Carb (or starch) blockers are made from vegetable sources, particularly white kidney beans. White kidney bean extract works by blocking or reducing the activity of the enzyme alpha-amylase . Amylase helps digest starchy carbohydrates, which are found in foods such as potatoes, bread and pasta. When amylase is blocked, those carbs pass through the body undigested, so you don't absorb the calories.
Some starch blockers such as acarbose (Prandase®, Precose®) are available only on prescription. These are used for blood sugar control in people with type 2 diabetes. However, there are also starch blockers that are sold without a prescription as weight loss supplements. These may also contain other ingredients, such as appetite suppressants and stimulants, some of which may be dangerous for some people.
Most commercial preparations contain Phase 2® product, a standardised white bean extract. Recommended dosage: 1-2 tablets (each containing 500 mg) x 3 times a day, taken before meals with liquid, for a total of 1,500 to 3,000 mg per day.
Manufactures of XLS Medical Carb Blocker  claim that their formulation will help reduce calorie absorption from carbohydrates by two thirds (66%). They also claim that it can help you reduce your waist circumference by 8cm in 4 weeks. However, they do not make any claims in relation to overall weight loss.
The truth is that, when it comes to shedding pounds, the evidence is not as clear. Several studies conducted in the '80s showed that the bean extract in carb blockers had no effect on weight loss. Subsequent studies, conducted at the Mayo Clinic in America, determined that much higher doses than those found in most commercial carb blockers — namely, 4,000 to 6,000 mg of bean extract and 4,000 mg of wheat germ extract — did slow carbohydrate digestion  and reduced the increase in blood glucose levels following a meal . Unfortunately, the carb blockers that can be found on the market contain 500 mg of these extracts at most, which is not even close to the supposedly "effective" dosage.
More recently, clinical studies using commercial preparations have shown that Phase 2 may promote weight loss only when taken concurrently with meals containing carbohydrates .
No serious side effects have been reported from ingestion of white bean extracts . However, since alpha-amylase inhibitors prevent the breakdown of complex carbohydrates - those carbohydrates will pass through the intestine into the colon. In the colon, bacteria will digest the complex carbohydrates, and this may initially cause gas, bloating, stomach cramping and diarrhoea.
If you are expecting a miracle weight loss solution, the chances are that you will be disappointed. For carb blocking supplements to be effective they need to contain potent and scientifically proven extracts. And, even then, they will only be somewhat effective provided you monitor your carb intake and follow a restricted-fat/calorie diet.
But don’t despair. If you are looking to lose some weight, or manage blood sugar levels, there is another option: add fibre to your diet to help you feel fuller for longer. Start by replacing simple carbohydrates with complex carbohydrates and look for foods with a low glycemic index (GI). Low GI foods that are high in fibre include whole grains, leafy vegetables, most fruits, and legumes. I can’t promise that you will be able to squeeze into that skimpy swimsuit before the end of summer. What I can say is that, by increasing your fibre intake and adopting a generally healthy diet, you will lose weight gradually. More importantly, you’ll be able to keep it off!
Written by Victoria Trowse