Can transgender women breastfeed?

THE MALE breast is an evolutionary enigma. Human males are born with nipples and milk ducts—traits that appear early in embryonic development, before sex differentiation. What is more mysterious, however, is why male mammals of all kinds have not evolved to use this equipment to feed their offspring. The plumbing is intact: human males are known to be able to produce a milky nipple discharge in rare circumstances. That phenomenon is now in the spotlight after one trust in England’s National Health Service (NHS) stirred controversy in February by concluding that milk from transgender women—that is, biological males who identify as women—was the “ideal food for infants”. Is this true?

Anecdotes suggest that men have occasionally fed infants. The book “Anomalies and curiosities of medicine”, published in 1896, offers an account of a South American peasant who fed a child for five months when his wife fell sick after delivery. The secretion of a milky fluid dubbed “witch’s milk” is also known to occur in newborns of both sexes due to hormonal surges. (The name came from the belief that witches would steal this milk for use in magic.) Later in life, the production of milky discharges can be triggered by tumours of the pituitary gland, hormonal imbalances and the use of some drugs such as Thorazine, an antipsychotic. When this happens it is termed “galactorrhea”—as it is unrelated to pregnancy or birth and so is not, strictly speaking, lactation. Though intriguing, such occurrences do not suggest an adaptive capacity for male lactation.

Transgender women have at times induced milk production. In one such case in 2018 a 30-year-old transgender woman in New York induced milk production by taking two hormones, estradiol and progesterone, which are associated with pregnancy and delivery, in doses that mimic their rise and fall in biological women. Doctors gave her drugs that stimulated the production of prolactin, a milk-producing hormone, and told her to regularly use a breast pump. After three months, she produced eight ounces (237ml) of milk per day. By comparison, a nursing mother would produce between 20 and 40 ounces of milk every 24 hours in the first month after birth. That is largely explained by evolution: women’s breasts gradually develop for the task, by growing storage, mammary glands, a duct system and subdivisions of breast tissue, as well as plumping during puberty.

But whether male milk is equivalent to that produced by females is unclear. That assertion made in a letter from the medical director of University Hospitals Sussex NHS Foundation Trust, disclosed on February 18th, caused an uproar. Although male and female milk are likely to have similar quantities of fats, proteins and carbohydrates (due to the presence of milk-producing cells in both sexes), the exquisitely complex nature of female breasts and breast milk makes the question a difficult one.

What is, however, established, is that breast milk from women is better for infant growth and nutrition than any common substitute. Modern science has as yet been unable to replicate its myriad components in formula. In addition to macronutrients such as proteins and fats, breast milk contains vitamins, minerals, hormones, growth factors and immune-protective components such as immunoglobulin and lactoferrin, which help prevent neonatal infections. It also contains a rich ecosystem of microbiota and microRNA that is transmitted into the infant’s gut.

There is also some evidence that backwash from the saliva of a sick infant can trigger a protective immune response in the milk the mother delivers to the child. Such dynamic changes in the composition of breast milk in response to illness suggest a co-evolved system between breast and baby. This goes some way towards explaining the huge health benefits of breastfeeding, which have formed the basis of public-health advice. Breastfeeding by transgender women can help them bond with a baby. However, it would be prudent to assume, at least until there is more evidence, that their milk might not have all the nutritional benefits of female breast milk. If it did, evolution would probably have favoured its use some time ago. ■