Water flavored with a small amount of lemon is typically negligible in calories and for many people does not meaningfully disrupt fasting goals. However, large amounts of lemon juice, added honey/sugar, or lemonade-style drinks add calories and effectively end the fast. Lemon acidity can worsen reflux/gastritis when taken on an empty stomach. If you want a strict “zero-calorie” fast, plain water is the cleanest. Lemon water is not a “detox”; it is simply a way to make hydration easier. Real outcomes come from overall energy balance and food quality.