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The Mishnah (Eruvin 26b) states that water and salt can't be used as the food for eruv chatzeiros or eruv techumin (thus Rashi; according to Tosafos only the latter). Rashi explains that this is because they are לאו מידי דמזון, not a food item.

Then, on 27a, the Gemara states that "truffles and mushrooms" can't be used for this purpose either. Rashi doesn't comment on this here, but on Kiddushin 34a (ד"ה והאיכא) he states וקים להו לרבנן דלא זייני - the Rabbis know as a fact that they are non-nutritive.

What does this mean? If we look at it from the point of view of modern nutritional data, then mushrooms - raw, and even more so cooked - contain carbohydrates, protein, and some vitamins. If, instead, we define "food item" and "nutritive" simply in terms of whether they satiate a person, then don't mushrooms do that too?

(The notes in Hagahos HaGra to Eruvin there reference Gra's comments to Shulchan Aruch, Orach Chaim 386:5, where he argues that indeed the Gemara means to apply this exclusion of truffles and mushrooms to a different halachah, about what can be purchased with maaser sheni money, where the item bought has to be something that directly or indirectly "grows from the ground." However, I'm not sure how he would understand Rashi's explicit statement in Kiddushin.)

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I've often wondered this myself, and you've worded it very well. I'd upvote this more than once this if I could legitimately. – msh210 Feb 1 '12 at 4:33
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It's not unlike the Gra to just argue on Rashi. – Double AA Feb 1 '12 at 5:04
    
Are you willing to accept as an answer that Rashi is just mistaken? – avi Feb 1 '12 at 8:37
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@avi: he had to have meant something. (To say "he was mistaken in that he didn't know 21st-century food science" is one thing; to say that he made a completely meaningless statement is something else entirely.) Besides, it's not just Rashi; you still have to account for the Gemara in Eruvin (and the quotation of it in Kiddushin - incidentally, in the latter place nothing is said at all about maaser sheni money, making it difficult to assume that Gra's emendation could apply there). – Alex Feb 1 '12 at 15:06
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@avi: but then that just pushes back the question a bit. Why would they have thought that mushrooms are, in that regard, unlike pretty much everything else edible? (We find people collecting mushrooms to eat after a drought ended - Taanis 23a - although I suppose that could be taken either way: that they thought of it as food, or that it was something they'd eat only out of desperation.) Interesting point about poisonous vs. nonpoisonous ones, though - maybe indeed that's an answer. – Alex Feb 1 '12 at 15:38

I noticed you have been waiting long for an answer and I have a theory to offer, with some rabbinic support. Perhaps this is what you are looking for, or it might just set you or another member on a path that might lead to the answer you are looking for.

I suspect the rabbis believed that a mushroom is non-nutritive because in Genesis 1:29 God is recorded as giving to man every vegetation that has seeds and every tree bearing fruit with seeds. It may be that since mushrooms do not have seeds or fruit with seeds, but rather are a fungus with spoors, they are not considered as given by God for food and thus percieved to be "non-nutritive".

Ramban offers this commentary on Genesis 1:29 discussing the difference between seeding vegetation and fruits given to man and non-seed bearing and non-fruiting green herbs given to animals. If Ramban is conscious of God's giving to man specifically vegetation with seeds, and he considered herbage without seeds designated to animals, then certainly the mushrooms would fall outside of that category. If one considers it not food for human consumption, Perhaps that came to be thought of or expressed as non-nutritive, not nutritious.

If you have access to what other rabbinical writings say on this verse you might find something more direct. I just suspect this is the logical starting place.

I hope you find what you are looking for.

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I edited to show the thought flow/logic. I am not familiar with the timeline of rabbinic writings though; so, I'm not sure what precedes what. Perhaps this answer could be make a community answer and others who know better than me could help put the time line together to provide a more logical answer, and maybe add in other rabbinic reflections on this verse if they exist. – Sarah Jun 20 at 13:08

I spoke to a Rav yesterday who explained to me that it is a food that does not satiate.

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In what sense, though? Surely, if a person ate enough mushrooms, he'd feel full. – Alex Apr 23 '12 at 15:11
    
Perhaps not. Maybe try it! – Gershon Gold Apr 23 '12 at 15:36
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@Alex - I would rather translate וקים להו לרבנן דלא זייני as "the Rabbis maintain that it does not 'sustain'," which should be interpreted as "it does not meet the Rabbis' halachic standard for what is considered nourishing food." Perhaps this is partly because mushrooms are a low-calorie food. – Toras EMES 613 Apr 27 '12 at 22:47
    
Mushrooms have more calories than cucumbers, are those also not acceptable? – A L Jun 15 at 2:13

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