You don't give up groceries (or medicine or heating oil or....) to give money to adult children. You don't have to feel guilty about it. You don't owe them an explanation. You can opt to tell them you are on a fixed income but I'm assuming they know you don't have a job. Send a card (buy 2 for $1 at the local dollar store or discount house) and buy a small booklet of stamps. Include a few words about how much you love them or share a special memory about them when they were younger.
If anyone complains, they are not the fine offspring you think you raised.
Since they don't do anything for you (which could be because they weren't raised to go the extra mile for someone else, or because they are clueless/thoughtless, or because they don't know it's important to you), they can't possibly be offended. If they are, you can take the opportunity to say it's been very one-sided or that you are quite broke and unable to afford groceries. Maybe they are in a position to help you, maybe not, but certainly they will no longer be in a position to expect more from you.
Hopefully they are good and decent sons who wouldn't want you to jeopardize your health o send them cash. Hopefully they are working. And it's time for them to stand on their own two feet economically.