Mass cannot be changed into energy. Maybe it can down the hall in Mr. Einstein's physics class, but here in my nutrition class we will not violate the classical laws of the universe.
Mass is converted to energy by every chemical reaction! Say you have a mixture of rust and aluminium powder, which you ignite (the thermite reaction). It results in iron, aluminium oxide and heat. The resulting matter will weigh a tiny bit less, by exactly the energy produced divided by c^2