In the Modern Orthodox day schools I attended in my childhood, I heard numerous times that there is a mitzvah to eat healthy and get exercise. The verse that teachers would quote for this mitzvah was either u-shemor nafshekha (Deuteronomy 4:9) or ve-nishmartem me’od le-nafshoteikhem (Deuteronomy 4:15). Both of these verses can be approximated with the same translation, “Guard your nefesh.” However, if we look at these verses closely, both in their context in the Written Torah, and the history of their interpretations in the Oral Torah, we will see that a broad mandate for healthy living is far from obvious.
U-shemor Nafshekha in the Bible
We begin by quoting the context of these verses in Deuteronomy 4:5-19:
- See, I have imparted to you laws and rules, as the Lord my God has commanded me, for you to abide by in the land that you are about to enter and occupy. 6. Observe them faithfully, for that will be proof of your wisdom and discernment to other peoples, who on hearing of all these laws will say, “Surely, that great nation is a wise and discerning people.” 7. For what great nation is there that has a god so close at hand as is the Lord our God whenever we call upon Him? 8. Or what great nation has laws and rules as perfect as all this Teaching that I set before you this day? 9. But take utmost care and watch yourselves scrupulously (u-shemor nafshekha), so that you do not forget the things that you saw with your own eyes and so that they do not fade from your mind as long as you live. And make them known to your children and to your children’s children: 10. The day you stood before the Lord your God at Horeb… 12. The Lord spoke to you out of the fire; you heard the sound of words but perceived no shape— nothing but a voice…. 15. For your own sake, therefore, be most careful (ve-nishmartem me’od le-nafshoteikhem)—since you saw no shape when the Lord your God spoke to you at Horeb out of the fire— 16. not to act wickedly and make for yourselves a sculptured image in any likeness whatever: the form of a man or a woman, 17. the form of any beast on earth, the form of any winged bird that flies in the sky, 18. the form of anything that creeps on the ground, the form of any fish that is in the waters below the earth. 19. And when you look up to the sky and behold the sun and the moon and the stars, the whole heavenly host, you must not be lured into bowing down to them or serving them….
In context, the verses are clearly an imperative against theological corruption, not physical corruption. As all of the medieval exegetes make clear, the Torah is warning the reader not to forget the commandments that God gave and not to make any physical image of God (see, for example, Ramban and Bekhor Shor on 4:15). There is no indication anywhere in this section, or in the commentary of the parshanim, of anything having to do with maintaining the health of one’s body.
U-shemor Nafshekha in the Talmud
Of course, Scriptural context only takes us so far in a Halakhic discussion. Far more important is to see how Hazal interpreted it in the Oral Torah. The first place we encounter it is in a peculiar story in Berakhot 32b-33a:
The Sages taught: There was an incident, involving a particular pious man [hasid] who was praying while traveling along the road when an officer came and greeted him. The pious man did not respond with a greeting. The officer waited for him until he finished his prayer.
After he finished his prayer, the officer said to him: “You good for nothing! Isn’t it written in your Torah: ‘Take utmost care and guard yourself diligently’ (u-shemor nafshekha; Deuteronomy 4:9)? And it is also written: ‘Take therefore good heed unto yourselves’ (ve-nishmartem me’od le-nafshoteikhem; Deuteronomy 4:15)? Why did you ignore the danger to your life? When I greeted you, why did you not respond with a greeting? Were I to sever your head with a sword, who would hold me accountable for your spilled blood?”
The pious man said to him: “Wait for me until I will appease you with words.” He continued, “Had you been standing before a flesh and blood king and your friend came and greeted you, would you return his greeting?”
The officer said to him: “No.”
The pious man said: “And if you would greet him, what would they do to you?”
The officer said to him: “They would cut off my head with a sword.”
