"Old rule"?

What does “Old rule” mean? Does it have to do with fasting?

Thank you!

What’s the paragraph or sentence you saw it used in?

Moral Theology forum, Vegetarianism thread:

If one is Catholic, I would think that conforming to the Old Rule, so long as it does not impair health (and as I recall, the Rule makes allowances for that) would be a rewarding practice. And whether or not one is Catholic, yes, there are the modern husbandry issues, as well as the ‘ecological footprint’ ones to consider (which may possibly be seen as a form of charity), and very often, the sheer health benefits, depending on ones constitution.

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