Protein powder. I recommend going for a flavorless one or vanilla. Either of these go pretty well in most smoothies. Chocolate, strawberry, or other highly distinct flavors will limit the amount of tasty smoothies you can make.
Natural peanut butter. This is probably my most favorite way to add protein to a smoothie because it is natural and adds a nice (but not overwhelming) flavor. Go for the natural peanut butter because processed ones tend to have added sugar and hydrogenated fats.
Soy milk. Soy milk used to be a pretty popular replacement for cow milk but has gotten a slightly bad reputation over the years largely because of its association with genetically modified soy beans (GMO). However, if you can find non-GMO soy milk, you should be fine.
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