The pious man said to him: “This is a matter of kal va-homer [a fortiori inference]—You, who were standing before a king of flesh and blood, who today is here but tomorrow he is in the grave, would have reacted in that way. I, who was standing before the Supreme King of kings, the Holy One, Blessed be He, Who lives and endures for all eternity—all the more so.”
The officer was immediately appeased and the pious man returned home in peace.
While this passage clearly does interpret the verse as an admonition to avoid behavior that will lead to bodily harm, the interpretation comes not from any rabbi, but from a Roman officer, and the hasid offers a kal va-homer that would be more applicable to this case than the Roman’s suggestion anyway. At best, we can perhaps infer from the rabbis’ lack of arguing with the interpretation itself that it was accepted. Most rishonim have little to say on this passage and very few clues as to whether they take it as halakhically authoritative or not. Even if we assume they did, it certainly did not have any prominent place in most of the practical halakhic guidebooks or codes of the Middle Ages.
Rambam and Shulhan Arukh
There are a small number of rishonim who do cite this verse as a source for a prohibition on causing bodily harm, most notably Rambam. He codifies it in Hilkhot Rotzeah U-shemirat Nefesh (11:4-6) following the requirement to have a parapet, or fence, around one’s roof:
Halakhah 4: Both a roof and anything else dangerous that a person is liable to stumble on and die, for example, if a person has a well or a cistern in his courtyard, whether they contain water or not, a person is obligated to make a wall ten tefahim high around them or make a cover for them, so that a person will not fall in and die.
So too, any obstacle that contains a mortal danger, it is a positive mitzvah to remove it, and to protect oneself from it, and to be very careful regarding these matters, as it says, ‘Take utmost care and guard your nefesh.’ If a person does not remove it and leaves a dangerous obstacle, he negates a positive commandment, and violates the negative commandment: “Do not cause blood to be spilled.”
Halakhah 5: Many things were prohibited by the sages because they involve a mortal danger. Anyone who violates them, and says: ‘I will endanger myself, and what does this matter affect others?’ or ‘I am not careful about these things,’ we punish him with lashes for rebelliousness.
Halakhah 6: These are they: a person should not place his mouth over a pipe through which water flows and drink, nor should he drink at night from rivers and lakes, lest he swallow a leech without seeing, nor should a person drink water that was left uncovered, lest a snake or other poisonous crawling animal have drunk from them, and as a result of drinking it, the person would die.
While Rambam calls u-shemor nafshekha a positive commandment, it is notable that in his list of the commandments at the beginning of the section, he does not mention it. Additionally, he does not indicate any new obligations or prohibitions on the basis of u-shemor nafshekha that would not have already been included in the negative commandment of “Do not cause blood to be spilled.” It seems like u-shemor nafshekha is being brought as an additional value or motivation for the rabbis to enact the subsequent rabbinic prohibition, rather than as a formal command bearing any specific imperative in its own right. When he says in Halakhah 5 that a person who violates these prohibitions can be punished with lashes for rebelliousness, the simplest explanation is that he is referring to the lashes the rabbis can give for the violation of any rabbinic commandment.
It is worth noting that Shulhan Arukh does seem to read Rambam somewhat more broadly than this. In Hoshen Mishpat 427, he begins in se’ifim 7-9 by quoting the aforementioned halakhot of Rambam almost verbatim. However, in se’if 10, he adds:
Anyone who violates these things, or things similar to them, and says: ‘I will endanger myself, and what does this matter affect others?’ or ‘I am not careful about these things,’ we punish him with lashes for rebelliousness. And one who is careful about these things will receive a blessing of good.
The addition of the phrase, “or things similar to them,” makes it clear that he assumes one can receive lashes not merely for the specific rabbinic prohibitions of placing one’s mouth over a pipe, drinking from rivers and lakes at night, or drinking water that has been left uncovered, but for any similarly dangerous activities. He, therefore, clearly understands that the lashes are for the violation of u-shemor nafshekha and not for the violation of the specific rabbinic prohibition.
