I don't think humans have a digestive system capable of using leafy or cruciferous veggies as a major source of calories... nearly all of their calories are in the form of fermentable fiber which supplies energy by fermentation in colon to short chain fats. Unlike hindgut fermenter herbivores our colon isn't large enough to supply much energy this way.
Dying or becoming severely physically/mentally ill is very likely going to significantly lower the output of your utility function, so it would probably be a very bad idea to ignore the low-hanging resources which can significantly extend the time for which you are alive and well. I have attempted to search LessWrong for a list of such resources, and haven't been able to find one.
Are there any books, websites, or posts that contain significantly low-hanging fruit in this area? If so, please list them in the comments below.