You can eat healthily at MacDonalds, but it is difficult and you have to know what is what.
Don't eat the bad, and do eat the good.
To me, "good" means that it has protein, or vitamins and minerals, and it does not have much carbohydrate (sugar).
Most of what you can eat at MacDonalds is BAD, but the hamburger itself and the lettuce and tomato will not put up your blood sugar. Diet Coke will not put up your blood sugar.
Regular Coke will put up your blood sugar.
It is up to you to choose what you let pass into your mouth... so let pass Diet Coke instead of regular Coke. Don't let the French Fries in... don't let the bread in... Chocolate milk shake is not great, but it is better than chocolate cake.