One of those questions without a straight answer... that is because different people/babies have different sensitivity levels to foods that cause trouble - some just have intolerances which have a high threshold level, allowing them to have a certain amount of the product before a reaction is seen.  Others have true allergies that can range to mild all the way to severe.  Butter is indeed mostly fat, but there is no way to completely isolate it from the casey/whey and some minimal amounts may still be found in butter (and no bacteria there to handle it).  If the issue with dairy is severe, some babies might react even to butter.  If parent really wants to rule it out, and they are ok with this idea, best solution is a challenge trial: a couple of weeks butter free, followed by a butter challenge to see if there is a reaction.
Tina Schallhorn, RD LDN CLC
Pediatric Dietitian, soon to be first time mom.

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