Is it better for weight loss to have one day of water fasting or one day of drinking only protein shakes

Neither is definitively better for weight loss: the bigger factor is whether the day creates a calorie deficit. Water fasting may make that deficit larger, while protein shakes can help you get protein that supports lean mass. 1 2

1. A 24-hour water fast: More extreme fasting may be less favorable for maintaining lean mass; the evidence here does not establish that a single 24-hour fast is better for weight loss than a protein-shake-only day. 3 4

2. Protein shakes: Protein supports lean tissue, and two or three high-quality protein meals may help provide it; resistance training is an important tool for maintaining lean mass. 5 6