Well I guess it depends if yoiu are adding a tablespoon or a cup?

If you take in more calories than you burn.... well you know the saying.

Also, take care that this shake doesn't have a lot of high-glycemic carbs at the same time, otherwise this is inviting fat storage.

is oatmeal a high glycemic carb? I like plain oatmeal in my shakes and banana.

Oatmeal(the regular kind) is definitley low glycemic.

as long as it's all natural peanut butter and not garbage like Skippy's