more balanced way, and the baby will obtain a better blend of foremilk and hindmilk.

It does not take very long for the milk supply to decrease. It is so much easier to lose a milk supply than it is to maintain or increase one. During the period when the milk supply is being reduced the baby may become more fussy until the milk supply has actually decreased. Stools may initially become more liquid, more green, and more mucusy. This does not usually last longer than twenty-four to forty-eight hours. The mother should use breast massage to move milk forward into areas of the breast cleared by the baby during feeding in order to prevent backed up milk and blocked ducts. Any residual milk which is causing pain from over-distension of the ducts should be expressed in the smallest amounts needed to relieve the mother’s discomfort.

Each mother and baby needs individual assessment in determining the best strategies to follow. Factors which should be considered include the size and storage capacity of the mother’s breasts, the baby’s behaviour at breast during a feeding, an analysis of the baby’s actual intake at each breast according to an accurate electronic scale, the baby’s growth along the weight curve in accordance with his length, and an assessment of the baby’s stools and urine output.

In summary, recommendations to mothers for managing an oversupply of milk should follow the baby’s feeding cues as much as possible. If it is apparent the baby will not take any milk from the second breast, feeding time on the first breast should be restricted to ensure some milk is removed from the second breast. Ideally, milk removal should be as equal as possible from both sides. Residual milk causing uncomfortable pressure in the breasts should be relieved with massage and as little milk expression as is necessary. The goal should be to establish a milk supply which is true to the baby’s needs while ensuring maternal satisfaction with breastfeeding. This will allow for optimal infant health during the period of exclusive breastfeeding in the first six months and provide the basis for mothers to be able to continue breastfeeding until the baby is at least two years old.

Written by Joan M. Fisher, RN, BN, MEd, IBCLC