“How do infants extract milk during breast-feeding? We have resolved a century-long scientific controversy,” say David Elad and his team from Tel Aviv University in Israel in the opening of their new PNAS article.
Until now, researchers have debated what forces from a suckling baby actually draw milk out of a woman’s breast. Is it peristalsis–a mouthing akin to milking a cow’s udder–or is it suction, a negative pressure created by the baby’s mouth?
The answer, Elad and colleagues say, is suction. To come to this conclusion, the team used two approaches. The first was to gather ultrasound movies of nursing babies–and to quantitatively analyze the frames to get an objective understanding of the process.
“When a baby is breastfeeding, first of all he/she is latching on to the nipple and areola complex, which means the baby is sealing the mouth–the baby is extracting into his/her mouth the nipple and areola complex,” notes Elad. Once a baby has latched on, the complex is stretched twice as long as the free nipple, as previous studies showed.
The observations also showed that the front part of a baby’s tongue acts like a rigid plate, moving up and down with the jaw as the baby nurses. Meanwhile, the elongated nipple moves back and forth in the mouth; the team measured this distance. The up and down motion of the front of the tongue, “definitely does not represent a peristaltic wave of contraction,” the authors write. “That is to say, the whole nipple is periodically compressed against the hard palate.” Taken together, the findings point to suction.
These observations allowed the team to do the second part of their analysis: creating the first computer model of breastfeeding mechanics. “We showed what this sub-atmospheric pressure should be in order for the nipple to be two times longer in the mouth,” Elad says, and they showed how it fluctuated with the motion of the nipple. It oscillated between about -20 and -40 millimeters of mercury after the baby latched on.
Oscillating pressure is not necessary: a constant pressure would work fine to extract the milk, Elad notes. But the baby has to do more than just draw milk. While a baby nurses, it is also swallowing and breathing, so the changing pressure and the jaw motion is probably a part of this rhythm. “If you watch movies of a baby nursing,” he says, “it’s just moving the mandible and it’s so relaxed.” Some literature reports say that babies drinking from artificial nipples on bottles have lower oxygen saturation than breastfeeding babies, perhaps because it is harder to settle into this ideal rhythm while bottle-feeding, he says.
Elad can’t say whether this mechanism applies to other animals. His next steps are to apply the same methods to studying babies with breastfeeding problems to try to gain insights into the problems’ underlying cause.