Regardless of whether one takes the Shulhan Arukh’s expansive reading of the lashes or a narrower view, one thing is clear. The only things included in this commandment are behaviors that could pose an imminent mortal danger. There is no broad mandate for a good diet and healthy lifestyle. This point is perhaps made most clear not where Rambam mentions u-shemor nafshekha, but where he does not mention it. In Hilkhot De’ot 4:1, Rambam does discuss the importance of a generally healthy diet and lifestyle. He introduces the section by saying:
Since having a healthy and sound body is among the ways of God—for it is impossible to understand or to know [anything of the knowledge of the Creator] if he is [too] sick—a person must distance himself from things that harm the body and behave in a manner that makes him healthy and strong.
Rambam here appeals to an intuitive logic that one cannot properly serve God through learning Torah and doing Mitzvot if his body is not healthy. He makes no mention of any specific Biblical command. Surely, had Rambam thought healthy diet and lifestyle were included in the command of u-shemor nafshekha, this would have been the place to mention it.
Kitzur Shulhan Arukh
The first time we find any usage of u-shemor nafshekha as mandating anything beyond avoiding imminent mortal danger is in the 19th century in Kitzur Shulhan Arukh. He begins his section on the importance of a healthy lifestyle as follows:
Since having a healthy and sound body is among the ways of God—for it is impossible to understand or to know anything of the knowledge of the Creator if he is [too] sick—a person must distance himself from things that harm the body and behave in a manner that makes his body healthy and strong, as the Torah states, ‘Be most careful and guard your nefesh.’
He takes the language from Rambam Hilkhot De’ot about the importance of maintaining a healthy lifestyle and the intuitive logic motivating it, and conflates it with the biblical command of ve-nishmartem me’od le-nafshoteikhem. One can only speculate as to what motivated him to do this. Perhaps the logic that was intuitive for Rambam in the cosmopolitan medieval Islamic world in which he lived was not as intuitive in the shtetl world of Eastern Europe, and therefore people would not be motivated without the force of a Biblical verse. Regardless, Kitzur Shulhan Arukh was an immensely popular work among the general Jewish population in Eastern Europe, and there is no doubt that this is the source for the now common understanding of the verse that I received from my elementary school teachers.
Iggrot Moshe and Other Contemporary Poskim
While Kitzur Shulhan Arukh’s broad expansion of the scope of u-shemor nafshekha took hold in the minds of the general public, it is unclear how many poskim actually accepted it. Rabbi Moshe Feinstein, in his Iggrot Moshe, authored numerous responsa on medical and health issues, and only once does he reference u-shemor nafshekha: in his 1981 responsum on smoking. While he advises strongly against smoking, he argues forcefully that it would not fall under the prohibition of u-shemor nafshekha. He writes:
It appears obvious that regarding something that causes no detriment to the health of many people, such as types of food that people enjoy very much—like fatty meat, or very spicy food—even though it may harm the health of some people, there is no prohibition to eat it on account of danger, since the majority of people are not endangered by it. See Rambam in chapter 4 of Hilkhot De’ot, where he describes which foods and drinks are good for the health of the body or bad for the health of the body. He does not use the language of prohibition, neither Biblical, nor Rabbinically prohibited by the Sages, as he did when he wrote about removing an obstacle that presents a potential mortal danger in Hilkhot Rotzeah 11:4 that it is forbidden to place it there and a mitzvah to remove it—even if it is a danger only to himself— based on the positive Biblical command of, ‘Take utmost care and guard your nefesh,’ and the negative command of, “Do not cause blood to be spilled.”
He explicitly rejects the approach of Kitzur Shulhan Arukh that includes Rambam’s general health advice from Hilkhot De’ot under the rubric of u-shemor nafshekha.
