High-protein/low-carb diets can lead to quick weight loss in the first few days, but people almost always regain the weight once they resume their normal carb intake.

Eating too few carbs can cause:

  1. Gastrointestinal problems. A good amount of the fiber we need comes from carbohydrates. Continued low-carb eating can result in constipation, diverticulosis (a condition marked by painful infection in the intestines) and increased risk of certain cancers.
  2. Cardiovascular problems. Because these diets call for eating a lot of protein -- most easily found in foods like red meats and cheeses -- individuals take in much more cholesterol and saturated fat than they normally would. A less dire consequence of low-carb eating: bad breath.