Y'all remember Stargate when Kurt Russell and James Spader meet the nice Egyptian descendants and offer them chocolate as a sign of friendship, right?
Well, isn't the ability to process lactose in adulthood a relatively recent (c. 3000 years ago) genetic mutation that originated predominantly in European farmers? Since the alien humans are descendants of Egyptians from 10,000 years ago, wouldn't they be lactose intolerant and therefore likely to get diarrhoea as a result of their chocolate bar gifts?
Maybe it should be in the standard “meeting alien” protocols: don't offer them milk chocolate, only dark or white chocolate, in case they're lactose intolerant?
Just a thought.