Before we start debating on this issue, consider the fact that we belong to the solar system. Sun is heading this system and all the planets and satellites revolve around it. For the system to sustain, it is very important the basic rules have to be strictly adhered to. Suddenly if the earth stops, it can create havoc. Along with revolving around the Sun, the earth rotates around its axis too.
Let us presume that the revolution of the earth around the Sun is destiny and its rotation is karma. But the earth doesnt have any control over the rotation too. So is the case with all other beings except humans. Plants have no control over their growth, flowering etc., similarly animals dont have control over their actions. If they are hungry and there is food in front of them, they will eat.
Human beings, the highest of all creations, have been endowed with the power of discrimination. He can decide to observe a complete fast in spite of the fact that he may be hungry and food may be available to him. He has control over the karma (earths rotation) whereas the destiny (earths revolution) is fixed. It is because of the destiny that he takes birth in a particular country to certain parents.
Those who are believers of destiny will argue that a decision to observe fast is also fixed by destiny. This group consists of great escapists. They dont want to shoulder any responsibility for their actions and so this argument suits very well. Our Vedas teach us to take responsibility of our actions and to not expect any fruits thereof. This attitude is sure to bestow mental peace and inner contentment.
Do your actions with the faith that everything depends on your actions, but believe that the results are decided by the destiny. This would seem a balanced approach to follow.