Our Gemara on Amud Beis quotes medical advice from the Great Amora, Rosh Yeshiva and Physician, Shmuel:

אָמַר שְׁמוּאֵל: נוּן סָמֶךְ עַיִן — נוּנָא סַמָּא לְעֵינַיִם!

as Shmuel said an acronym: Nun, samekh, ayin, which stands for: Nuna samma la’einayim, which means: Fish is a medicine for eyes. 

Maharsha says that the Jews, when they were complaining about the Man, also hinted at this (Bamidbar 11:5-6):

זָכַ֙רְנוּ֙ אֶת־הַדָּגָ֔ה אֲשֶׁר־נֹאכַ֥ל בְּמִצְרַ֖יִם חִנָּ֑ם אֵ֣ת הַקִּשֻּׁאִ֗ים וְאֵת֙ הָֽאֲבַטִּחִ֔ים וְאֶת־הֶחָצִ֥יר וְאֶת־הַבְּצָלִ֖ים וְאֶת־הַשּׁוּמִֽים׃ 

We remember the fish that we used to eat free in Egypt, the cucumbers, the melons, the leeks, the onions, and the garlic.

וְעַתָּ֛ה נַפְשֵׁ֥נוּ יְבֵשָׁ֖ה אֵ֣ין כֹּ֑ל בִּלְתִּ֖י אֶל־הַמָּ֥ן עֵינֵֽינוּ׃ 

Now our souls are dry. There is nothing at all! Nothing but this manna in front of our eyes!”

The Jews were saying that Fish at least was healthy for the eyes, but the Manna could not offer that benefit.

There are other fascinating ways that the rabbis understood the complaints of the Jews about the Manna.  While Maharsha sees the Jews as at least having some intellectual basis for their complaint, others see the Jews’ complaint as rooted in an almost allergic response to being in a situation where they are forced to deny certain physical desires and needs.  For example, Rabbenu Bechaye, quoting Yoma (75a, Rashi and Sifri), says that the Jews mentioned these five species (cucumbers, melons, leeks, onions, and garlic) because the Manna could taste like anything except these five because they were harmful to pregnant women.  The Gemara (ibid) also mentions that when the Jews complained about the “Fish” they could eat for free, it was euphemistically referring to the sexual freedom and immorality they had in Egypt.  Ramban says their chief complaint was that they had to constantly be on the lookout for food, that is they never knew if they would get the food the next day. (This is the reference in the verse to eyes.) The Gemara Yoma (75b) describes how they were unnerved by the fact that since the Manna was pure food, they never had to excrete waste. And Yoma (74b) also says the complaint was that despite the fact that the Manna could taste like anything, it LOOKED the same and that was monotonous. Finally, the Gemara (Yoma 75a) tells us that the Manna acted as a kind of lie detector.  That is since it would only give the exact portion to each household, it could “out” anyone committing sexual immorality and theft, as the Manna would appear as a tell-tale by the house of where it belonged (you can look at the Gemara for the exact descriptions about how it played out.)

There is a common thread in all these peshatim, which is why they all might agree with each other and have sources in the conscious and unconscious expression of the Jews’ complaint within the text.  The Jews were forced by the Manna to live on a higher level than they felt ready to.  They had to be honest, faithful, trusting in Hashem (for sustenance), be fearful of sin (as the Manna could be cut off at any time or they could be outed), and in general, exist with more spiritual gratifications.  This is literally or metaphorically represented by not having the biological urge to excrete.  

We may surmise that Hashem “had no choice” but to extricate the Jews from Egypt at that time, as otherwise they might have been spiritually or physically annihilated.  However, it came at a cost. The Jews were not ready. It is like a doctor who has to induce labor early because of danger to the child or mother, even though the child will be premature and suffer other dangers. The Jews were dragged kicking and screaming into this spiritual reality, (or at least a segment of the population), and continued to fight all the way through.  There are two lessons from this:

1. Dependence breeds hostility.  It might be necessary at times, but we cannot afford to be naive that a price will be paid.
2. Spirituality, though delightful and sublime when you are ready for it, can be ironically a living Gehenom for those who are not ready for it.