This is true fasting…

The essence and power of abstinence is not in refraining from food, but in expelling from the heart every remembrance of evil and other such things. That is true fasting, and what – above all else – the Lord demands.
— Saint Leo of Optina (1768-1822)

Restrain your stomach…

If you have promised Christ to go by the strait and narrow way, restrain your stomach, because by pleasing it and enlarging it, you break your contract. Attend and you will hear Him who says: “Spacious and broad is the way of the belly that leads to the perdition of fornication, and many there are who go in by it; because narrow is the gate and strait is the way of fasting that leads to the life of purity, and few there be that find it.”
— Saint John Climacus (c. 525-606)

Observe abstinence…

As long as the flesh is in full health, let us observe abstinence at all times and in every place. When it has been tamed (which I do not suppose is possible this side of the grave), then let us hide our accomplishment.
— Saint John Climacus (c. 525-606)

Do you fast…

Do you fast? Give me proof of it by your works.If you see a poor man, take pity on him.If you see a friend being honored, do not envy him. Do not let only your mouth fast, but also the eye and the ear and the feet and the hands and all the members of our bodies. Let the hands fast, by being free of avarice. Let the feet fast, by ceasing to run after sin. Let the eyes fast, by disciplining them not to glare at that which is sinful. Let the ear fast, by not listening to evil talk and gossip. Let the mouth fast from foul words and unjust criticism. For what good is it if we abstain from birds and fishes, but bite and devour our brothers?
— Saint John Chrysostom (347-407)