Adding protein in smoothie?

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I love smoothie, that is one of the way to bring many fruits and vegetables in one's diet, usually I use just fruits and vegetables and adding sesame seeds or ginger or honey. But those things has no protein. Is there a way to bring protein to smoothie? Using soy seems to be the alternative but I have heard the health risk from eating soy (look at danger of soy) because soy has phytoestrogens which disrupt endocrine function. Any other alternative to incorporate protein in smoothie?