I can't believe how many times this subject gets to put the forefront of Yoga. Classically, Yogis have eaten a diet consisting of mainly vegetarian cooking. Let me explain the reasoning for this. It is based on ancient Vedic writings.

A long time ago, the writers of the Vedas looked a lot to nature to make their comparative assumtions. For instance, many Yoga poses are named after animals. Looking at how animals stretched themselves, the writers of the Hatha Yoga Pradipika incorporated this into a system meant to help prep people for meditation. In the same manner, looking to the animal world, the ancients looked at the length of digestive track relative to the body proportion of what the animal ate. The human digestive system was somewhere in between that of herbivores and carnivores, hence an omnivorous diet was considered best. Then, somehow vegetarianism became integrated with Yoga through Hindu tradition.

So what should you eat? Eat whatever. The fact is, we all have different body constitutions and if a certain diet lead to enlightenment, that would be all we have to do. I have found no advantage to being vegetarian. I can understand vegan-ism and the intent not to do harm to living things, but in Yoga, the intent to harm is an internalized samskara. In my opinion, if you start doing meditation, you may move in that direction. If not, don't beat yourself over it. Enjoy the steak or burger. Use your common sense and do what works best for you at the time.