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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