Say one short sentence, give no reason, and ask them something else. That is the technique, and it works because a reason is an opening and a sentence with no reason in it has nothing to argue with.
"I'm good, thank you. Did you make the sauce, or is that a family thing?" Eight words and a question, and the subject has moved. Nobody at that table will think about it again, which is the outcome you actually want. If you would rather not have the moment happen at all, the reliable structural fix is to arrive holding a dessert of your own, which is what our 12 desserts to bring to a party exists for.
The rest of this is what to say, in what order, and what to do when the person does not take the exit.
Why a reason makes it worse
The instinct is to explain. It feels more polite, and it is the thing that lengthens the conversation every single time.
"No thanks, I'm off sugar at the moment" is an invitation, whether you meant it that way or not. It hands the table a topic with two sides. Somebody will say they read that sugar is fine in moderation. Somebody else will say their cousin did keto. A third person will ask whether fruit counts. None of these people are being rude, and the conversation is now twenty minutes long and about you.
A sentence with no reason in it cannot be argued with, because there is nothing in it to disagree with. "I'm good, thanks" is not a claim about nutrition. It is a decline, which is a normal thing that happens at tables.
The sentences, roughly in order
Start at the top and only go down if you have to.
"I'm good, thank you." Complete. Add a question about something else and it is finished.
"I've had plenty, it was great." Best when you actually ate the meal, because it is true and it compliments the cook at the same time.
"Not tonight, but it looks great." The escape hatch that implies a future yes without promising one, useful with a host who takes food personally.
"I'm not eating sugar right now." Only when the person genuinely needs to know, usually because they are about to make you something or are hosting you for several days. It gives a reason, so expect a follow-up question.
"Doctor's orders, but I'll take a coffee." The one that ends discussions fastest, because almost nobody argues with a doctor. Use it when it is true and you want the topic closed.
Notice that none of these describe a diet. The moment you name a plan, you have handed someone a thing to have an opinion about.
You do not owe anyone your medical history
If the reason is prediabetes, PCOS, insulin resistance, a gut condition, or anything else you got from a doctor rather than from a magazine, you are allowed to keep that to yourself at a dinner table.
That is worth saying plainly because a lot of people feel they have to disclose to be believed. You do not. "I'm good, thanks" is a complete answer from a person with a diagnosis and from a person without one, and nobody is entitled to know which you are.
If the reason is PCOS specifically, it is also worth knowing that the cravings themselves are a mechanism rather than a discipline problem, which is the subject of why PCOS makes you crave sugar. That is a useful thing to understand for yourself. It is not a thing you have to explain to a table.
Telling people is also fine, and sometimes easier, especially with family you will see repeatedly. That is a decision about who you want to tell, made once, and not something you should feel obliged to redo every time a plate comes round.
The person who keeps pushing
Occasionally somebody will not let it go. There is a ladder for this and it has three rungs.
Repeat yourself, word for word. Not a new explanation. The same sentence, same tone, with a small smile. "I'm good, thank you." Most people stop here, because repetition without irritation signals that there is no new information coming.
Name it lightly, once. "You're going to have to let me win this one." Said warmly it lands as a joke, and it also tells the whole table that the subject is closed. It works because it does not accuse anyone of anything.
Leave the topic physically. Get up, help clear something, start a conversation with the person on your other side. Two people cannot have a debate if one of them is in the kitchen.
What does not work is out-arguing them. Winning a nutrition argument at a dinner table costs you the evening and changes nobody's mind. The goal is not to be right, it is for the subject to end.
The host who looks hurt
This is the one that actually stings, because the person is not being difficult. They made something and they wanted you to have it.
The fix is to be specific about something you did eat. "The chicken was so good, I went back twice" does far more work than any amount of apologising about the dessert, and it is not a lie if you did go back twice. Hosts want to have fed you. Give them the evidence that they did.
If you can, take a piece for later and mean it. A wrapped slice going home with you is a genuine kindness to a person who cooked, and what happens to it after that is nobody's business.
When the table turns it into a debate
Sometimes the question is not about you at all. Somebody wants to talk about sugar, or seed oils, or what they read about sweeteners, and you have been nominated as the expert because you are the one not eating the cake.
You are allowed to decline that job. "Honestly I just follow what my doctor says" or "I have no idea, I just know what works for me" both end it, and neither one requires you to defend a position.
If you do want to talk about it, that is a different evening and usually a smaller group. The mechanism stuff is genuinely interesting and there is a lot of nonsense on packaging, which is why we wrote about how to read a sugar-free label. A dinner table with nine people at it is the wrong venue for it.
What to do with the plate in front of you
If a slice arrives anyway, take it. Say thank you. Eat some, or move it around, or leave it.
Nobody is watching your plate. People believe they are being observed far more closely than they are, and the half-eaten dessert at the end of a party is the least noticed object in the room. Refusing a plate as it is being handed to you is more visible than accepting it and not finishing it.
Family, which is the hard version
A stranger asking once is easy. A relative who takes it personally, every holiday, is a different problem, and no clever sentence solves it in the moment.
That one gets handled outside the meal, in a quiet conversation, once. Something like: this is not about your cooking, I love your cooking, and I would rather it stopped being a thing every time. Said privately, without an audience, it usually works. Said at the table, with everyone listening, it becomes an argument about respect and nobody wins those.
And if the reason underneath it is medical, that private conversation is also where you decide how much to share. Anything about what your body is doing belongs between you and your care team, not between you and the table. I can write about what is in a dessert. What any of it means for you is theirs to answer.
The thing that removes the question entirely
Arriving with food is the structural fix, and it beats every sentence in this post.
If you walk in holding a tray, you are participating before anyone has a chance to notice what you are not eating. You have a reason to be near the food table. When the dessert goes round, you take one of yours, which is an entirely normal thing for the person who brought them to do.
The question never gets asked, so none of the sentences above are needed. That is why the bring-something strategy is worth the forty minutes it takes on a Saturday morning.
The short version
One sentence, no reason, then a question about something else. Repeat it word for word if you need to. Compliment something you did eat. Do not take the expert job, do not argue about nutrition at a table, and do not feel you owe anyone your diagnosis.
And when you can, arrive holding something, so the conversation never starts.
