It will defeat the purpose of fasting significantly as the goal is to reduce insulin resistance and get into ketosis and eventually autophagy as fast as you can, you will still lose weight if that’s ur primary goal, but at a lower rate and your ketosis will be slower.
You will lose weight and it will drop fast. I’ve done it without thinking, I was really busy and stressed and visiting Starbucks 4 times a day for weeks. Anytime you are in a calorie deficit you will lose weight. But you won’t look healthy. You look tired and aged from the coffee and sugar. You won’t look glowing and rested as you would with a water fast or eating raw vegetables.