Yes, humans are omnivores, and we function best eating both animals and plants, and I remember that even Adamo's blood type diets include both meat and plants for people of different blood types, with different varieties of these meat and plants being recommended for different blood types.
Like what the article mentioned, the optimal diet for a person depends on the current metabolic health too, and I came across another article that discusses the same subject. http://nicolesnaturalnews.
I also noted from the article that the author Kris Gunnars personally thinks that "vegan diets can have health benefits for a lot of people… at least in the short term, before the nutrient deficiencies kick in (which can be partly circumvented by supplementation)."
Similarly, this article "The vegetarian myth" says that "There is a time for detoxification, and a time for rebuilding and nourishing. Vegan/vegetarian diets are great for detoxification, but this should only last a week to a couple months. Then, after the body is done detoxifying, it needs nourishment and a rebuilding of nutrient stores."
So, maybe for most cases, vegan diets are best only for a temporary season of detoxification, before one needs to include meat again for nourishment and rebuilding of nutrient stores. As Kris Gunnars also wrote, she is "100% certain that a plant-based diet that includes at least a little bit of animals (the occasional whole egg or fatty fish, for example) will be much healthier in the long-term than a diet that eliminates animal foods completely" since such meat and dairy contain important nurients such as iron and animal protein.
Is Red Meat Healthy?