What is the relation between kosher and healthiness? G-d wants us to eat properly but also to be healthy. For example, it is better to try not to eat too much meat (even if it's kosher), and more vegetables and fruits.
So does kosher mean we are eating healthy?
Sefer Hachinuch 73 says (in my own loose translation, and with emphasis added):
Among the bases for this command [of not eating an animal that was slaughtered and then found to have been close to death] is as follows. The body is a receptacle for the soul; through the body, the soul does its work. Without the body, the soul's work can never be completed. That's why it enters the body: for its own benefit (for God only does good, never bad). The body in the soul's hand is like the tongs in the blacksmith's, with which he makes things.
Now, when the tongs are strong and well-aligned so it can grasp the items, the blacksmith can make good things; otherwise, the items being made will never come out good and nice. Likewise, when the body is missing anything, the intellect suffers accordingly. Therefore, the Torah banned all things that cause the body detriment. Along these lines we can explain simply the Torah prohibition on all forbidden foods; and if there is any among them such that we (and the doctors) don't know the damage they cause, don't be shocked: the Doctor who banned them is wiser. How foolish and confused is he who thinks the only damage in something is that which he comprehends!
Know further that it's for our benefit that the damage in these foods was not revealed to us. For people would then arise who consider themselves very wise and say "oh, that damage? That's only damaging in that climate, or for such people". Lest people be fooled by this, the inherent damage was not revealed to us.
Not necessarily.
There is a misconception that Kosher food is more healthy than non-kosher. However, poisons can be kosher, while perfectly healthy salads with some dead bugs are not.
The laws of Kosher ensure one's spiritual health rather than his physical one.
I'm first just going to just copy from this essay by R. Gil Student (hope that's allowed):
Ramban (Lev. 11:13) states that non-kosher animals are physically unhealthy. Rashbam (Lev. 11:3) agrees and Rambam (Moreh Nevukhim 3:48) offers this as a secondary reason for these [dietary] commands.
However, Abarbanel (Lev. 11:13, p. 65) and Akeidas Yitzchak (ch. 60) argue against this approach. First, if this were true, the Torah would be merely a book of medicine. Second, we see that gentiles eat non-kosher animals and do not suffer health-wise. And third, why aren’t poisonous animals mentioned explicitly in the text?
I [R. Gil Student] don’t find these objections particularly convincing. The Torah requires building a ma’akeh (guardrail) around your roof even though that is merely safety related. No one argues that this somehow diminishes the Torah’s value. And the argument is not that non-kosher animals are poisonous, just that they are less healthy. Perhaps they contribute slightly to an earlier death but social differences mask that contribution. Only a controlled study, something with which the Medievals were unaware, could truly answer this. And perhaps the Torah already prohibited eating poisonous food in its general admonition to take care of your health. Refraining from poison is obvious. Refraining from these animals, which we still do not know whether they are truly unhealthy, requires a new prohibition. Not that I find the approach particularly convincing but I believe these two arguments can be rebutted.
I have two things to add to this: one, is that R. Yosef Bechor Shor also adopts this approach in his comments to Shemos 15:26.
Second, is that I've heard in the name of R. Moshe Shapiro (and I believe that this idea is printed somewhere in the writings of R. Akiva Tatz) that if one adopts this approach, the proper perspective is not to say that the Torah prohibits these animals because they are unhealthy. Rather, the truth of the matter is that the Torah (or some form of it) preceded the creation of the world, and therefore only because the Torah prohibited these animals, their nature is that they are unhealthy. One can, of course, argue which caused which (as one might do regarding morality) but I've always found this to be a fascinating idea.
Regarding the raltionship between kosher and health, besides for the obligation to eat kosher, Jews also have an obligation to take care of themselves, which means eating healthily. Some sources that state this explicitly include the Rambam (Rambam, Hilchot De'ot 4:1), Tur (O.C. 155), and Kitzur Shulchan Aruch (siman 32)
