The source of all protein in your food web is plants. The source of all calcium is the Earth itself, mainly as harvested by plants. Animals only add inefficiency; they are not what enable "human athleticism", they only make it more expensive and harmful.

Neither of those points address the ethics or politics of harvesting vulnerable individuals for flesh and secretions. Doing so clearly requires stepping outside of values like compassion and fairness, and so requires stepping outside the leftist political sphere. Consuming animals is an exception to leftist politics, not some kind of special case.