I've always known that the above argument is specious, but it wasn't until I took a human nutrition course last semester that I found this awesome counter:
Native Americans in Mesoamerica have traditionally eaten a corn-based diet. For reasons unknown to Europeans, they soaked it in limewater before they ate it.
When Americans, usually in the southern states, started eating a primarily corn-based diet, they developed a pretty nasty condition known as Pellagra, which causes severe skin problems, dementia, and death within 3-5 years.
What's the deal, here?
It turns out that Pellagra is caused by niacin (vitamin B-3) deficiency. Niacin is found in corn, but only in a bound form that's indigestible - unless you soak the corn in lime water first.
So, let's compare the divine advice God is giving ancient humans: Israelis are told to hold off a week before they mutilate baby boys' genitals (which, even with the waiting must have caused lots of deaths. Babies didn't do so well however many thousands of years ago). Native Americans are told to soak their corn in limewater to release an essential vitamin so they don't turn black, go crazy and die.
Who are the chosen people, again?