Is one permitted to make a neder to eat less if he’s obese? Somebody who’s tried everything, and knows that if he would be noder he would 100% be able to keep it. It’s very important for his health that he lose weight, and he feels that this is the only way.
Before getting to your question, there is no such thing as making a neder to eat less, that might be included in a shevuah, but not a neder. In order to make a neder he has to make the item forbidden to him, similar to hekdesh, such as saying that potato chips, pretzels, danishes, soda, etcv. are forbidden to me…
Being obese is a very serious situation, and you should do everything in your power to help yourself. On a practical level though it is a not a good idea, and don’t do it. Before you are in the situation it sounds nice, but making nedorim is a very serious aveiro, especially when the chances are significant that you won’t be able to keep it. If you want to make sure you don’t eat, let’s make a deal. You will send me, (or anyone else that you aren’t particularly interesting in giving money to) $50 dollars, for my personal use, not to tzedakah, but as a present to me as a fine for overstepping the rules of what you may eat. You will deposit the money that day into by personal bank account, via electronic transfer on the very day that you violated the terms of what you may eat. If you violate the terms more than once a day it will be $50 for each time you violate the terms and payment is due on that day. I’m sure that this sounds rough, but it will work. By the way, breaking a neder has much worse of a consequence than the couple of hundreds of dollars that this deal will cost you.