You write to ask “what is the original health rationale” for the Jewish law about not eating meat and milk together?
While I have frequently heard that the dietary laws of Kashrut serve health purposes (or once did) the truth is that there is no strong reason to think that the rationale for these laws was ever health. Let me clarify.
The Torah prohibits “seething a kid in its mother’s milk.” From tradition, and from the repetition of this commandment three times in the Bible (Exodus 23:19, Exodus 34: 26, Deuteronomy 14: 21), we learn that this refers to a prohibition on cooking, eating or even benefiting from a mixture of milk and meat (i.e. selling it for money). For observant Jews, this is part of Torah law which is accepted on both divine and rabbinic authority (Hullin 113b, 115b). I cannot work at selling cheese burgers, for example, even if I never eat them, since this would be a form of benefit. This was later extended by the rabbis to milk and chicken.
The Torah never claims that meat and milk are prohibited for reasons of physical health. The Torah does however say several times in connection with the dietary laws that their purpose is sanctity, which is generally taken to mean being set apart the way God is set apart. “Be thou therefore holy as I the Lord your God am holy.” Holiness is not the same as ethics (though it presumes ethics) and is not the same as physical health, though I also assume that it would not be contrary to physical health to observe the dietary laws. These laws promote self-discipline and control of appetite and Jewish mystics (but not philosophers) have sometimes argued that forbidden foods are spiritually stultifying. This could be true, though I don’t know of any independent evidence for it, but it is beside the point from a Jewish law point of view. As Maimonides writes in his Eight Chapters, a person should say, “I really wish I could eat that forbidden food, but what can I do, since my Father in Heaven has forbidden me!” Maimonides does not think this is true, say, of murder or theft, which any rational and moral person ought to feel repelled by. My point is that, at least for Maimonides and those who follow him, a commandment like not eating milk and meat together does not have to have an obvious reason in order to be binding. We observe it because we are committed to the Torah. Reasons do not need to be known in order to be effective.
This does not of course mean that one cannot find a rationale if one works enough at it or that such rationales are without significance. Maimonides himself felt that many of these laws were based on the rejection of ancient idolatrous practices (Guide of the Perplexed III: 48). Others however have suggested a different kind of moral rationale. Milk, these writers suggest, is a product that sustains life, and they argue that even though we are permitted to eat meat, still we ought to cringe at the irony of cooking it in a substance designed to nurture and support young animals. Thus the language of not seething a kid in its mother’s milk is meant to convey something more than the actual law of not cooking meat and milk together—it helps to suggest the reason for the law’s institution. Though we are permitted to eat meat, we ought to be developing our moral sensitivity even for animals and one way of doing this is to keep separate these different kinds of foods. Rabbi Abraham Isaac Kook writes in several places (see his essay “On Reasons for the Commandments” translated in Ben Zion Bokser’s excellent volume) that this project of sensitization to suffering should continue to grow and develop and may one day lead us to eschew eating meat altogether.
I hope that you find this answer helpful. This is the perspective I find most meaningful from the perspective of Orthodox Judaism. It is interesting that this approach is quite compatible with that suggested by some modern anthropologists, and I refer you to the article “The Abominations of Leviticus” by Mary Douglas, which is published in her fine volume, “Purity and Danger.” For Douglas, the dietary prohibitions have essentially a symbolic meaning, and one that was conducive the acquisition of holiness in biblical terms.