Rabbi Shmuel Wosner, in his Shevet Ha-levi, argues that smoking cigarettes would be included in the prohibition of u-shemor nafshekha. However, from the way he presents it, it would seem that the most likely explanation is that he sees a higher level of danger in cigarette-smoking than Iggrot Moshe did, not that he has a more expansive view of what is included in u-shemor nafshekha. He writes:
Rambam says in Hilkhot Rotzeah U-shemirat Nefesh 11:5, ‘Many things were prohibited by the sages because they involve a mortal danger. Anyone who violates them, and says: “I will endanger myself, and what does this matter affect others?” or “I am not careful about these things,” we punish him with lashes for rebelliousness.’ Included in this list are types of food and drink that the Sages prohibited because they present a potential mortal danger, and Rambam listed them there, as well as Ritva in Shevu’ot 27, and wrote that eating these foods that damage the body are included in the Biblical prohibition of, ‘Take utmost care and guard your nefesh.’ It is the responsibility of the sages of our time to proclaim and make known the grave danger involved in smoking cigarettes. It has been verified through thorough investigation beyond all doubt that hundreds of thousands die before their time because of smoking cigarettes. It is also well-known that it is a major cause of severe illnesses of the lungs and heart, and many other things. This has been found in the research of doctors in every country in the world. It is therefore clear that the Halakhah is that it is absolutely forbidden to begin smoking. (Shevet Ha-levi 10:295)
Note the extent to which he goes to compare smoking to the things which Rambam prohibits in Hilkhot Rotzeah and makes no mention of Rambam in Hilkhot De’ot.
Several other poskim as well, such as Rabbi Ovadia Yosef and Rabbi Yitzchak Weiss, make mention of u-shemor nafshekha only in contexts where actual mortal danger is being discussed. Their rejection of Kitzur Shulhan Arukh is not explicit the way it is in Iggrot Moshe, and we cannot prove anything absolutely from silence. Nevertheless, we certainly have no proof that any of these major poskim, in their extensive responsa literature, accepted a broad mandate for a healthy lifestyle as included in the mitzvah of u-shemor nafshekha.
In contrast with the approach of these poskim is the approach of Rabbi Eliezer Waldenberg in his Tzitz Eliezer. Unlike Iggrot Moshe, who dealt with many medical questions, and only mentioned u-shemor nafshekha once, the verse features prominently in many of the medical responsa in Tzitz Eliezer. Many, if not most, of these uses are not surprising. They relate to classic cases of mortal danger, and fit with the simple reading of Rambam and Shulhan Arukh. However, there are several cases that do not fit this rule, which give us insight into his overall approach. In Tzitz Eliezer 17:2, he deals with a patient who did not want medical treatment because he views it as a lack of faith in God. The doctors want to know if they should attempt to persuade him otherwise. He writes in response:
It is certainly permitted to persuade him to accept medical treatment. Additionally, explain to him that it is a foolish piety not to accept the needed medical treatment, because the Torah said, ‘Be most careful and guard your nefesh.’
He makes no distinction between whether the medical treatment is for a situation of imminent mortal danger or not. Indeed, if the patient viewed it as a lack of faith to accept medical treatment even in cases of imminent mortal danger, all the more so we can presume he would refuse it if there was no imminent mortal danger. We see from Rabbi Waldenberg’s response that he includes in ve-nishmartem me’od le-nafshoteikhem a broad mandate to maintain one’s health and do whatever the doctors believe is necessary, not merely to avoid situations of imminent mortal danger. In doing so, he is clearly following the approach of Kitzur Shulhan Arukh.
In Tzitz Eliezer 15:44, Rabbi Waldenberg is asked about marrying relatives (e.g., first cousins). The relatives in question are people whose relationship would not be considered incestuous under Torah law. Nevertheless, the doctors say that marrying these relatives will increase the likelihood of having children with severe birth defects. In summarizing his response, he writes:
It is impossible to say that marrying these relatives is absolutely forbidden…but it is certainly permitted [for doctors] to warn about the risks. One who decides to refrain [from the marriage] is not violating any prohibition since he is concerned for his nefesh because of how strictly we treat danger. He will receive reward for this refraining, since he is doing it in fulfillment of ‘Be most careful and guard your nefesh.’
