If you absolutely have to eat fast food and I don't know why that would ever be, but if you have to here are some counter measures. First you can take some digestive enzymes but the need to be from a whole, raw food source. Second you need to take any where from 4 to 8 pro biotic tablets. These need to be anywhere from 6 trillion different types of good bacteria and higher.
Both of these will aid your body in removing the crap you just ate, and helping you to digest a bit more of it as well. My advice is to use the same rules when eating out that you do when searching for raw foods at a grocery store. Go to a grocery store instead of a fast food restaurant there never to far apart. If not, only eat things living like a salad or fruit no french fries ever! There is no excuse for you to eat those or any other deep fried heart stopper. If you want a burger eat it without the bun even though it has been proven that fast food place receive lower grade beef than other restaurants.
One thing above all else don't go to a fast food restaurant you can make it home you will be fine call it a fast if you have to, whatever it takes.
You will be better for it!