[marton78]

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

[dTal]

The real pedantry is in the comments.

On the patented Special Relativity Diet, by eating nothing and converting your rest mass to energy, you can lose... a whopping 93 nanograms a day!