the wonder of breasts

Guardian | 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."