Is there a halachic issue with intermittent fasting and/or with restrictive diets such as keto or paleo? Or even diets in general? Excluding the issue of washing for Shabbos/Yom tov?
Shalom, Thank you for your question. Before answering allow me to make a disclaimer – it is certainly a religious obligation to take care of one’s health, including finding a healthy diet. However, as to exactly which diets are healthy for each person is an issue that one needs to turn to a health professional. This is all the more true when it comes to fasting – please consult with a doctor as to the advisability of undertaking any such fasts. Having said that, let’s turn to the halachic issues that might be involved in fasting, and then in diets. When it comes to fasting, Judaism traditionally understands that there is a religious place for fasting. This is in order to help attain forgiveness for sins, to arouse remorse and atonement, to mark certain tragedies in Jewish history etc. I assume from your question that you are referring to fasting for health reasons. While we do not find specific sources mentioning such fasts, if one were to be certain that such fasts were in fact beneficial for one’s health, they would be allowed. However, there are certain limitations. Firstly, if by undertaking such a fast it would limit a person’s ability to perform other mitzvot (through lack of strength), then it is usually forbidden. So, if by fasting a person will not have the strength they need to pray, learn or do their normal acts of kindness to others, one should refrain from fasting. Secondly, there are certain days that fasting is forbidden on (except for certain rare instances that do not apply here). One is not allowed to fast on Shabbat or Festivals. Also, fasting on a Friday is forbidden if it will mean that one cannot partake of the Shabbat meals as a result of their fasting. As to dieting – in general Judaism does not prefer one diet over another (except for keeping kosher of course). We do find that the Rabbis in the Talmud suggested certain foods, and times for eating. However, a person should follow the medical advice of their day and age, and what is best for their particular situation, as in matters of health each place, time and person will have their own specific advice for their best health, and the Talmud was only referring to its time and location. There are several points one needs to taking into account. Firstly, there is a command to enjoy the Shabbat with special meals. One can certainly keep to a diet on Shabbat – but one should find a balance between their diet and the Shabbat enjoyment. To keep to a restrictive diet that brings one to a place of feeling “starved” or in a battle of wills against their desire to eat certain foods, is certainly not in the spirit of Shabbat (and I’m not so certain that it is in the spirit of a good diet! But I’ll leave that to for the dietitians). One should organize their Shabbat meals, while staying within their diets, so as to have some special foods on Shabbat – perhaps planning their weeks intake giving Shabbat a bit more than the other days when one could cut back. Or enjoying some special foods just on Shabbat – even if only in small measures. Also, when it comes to Shabbat a person has to both have Kiddush and wash and have bread for the Friday night meal, and the Shabbat day meal (it is preferable to wash and have bread for the third meal also, but this can be fulfilled without bread if one finds it difficult to eat bread). If Kiddush wine is not part of your diet plan, you can fulfill the mitzvah by letting someone else say Kiddush and include you. In such a case you do not have to drink yourself (it is preferable to have even just a small taste of the wine, but from the letter of the law you can forgo it). When it comes to eating bread – unless you have a medical condition that prohibits eating bread, you should eat bread at both these meals. If bread is not included in your diet, you can eat the minimum amount. That is a “keziyit” - about the size of half a slice of sliced bread. This should be eaten in the space of 4 minuets (rather than in small amounts over the course of the whole meal). Of course, you can choose the best bread for your diet – whole grain, sugarless, etc. You do not have to eat the traditional white challah found on most Shabbat tables. If you are making the blessing you will need two complete loaves of your healthy bread for each meal. I hope this brief outline will be of help to you – and please write back with any other questions you have. May you be blessed with only good health.