Our culture is obsessed with breasts, yet we know remarkably little about them. But their secrets are starting to be unravelled, and nothing is more astonishing than breast milk

We love breasts, yet can't quite take them seriously. Breasts embarrass us. They're unpredictable. They're goofy. They can turn babies and grown men into lunkheads.

They appear out of nowhere in puberty, they get bigger in pregnancy, they're capable of producing prodigious amounts of milk, and sometimes they get sick. But for such an enormously popular feature of the human race, it's remarkable how little we know about their basic biology.

The urgency to know and understand breasts has never been greater. Modern life has helped many of us live longer and more comfortably. It has also, however, taken a strange toll on our breasts. For one thing, they are bigger than ever. We are sprouting them at younger ages. We are filling them with saline and silicone and transplanted stem cells to change their shape. This year marks the 50th anniversary of the first silicone implant surgery in Houston, Texas.

More tumours form in the breast than in any other organ, making breast cancer the most common malignancy in women worldwide. Its incidence has almost doubled since the 1940s and is still rising.

But breasts are often overlooked, at least for non-cancer scientific research. The Human Microbiome Project, for example, is decoding the microbial genes of every major human gland, liquid and orifice, from the ears to the genitals. It neglected to include breast milk.

I wanted to know more, so I went to the 15th meeting of the International Society for Research in Human Milk and Lactation in Lima. Many attendees were molecular biologists, biochemists or geneticists who are deconstructing milk bit by bit. Until recently, it was thought breast milk had around 200 components. These could be divided into the major ingredients of fats, sugars, proteins and enzymes. But new technologies have allowed researchers to look deeper into each of these categories and discover new ones.

Scientists used to think breast milk was sterile, like urine. But it's more like cultured yoghurt, with lots of live bacteria doing who knows what. These organisms evolved for a reason, and somehow they're helping us out. One leading theory is they act as a vaccine, inoculating the infant gut. A milk sample has anywhere from one to 600 species of bacteria. Most are new to science.

Then there are the sugars. There's a class of them called oligosaccharides, which are long chains of complex sugars. Scientists have identified 140 of them so far, and estimate there are about 200. The human body is full of oligosaccharides, which ride on our cells attached to proteins and lipids. But a mother's mammary gland cooks up a unique batch of "free" or unattached ones and deposits them in milk. These are found nowhere else in nature, and not every mother produces the same ones, since they vary by blood type. Even though they're sugars, the oligosaccharides are, weirdly, not digestible by infants. Yet they are a main ingredient, present in milk in the same percentage as the proteins, and in higher amounts than the fats. So what are they doing there?

They don't feed us, but they do feed many types of beneficial bacteria that make a home in our guts and help us fight infections. In addition to recruiting the good bugs, these sugars prevent the bad bugs from hanging around. "The benefits of human milk are still underestimated," said Lars Bode, an immunobiologist at the University of California, San Diego. "We're still discovering functional components of breast milk."

A quick survey of what biotech companies are doing shows the range of benefits being attributed to human milk. Prolacta Bioscience is concentrating and pasteurising donated human milk – and then selling it – as an "immunonutrition" supplement for premature babies, who do much better on breast milk than on formula. One of the most sought-after components of human milk is a glycoprotein called lactoferrin. Known to have anti-inflammation, antioxidant and anti-infective properties, it's an iron-binding machine that outcompetes pathogens. One Japanese company markets capsules called "Lactoferrin Gold".

Also found in breast milk are endo-cannabinoids – note the root "cannabis" – which cause the munchies and may help entice infants to eat. But they also regulate appetite so infants don't eat too much. Interestingly, formula-fed babies have a notoriously higher caloric intake. It's one of the speculations about why we have a childhood obesity epidemic.

Bruce German, a food chemist from the University of California, Davis, told me these breakthroughs in the understanding of milk are fundamentally altering what we know about human health. "The whole story is part of a revolution in science, where the chemistry-dominated science of the 20th century is giving way to the biology-dominated science of the 21st century. Such a shift is sometimes difficult to appreciate."

If human breast milk, nature's perfect food, came stamped with an ingredients label, it would read something like this: 4% fat, vitamins A, C, E and K, sugars, essential minerals, proteins, enzymes and antibodies. It contains 100% of the recommended daily allowance of virtually everything a baby needs to grow, plus extras to help ward off a lifetime of diseases, from diabetes to cancer. Despite exhaustion, visiting relatives and dirty laundry, every time we nurse our babies, the "love hormone" oxytocin courses out of us like a warm bath.

But read down the label a little further, and the fine print sounds less appetising: DDT, PCBs, trichloroethylene, perchlorate, dibenzofurans, mercury, lead, benzene, arsenic. Because breasts store fat, they store toxic, fat-loving chemicals. When we nurse our babies, we feed them not only the fats and sugars that fire their immune systems, cellular metabolisms and cerebral synapses. We also feed them, in albeit miniscule amounts, paint thinners, dry-cleaning fluids, wood preservatives, toilet deoderisers, cosmetic additives, gasoline byproducts, rocket fuel, termite poisons and flame-retardants.

If, as Cicero said, your face tells the story of your mind, your breast milk tells the decades-old story of your diet, your neighbourhood, and increasingly, your household decor. Remember that old college futon? It's there. That paint in your bathroom? There. The chemical cloud your landlord used to kill cockroaches? Yup. Ditto, the mercury in last week's sushi, the benzene from your petrol station, the perfluorooctanoic acid (an anti-grease coating) from your latte cup and sofa upholstery, the preservative parabens from your face cream, the chromium from your nearby smoke stack.

