" How Safe Are Liquid Diets? Ideally, liquid diet drinks should give you a balance of nutrients you need throughout the day, but that isn't always the case. Very low-calorie diets (400-800 calories per day) in particular can be lacking in a balance of protein, carbohydrates, fats, vitamins, and minerals you need and should only be used under medical supervision. Missing out on essential nutrients can lead to side effects such as fatigue, dizziness, hair loss, gallstones, and heart damage. Also, if you don't get enough fiber, because you're not eating whole grains, fruits and vegetables, you can get constipated. You also can lose muscle if you don't get enough protein calories in your diet. Are Liquid Diets Used for Medical Purposes? Doctors sometimes recommend liquid diets before certain medical procedures, or for people who are obese and need to get to a safer weight before having surgery, including weight loss surgery. Those liquid diets are medically supervised."