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Biology: Mammary chronicles

Josie Glausiusz celebrates an environmental history of the human breast.

Breasts: A Natural and Unnatural History

Norton: 2012. 352 pp.$25.95/£16.99) 9780393063189 | ISBN: 978-0-3930-6318-9

The breast looms large in human culture and biology. The essential proteins and long-chain fatty acids in breast milk help to build babies' big brains, and the cornucopia of other components, such as virus-slaying macrophages and oligosaccharides that feed beneficial bacteria in the baby's gut, offer crucial immune protection. Unfortunately, breast milk can also contain pesticides, mercury, benzene and minuscule amounts of paint thinners, dry-cleaning fluids, rocket fuel and flame retardants.

This contaminant-crammed elixir is uniquely modern, as Florence Williams details in Breasts. This is no salacious tell-all, but a lively, absorbing, meticulously researched book covering all aspects of breasts, from anatomy to their role in evolution, attraction and infant bonding; changes during puberty, pregnancy and cancer; and Western society's passion for flaunting, grading and inflating them. At heart, however, the book is an environmental history of “how our breasts went from being honed by the environment to being harmed by it”.

Breast milk aided the evolution of the large human brain — but it can contain toxins. Credit: WESTEND61/REX FEATURES

Williams, a US science journalist, uses her own body as a research tool. She sends her milk to Germany to be tested for flame retardants, delves into her family history of breast cancer and visits a suave Texas surgeon for advice on silicone breast implants. To mimic a study on early puberty, she and her seven-year-old daughter, Annabel, valiantly try to give up plastic-wrapped food as well as products containing endocrine-disrupting phthalates — including Williams's car.

Intimate explorations of breast biology have a distinguished history. In 1840, British surgeon Astley Cooper published The Anatomy and Diseases of the Breast, in which he observed — after injecting dyes into more than 200 disembodied breasts — that blood is transformed into milk in grape-like lobules, inside tissue cavities called alveoli. The milk then enters a network of lobes that empty into 12 or so orifices in the nipple.

Unlike any other organ, human breasts do most of their development well after birth. These plump orbs are also unique in that no other primate is so endowed: females of other species develop swellings only during lactation. Evolutionary biologists have devised elaborate stories to explain the permanent adult presence of human breasts; the most popular is that they are an adornment, like a peacock's train, for attracting the opposite sex. Williams leans more towards the ideas of anthropologist Frances Mascia-Lees, who posits that breasts' ever-present fat reserves are easily mobilized during lactation to keep pace with the baby's rapidly growing brain.

The immune support offered by human breast milk is formidable. The average new mother produces roughly 454 grams of milk from each breast every 24 hours. This elixir is not unlike cultured yoghurt, carrying 100–600 species of live bacteria, most new to science. (Mysteriously, the US National Institutes of Health's Human Microbiome Project, which is decoding the genes of microbes from every major human surface or orifice, is not analysing breast milk.) One theory suggests that the bacteria in breast milk work as a kind of gut vaccine.

The stuff is so beneficial that companies that produce substitute breast milk are racing to replicate its ingredients, with little success so far. But Williams's investigation suggests that there may be good reasons to hope that they succeed: breast-milk toxins can include “the mercury in last week's sushi, the benzene from your gas station, ... the chromium from your nearby smoke stack”. Moreover, even a tiny dose of these contaminants can be harmful to babies, and such toxins have been implicated in low intelligence and cancer.

Williams reports that flame retardants, found in sofas, nursing pillows and infant car seats, can impede brain growth and affect thyroid hormones. When Williams's milk is tested for polybrominated diphenyl ethers (PBDEs), she learns that her levels are slightly above average for US women — and notes that mothers offload about 30% of their PBDE burden onto their babies if they nurse for a year. Williams breast-fed her two children for 18 months each.

It is not just infant development that may be affected: the author describes how hormone-disrupting phthalates and bisphenol A (BPA) may be advancing puberty in girls by prematurely switching on oestrogen receptors in breast tissues. Williams looks at possible reasons behind the rise in breast cancer — globally, the leading cause of cancer-related death in women, with 1 million diagnosed each year — including better detection, hormone-replacement therapy and exposure to untested chemicals.

In one alarming account, she reports on an epidemic of male breast cancer among US marines at Camp Lejeune in North Carolina. Over three decades, starting in the 1950s, fuel tanks leaked more than 3.8 million litres of petrol into the base's groundwater. One well, which supplied drinking water to 8,000 people, contained 76 times the legal limit of benzene, a known human carcinogen.

As Williams points out, breast milk boosted brain size in our ancestors, but those brains have helped us to change the environment — which, in turn, is channelling to infant brains toxins that may impede their development. There is hope for the future, however: in 2004, the United Nations implemented the Stockholm Convention on Persistent Organic Pollutants, in which 177 countries have agreed to ban or restrict such chemicals, including some PBDEs. The United States has yet to ratify the treaty.

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Correspondence to Josie Glausiusz.

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Glausiusz, J. Biology: Mammary chronicles. Nature 485, 306–307 (2012).

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