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.)