Protein and fat from healthy animals, no seed oils, and (almost) no processed grain food, you should be good to go for 100 years. Our gut bugs do want us to eat starchy carbs, but they should be natural, like rice, a few grains alone, not baked into crap, and best starches from vegetable roots like tubers. 1 serving or less of fruit a day. Rare dessert. If it doesn’t cause digestive problems, even a little natural, minimally processed bread would be ok.
It’s more a cumulative thing. If you eat a healthy, clean diet all week, of course you can share someone’s birthday cake.