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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
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
@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
@Alex it's perfectly reasonable that they thought that mushrooms had no positive health benefits. I'd be curious to know what the ratio of posionis or drug mushrooms and truffles were in Babylonia. – avi Feb 1 '12 at 15:12

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
@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

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