An udder breakthrough for science (Image: Imagebroker/FLPA)

Here’s a question you don’t hear in the breastfeeding debate: why doesn’t milk turn breasts to bone?

Charles Darwin suggested that lactation evolved through natural selection, starting when the ancestors of mammals gained a nutritional advantage from lapping up sweat-like secretions from glands under their mothers’ skin.

This idea had some grounding. Darwin would have studied monotremes – egg-laying mammals such as the echidna. Monotremes have nipple-less mammary patches, and these secrete a fluid that provides moisture to permeable eggs.

But milk contains 100 times the calcium of these gland secretions, and 1000 times the concentration of protein. Such an increase in calcium should cause calcification of the secretory gland, and the proteins in milk should form toxic fibrils.


Hard problem

In fact, it is a long-standing paradox that hard tissues such as bone calcify, whereas soft tissues do not – even though both tissues are fed by the same extracellular fluid.

To find out what was going on, Carl Holt at the University of Glasgow, UK, and John Carver from the University of Adelaide, Australia, built 3D models of the interaction between ions and proteins in biological fluids.

They found that caseins, nutritional proteins in milk, help to prevent calcium build-up by capturing calcium phosphate and squirrelling it away inside molecular aggregates called micelles.

Holt and Carver say that the concentration of these spherical micelles in milk may have increased over evolutionary time, producing a progressively more nutritious fluid.

Journal reference: Journal of Evolutionary Biology, DOI: 10.1111/j.1420-9101.2012.02509.x