Several, though not "holy" in itself, but befitting a holy person (saint-like behavior).

In this particular instances he means in the practice of Christianity, which in the Catholic and Orthodox version (or the unified version before the schism) had strict lent period, with rules about abstaining from meat (and, for those following the stricter rules, dairy products two in most days (and in the traditional versions, a bread+water diet only on some important holy days). Regular christians followed (and follow) those at their own preference/faith/tolerance level, the clergy, monks, and saints, follow them strictly. And there are several stories of saints who followed them year round.

But of course, veganism is also associated with holy behavior in Hinduism, Buddhism, and such.