His response here is fascinating. One the one hand, he recognizes the distinction between cases with imminent mortal danger and those without. He will not say something is absolutely forbidden without imminent mortal danger. On the other hand, he clearly sees actions related to one’s health (or one’s children’s health) as being a fulfillment of the same mitzvah of ve-nishmartem me’od le-nafshoteikhem, even when they present no imminent mortal danger (and perhaps no mortal danger at all—he does not elaborate on specifically which kind of birth defects he was concerned about). He is the only posek we have seen who seems to have such a category, and once again seems to be clearly adopting the approach of Kitzur Shulhan Arukh that all health concerns are really being addressed by ve-nishmartem me’od le-nafshoteikhem.
We can further see how Rabbi Waldenberg views this mitzvah in one of his responsa that deals with a situation he believes to present imminent danger. In the course of answering the question, he writes a brief excursus on how the mitzvah is derived:
Rambam Hilkhot De’ot 4:1 rules, ‘Since having a healthy and sound body is among the ways of God – for it is impossible to understand or to know anything of the knowledge of the Creator if he is [too] sick—a person must distance himself from things that harm the body and behave in a manner that makes him healthy and strong.’
On the basis of these words of Rambam, we can understand the words of Rambam Hilkhot Rotzeah 11:10, where he derives the positive mitzvah to remove an obstacle that could pose a mortal danger and to be very careful about this from the verse, ‘Take utmost care and guard your nefesh.’ (Deuteronomy 4:9). See Maharsha in Berakhot 32b, commenting on the story about the pious man, whom the officer challenged on the basis of this verse. Maharsha notes that this verse is speaking about forgetting the Torah, as it says, ‘lest you forget the things.’ Likewise the verse of, ‘Be most careful and guard your nefesh,’ is dealing with not believing in any physical form of God. Neither verse is dealing at all with a person guarding his nefesh from physical danger.
However, based on Rambam Hilkhot De’ot it makes sense how a warning to protect one’s body is also included, because when the body is not healthy and sound, this can also cause one to forget the Torah, as Rambam wrote, ‘For it is impossible to understand or to know anything of the knowledge of the Creator.’ (Tzitz Eliezer 21:8)
He makes explicit here the connection between Rambam Hilkhot De’ot and Rambam Hilkhot Rotzeah and that he sees them both as part of one big picture: a mitzvah to keep your body as healthy as possible in order to maintain proper knowledge of Torah. This explicit connection leaves no further doubt that Rabbi Waldenberg interprets Rambam in the same manner as the Kitzur Shulhan Arukh.
The notion of deriving a mitzvah to protect one’s body from u-shemor nafshekha or ve-nishmartem me’od le-nafshoteikhem is of questionable authority in the Talmud, and there may well have been rishonim who did not accept it at all. Nevertheless, Rambam and Shulhan Arukh, followed by most contemporary poskim, do accept such a mitzvah. However, based on the simple reading of Rambam and Shulhan Arukh, the scope of the mitzvah only includes avoiding situations with the potential for imminent mortal danger. While Rambam does talk about the importance of general health concerns, he appears to base this idea on logic and does not explicitly connect it to this mitzvah.
The first source to make the connection between general health concerns and u-shemor nafshekha or ve-nishmartem me’od le-nafshoteikhem is Kitzur Shulhan Arukh. His approach seems to have taken hold in the minds of much of the current Jewish population, especially elementary school teachers. Nevertheless, most contemporary poskim seem to have rejected such an approach in favor of the simple reading of Rambam, and Iggrot Moshe makes this rejection explicit. Only Tzitz Eliezer appears to have accepted the approach of Kitzur Shulhan Arukh as normative. It is worth noting, as well, the surprisingly few sources available that make use of u-shemor nafshekha at all, despite its prominence in popular discourse.
