I'm reading the Lenten Triodion, and in this particular version there is a forward by Archimandrite Kallistos Ware, where he states:
Until the fourteenth century, most Western Christians [i.e., Catholics], in common with their brethren in the Orthodox East, abstained during Lent not only from meat but from animal products, such as eggs, milk, butter and cheese. In East and West alike, the Lenten fast involved a severe physical effort. But in Western Christendom over the past five hundred years, the physical requirements of fasting have been steadily reduced, until by now they are little more than symbolic.
Unfortunately he didn't provide a source, although he is a prolific writer on Orthodoxy.