What is a carb blocker? A carb blocker is also known as a starch blocker. They are supplements of white kidney bean extract, or Phaseolus vulgaris extract. White kidney bean extract is an alpha amylase inhibitor. What are the effects of carb blocker? Carb blockers work by blocking the production of a digestive enzyme called alpha amylase. Alpha amylase is the enzyme responsible for the digestion of carbohydrates. Carb blockers are used as weight loss supplements because of this ability to reduce the production of alpha amylase.