Babies survive the period immediately after birth by feeding on the content of their own cells, research suggests.
Newborns face sudden starvation
Right after birth babies face sudden and severe starvation as they have lost their nutrient supply from the placenta, but have yet to drink milk.
It appears they bridge the gap by breaking down cells, and releasing essential nutrients.
The study, by Tokyo Metropolitan Institute of Medical Science, is published in Nature.
The Tokyo team studied newborn mice. They found that a cellular process called autophagy ramps up immediately after birth and remains high for several hours.
During autophagy, a cell breaks down its own components in small digestive sacs called autophagosomes.
The researchers developed a way to monitor autophagic activity by making autophagosomes fluorescent.
They found mice genetically engineered so that they could not form autophagosomes died within one day of birth.
The researchers believe autophagy is essential immediately after birth for the production of amino acids, which are used either as an energy source or as building blocks for new proteins.
Amino acid levels in the dead mice were significantly down.
Lead researcher Dr Noboru Mizushima said: "For mammals, birth brings inescapable severe starvation.
"The transplacental nutrient supply is suddenly interrupted at birth, and neonates face severe starvation until supply can be restored by milk nutrients.
"Our results suggest that mammals tide over such starvation by autophagic degradation of self-proteins, in other words, by eating themselves inside cells.
"Although we do not have direct evidence that human babies do the same thing, it is very likely that all mammals overcome the physiological neonatal starvation by inducing autophagy, at least in part."
Some experts believe autophagy, while playing a key role in keeping cells clean, may also be implicated in the development of neurodegenerative conditions such as Alzheimer's disease.
The researchers believe their work will aid research in this area.
Dr Wolf Reik, an expert in developmental genetics at the Babraham Institute in Cambridge, told the BBC News website the study was "interesting and intriguing".
"It is logical that a baby would need some sort of bridging mechanism to generate energy when its nutrition supply is cut off when it first leaves the womb.
"However, existing on internal proteins in this way would only be possible for a limited period, and a baby would quite quickly need to replace the lost nutrients from the outside."
Professor Al Aynsley-Green, of Great Ormond Street Hospital, said the finding was not surprising, given the magnitude of changes that took place in the body's systems in the period immediately after birth.