How do embryonic livers produce energy giving glycogen even though they lack the enzyme that produces it? Spanish researchers claim to have found the answer to this interesting biological mystery.

In adults, the liver stores glycogen that provides a steady supply of blood glucose when needed, for instance when fasting.

Glycogen production is controlled by an enzyme called glucokinase (GK) and mutations resulting in too much or too little GK lead to hypo or hyper glycemia.

It has always puzzled scientists how embryonic livers can store plenty of glycogen, even though they don't produce any GK. These livers start making GK only after newborns drink their first carbohydrate-rich milk.

Researchers at the University of Barcelona have now found that embryonic mouse livers circumvent the lack of GK by greatly overproducing another enzyme - hexokinase (HK).

At such high levels, HK can make glycogen. However, unlike GK, HK makes glycogen independent of blood-glucose levels and the researchers confirmed this by fasting pregnant mice and observing that the embryonic livers did not alter their glycogen accumulation.

By using HK, embryos safeguard their glycogen production from any changes in maternal diet to ensure abundant storage.

This is critical since glycogen is a newborn's principal source of energy during the time between birth and its first milk meal.