The Knights Templar was a Catholic military order that took part in the Crusades and in consolidating the rule of the Church in Europe. They were also unusually healthy for their time. While the average European in the Middle Ages would be lucky to see the age of 50, many of the Knights were fighting into their 60s. That may have been due to the rules they lived by. As monks, they took a vow of poverty and their diet was kept simple, unlike the rich dishes of the wealthy class. But as warriors, they had to stay in fighting shape and power their extensive training, so they were permitted an amount necessary to stay strong.
The knights’ diets seem to have been a balancing act between the ordinary fasting demands on monks, and the fact that these knights lived active, military lives. You couldn’t crusade, or joust, on an empty stomach. (Although the Knights Templar only jousted in combat or training—not for sport.) So three times a week, the knights were permitted to eat meat—even though it was “understood that the custom of eating flesh corrupts the body.” On Sundays, everyone ate meat, with higher-up members permitted both lunch and dinner with some kind of roast animal. Accounts from the time show that this was often beef, ham, or bacon, with salt for seasoning or to cure the meat.
Read about the rules for the Knights Templar, including their diet, at Atlas Obscura.