On a body-weight basis, the dietary doses our babies get are much higher than the doses we get. This is not only because they're smaller, but also because their food – our milk – contains more concentrated contaminants than our food. It's the law of the food chain.

Here's how it works: animals at the top of the food chain receive the concentrated energy and persistent chemicals of all the biota underneath them. Each member up the food chain takes in approximately 10 to 100 times the load of fat-loving toxins of its counterpart below. This is why a slab of shark meat contains more mercury than its weight in plankton. Ocean food chains are longer than terrestrial ones, so people who eat many marine carnivores carry higher body concentrations of some chemicals. The Inuit, although they live in the remote Arctic, are the most contaminated population on earth.

Breasts, it turns out, are a particularly fine mirror of our industrial lives. They accumulate more toxins than other organs, and process them differently. We first realised this in 1949, when a doctor named Morton Biskind, from Westport, Connecticut, examined a pregnant woman who had strange neuropsychiatric symptoms. He had been following scores of patients with acute poisoning from exposure to the pesticide DDT, which had hit the US market a few years earlier. He'd heard the substance was being found in the milk of cows, rats and dogs, so after the woman gave birth, he thought to test her breast milk. It was rich in DDT. In 1966 a Swedish researcher tested his wife's breast milk for PCBs – polychlorinated biphenyls, used to insulate electrical transformers – after he discovered them in dead eagle tissue. Two years later, Sweden banned them, with the US following in 1978.

Because of the widespread use and persistence of PCBs, they are still among the highest-concentration toxins found in breast milk, even from mothers born after the ban. One of few kinds of chemicals to be banned outright in the US, PCBs have been well studied. In humans, at elevated levels they can interfere with thyroid function. Studies also show a relationship between exposure to PCBs and breast, liver and gall-bladder cancer and lymphoma.

The flame-retardant PBDEs are similar. The most common flame-retardant found in human breast milk is a mixture known as "penta-BDE", which was manufactured in Indiana until 2004. The company was making over 20m pounds of it each year, most of it to drench flexible polyurethane foam. In 2004, however, the European Union banned this substance and another called "octa-BDE" , concluding they were likely to be chronically toxic in humans. The US government instituted a voluntary phase-out. Another compound, "deca-BDE", is set to be phased out from next year, but a major replacement, decaethane, is almost identical in structure. Most of us will continue to be sitting on and ingesting PBDEs and their chemical cousins for a long time.

The breastfeeding lobby is sometimes reluctant to highlight breast-milk contamination because they don't want women to have another excuse not to breastfeed. But despite the reassuring arguments to keep breastfeeding, I find myself unsettled. The amount of chemicals infants suckle through milk is not insignificant. Recent studies show that lactating mothers offload about 2%-3% of their total PBDE body burden per month to their offspring, or about 30% if they nurse for a year. I nursed both my kids for 18 months, and now I can't help but wonder if that was such a great idea. For other chemicals, the dump rate is even higher, with a range of up to 14% per month for dioxins, and up to 8% per month for PCBs. Mothers who breastfeed for a year also pass to their infants 90% of their body burden of perfluorinated compounds, known as PFCs. Used in the manufacture of products such as Scotchgard, Gore-Tex and Teflon, PFCs have spread across the globe, even ending up in polar bear tissue, and virtually never break down in the environment. An EPA panel said one type of PFC called PFOA was "likely to be carcinogenic in humans".

The Norwegian Scientific Committee for Food Safety is debating the country's breastfeeding recommendations, which would have been unthinkable a decade ago. "I don't think it will change the current recommendations [to breastfeed for a year], but maybe there's no benefit to breast-feeding after six months," says Cathrine Thomsen of the Norwegian Institute of Public Health. Consider the gravity of this statement. Norway has the single highest breastfeeding rate in the world, with 99% of new mothers doing it. At six months, more than half of all babies are still nursing. The country has banned advertising by formula companies. It grants paid maternity leave for 42 weeks. It is a country deeply committed to breastfeeding. And now it is rethinking it.

This is an edited extract from Breasts: a Natural and Unnatural History by Florence Williams, published by Norton, available for £13.59 on guardianbookshop.co.uk

Did you know … ?

• All mammals have mammary glands, but no other mammal has breasts as we do, sprouting at puberty and remaining regardless of our reproductive status

• Timmie Jean Lindsey, who had the world's first breast implants in Houston 50 years ago, is still walking around with the original specimens. She knows they've ruptured – she's been screened – but doesn't want them removed

• One Houston doctor from the early days of breast implants, Gerald Johnson, had his swimming pool designed to look like a breast, with a Jacuzzi as the nipple

• A woman who has her first child before 20 has about half the lifetime risk of breast cancer as a non-mother or a mother who has her first child after 30

• Rhesus macaque monkey mothers produce fattier milk for sons, but make more milk for daughters. In that matrilineal primate society, daughters learn from hanging around their mothers, and thinner milk means they stay close for more frequent feedings. The sons, who don't feed as often, have more time to play and explore – skills they'll need when they leave the group

• Bernadino Ramazzini, a Renaissance-era doctor, was the first to notice that breast cancer was more common among nuns, which led to the link with nulliparity

• The Marine Corps base at Camp Lejeune, North Carolina, had the most contaminated drinking water supply ever discovered in the US, and is the centre of the largest cluster of male breast cancers ever identified, with 73 cases

• In 1982, the American Society of Plastic and Reconstructive Surgery said "there is a substantial body of medical information and opinion … to the effect that [small breasts] are really a disease"