What effect would eating fast food during testing, like mcdonalds for example, have on urine tests, such as those for stones provided by litholink?

None: Fast food, while loaded with salt, grease, and calories, isn't going to affect your routine lab work. Since you're working on weight loss and are clever enough to have found HeathTap, why not get with a real dietician and / trainer who can help you? You're young and can look forward to many happy years if you get toned up now. Best wishes.