Eating Shemitta Fruit
The first two mishnayot of the eighth perek of shevi'it outline the permissible and forbidden uses of shemitta fruit (fruit which budded during shemitta). The mishna dictates that the fruits be employed in their standard manner and not diverted for some other use. The first mishna forbids the conversion of edible fruits to medicine while the second bans using wine or vinegar as ointments while permitting the use of oil as ointment. In the first instance, these materials are generally not used for this purpose and hence this change is forbidden.
The Ramban, in his comments to the Rambam's Sefer Ha-mitzvot, adopted a famous position regarding shemitta fruits. Most Rishonim claim that these fruits possess kedushat shevi'it, which limits the range of uses a person can employ. According to the Ramban, there is an actual mitzvat esei to eat these fruits (similar to ma'aser sheni and teruma – note, in fact, the comparison drawn between these items in the mishna 8:2). The Ramban bases his comments upon the employment of the term 'akhila' in two instances regarding shemitta fruit. First the Torah writes "The year of shabbat ha-aretz shall be for you to EAT." Most Biblical commentators assume that this pasuk does not legislate eating but rather allows or qualifies it. Rashi, (in his commentary to the Torah) for example, interprets that "you should eat from hefker and not from your storehouses," reflecting the prohibition of storing shemitta fruit. Another derasha in Torat Kohanim (cited by the Chizkuni) derives from this pasuk the prohibition of offering sacrifices from shemitta produce (lakhem le-akhla ve-lo le-havi menachot unesachim).
The gemara in Avoda Zara (62), however, infers that one must EAT the produce and not use it for profit, while a parallel gemara in Pesachim (52b) claims "le-akhla ve-lo le-hefsed' - it is to be eaten and not to be wasted. Most opinions understand these derashot as forbidding business ventures with shemitta fruit or wasteful uses. Once these deviations are prohibited it is obvious that the produce must be eaten in the normal fashion. However, there is no particular mitzva to eat the fruit, but only a commandment to avoid wasting it. According to the Ramban, the primary mitzva relates to the obligation to EAT. Obviously, one who sells for profit or wastes is not eating and thus does not fulfill the mitzva esei while he also violates a negative commandment.
The Ramban also cites the pasuk in Mishpatim: ve-okhlu evyonei amekha (the poor will eat the shemitta produce). He takes this verse as a positive command that the fruit must be eaten; otherwise the Torah would have employed a more generic term to convey that the poor receive the fruit.
In addition to the Biblical terms from which the Ramban infers his position, there is a gemara (cited by the Megillat Esther in his commentary to the Sefer Ha-mitzvot) which also implies this halakha. The Yerushalmi in Shvi'it informs us that we do not obligate an individual to eat shemitta products which have become spoiled (pat she'ipsha); the implication is that we do obligate the eating of non-spoiled produce.
Even if we adopt the Ramban's position, it should be clarified that this mitzva of eating is unlike other mitzvot of eating, such as eating matza during Pesach. Normally, the mitzva is a chiyuv gavra - an obligation incumbent upon an individual with regard to a particular item. On Pesach night each Jew has a personal obligation to consume a kezayit of matza. Even according to the Ramban, a person does not have an obligation to actively pursue eating shemitta fruit. The mitzva rather relates to the produce itself which must be eaten rather than wasted. According to most Rishonim there is no mitzva in the eating, but by eating one prevents a situation of waste, which would be prohibited. The Ramban takes this notion one step further by mandating actual eating. However, the eating is only to ensure that the food not be wasted (as the aforementioned derashot suggest).
Questions which immediately spring to mind based upon this distinction include the following:
1) Must a person eat a kezayit in order to fulfill the asei? Even if he eats less than a shiur for a ma'aseh akhila, he is still ensuring that the produce is eaten and not wasted.
2) Does the mitzva apply to someone who doesn't possess shemitta fruit? A person is obligated to acquire matza in order to fulfill his mitzva. Possibly, the same requirement would not apply to shemitta fruit. If a person has fruit he is obligated to eat and not waste, but he has no obligation to pursue the mitzva.
3) Would a berakha be recited if the mitzva does not apply directly to the person performing an act of eating?
It should be noted that this issue - the nature of the mitzva according to the Ramban - might depend upon the source of the mitzva. Assuming he derives it from the term in Behar – 'vehayta shabbat ha-aretz lakhem le-akhla,' it appears logical that the mitzva doesn't relate to the person proper but to the produce itself. If, however, the true source is the term in Mishpatim, "ve-akhlu evyonei amekha," then it might reflect a personal mitzva upon the person (since the active tense 've-akhlu' is employed, rather than the passive tense, 'lakhla').
For a similar example, in which there is a mitzva to eat but not one which is incumbent upon the person, please see methodology shiur #11 Eating Korban Pesach (5758) which cites the approach of the Beit Halevi about the differences between the mitzva to eat korban Pesach and that of eating regular korbanot.