Alex, dear brother,
I don't think the Church's fasts are designed to "weaken" the body as much as to just subdue it. Fasting is just a discipline that enables us to say to our belly: "Belly, you are NOT my god!" When combined with prayer and almsgiving, as it always should be, then we have a successful fast, one which should be neither above nor below our abilities and to which we should be gradually introduced. One should also take our Spriritual Father's advice into account and not go off on our own and arbitrarily dismiss the fasts or, going the other way, impose more on ourselves than the Church's fasting canons require.
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