Following a vegetarian or vegan diet may be difficult for some. After all, there are some nutrients that you can only find in animal-based food products. In addition, only a few plant-based foods are capable of providing protein, an essential nutrient that helps build and repair body tissues, among other functions. Fortunately, vegetarians and vegans can rely on dietary supplements to meet their daily protein requirements. One of the best protein sources for people who don’t eat animal products is spirulina.