The goal of pointing this all out is not, God forbid, to downplay the importance in Judaism of maintaining one’s health. Surely, as we are hopefully emerging from a global pandemic, I do not want anyone to come away from this article thinking that I am advocating for the Halakhah to be less strict with regard to our physical safety. Rather, my goal is to emphasize the significance of logic as the source of this importance. As we see from Rambam, and most contemporary poskim, not everything God expects of us can be subsumed under a specific mitzvah. God also expects us to use our heads, and figure out for ourselves what we need to do in order best to serve Him. This message has the potential to speak even more powerfully to the children in the elementary schools as to the importance of maintaining one’s health than two verses in Deuteronomy.
 The Hebrew word nefesh can be translated in the Torah as either body or soul. To preserve that ambiguity, I will simply leave the word untranslated as nefesh.
 For an overview of contemporary scholarship about Talmudic dialogues between rabbis and non-Jews, see Sarit Kattan Gribetz and Moulie Vidas, “Rabbis and Others in Conversation,” Jewish Studies Quarterly 19:2 (2012): 91-103, and the articles they cite there.
 The only other mention in the Talmud of this verse is Shevu’ot 36a, which cites it as a source for the prohibition against cursing oneself. There, Tosafot s.v. U-shemor Nafshekha Me’od understand this as being related to the prohibition against causing bodily harm, though they express astonishment that the Gemara would bring this verse to prohibit cursing oneself and never bring it to prohibit injuring oneself. However, see Hatam Sofer ad loc. s.v. U-shemor Nafshekha who interprets that Gemara in a way that does not at all relate to a prohibition against causing bodily harm.
 Prominent medieval works that do not include any reference to either of these verses in the context of not causing bodily harm include, but are not limited to, Sefer Mitzvot Gadol, Sefer Mitzvot Katan, Orhot Haim, Torat Ha-adam, Sefer Ha-hinukh, Sefer Hasidim, Abudraham, and Kol Bo.
 See also Shu”t Ha-rashba, siman 1, who cites it in a very similar context to Rambam.
 This and all subsequent translations are my own.
 See Shabbat 40b, Yevamot 52a, Ketubot 45b, Nazir 23a, Menahot 70a, and Hullin 141b for examples of the use of “lashes for rebelliousness.”
 See Jonathan Ziring “‘Eat, Drink, and be Merry, for Tomorrow We Die’: Is There an Obligation to Maintain Good Health?” Verapo Yerape 4 (2012): 184, n. 56, where he makes note of this difference (in my name), but is not convinced of its significance.
 The lashes cannot be for the violation of the negative command of, “Do not cause blood to be spilled,” because, as Rambam says in Halakhah 3 of this same chapter, it is a prohibition that does not contain an action. Additionally, had he been referring to lashes for the violation of a Biblical negative commandment, he would not have used the phrase “lashes for rebelliousness.”
 The bracketed phrase appears in the Vilna printing of the Mishneh Torah. However, see the notes in the Frankel edition, where it is indicated that it is absent from most manuscripts.
 Kitzur Shulhan Arukh 32:1.
 See, for example Y.L. Maimon’s introduction to Kitzur Shulhan Arukh (Jerusalem: Mossad Harav Kook, 1987), 13-14.
 Iggrot Moshe Hoshen Mishpat 2:76.
 See Yabia Omer Yoreh Deah 1:8 et al.
 See Minhat Yitzhak 8:148.
 See Tzitz Eliezer 8:15.7, 9:17.4, 9:17.5, 10:25.7, 10:25.21, 12:43, 15:37, 17:8, and 21:8.
 This article only addressed one aspect of the halakhic obligation towards one’s health; for a full treatment of the issue, including other halakhic principles that are at play in this discussion, see Jonathan Ziring “‘Eat, Drink, and be Merry, for Tomorrow We Die’: Is There an Obligation to Maintain Good Health?” Verapo Yerape 4 (2012): 172-192.