Supplements made from white kidney beans ( Phaseolus vulgaris ) are sold as starch blockers, supplements said to interfere with the digestion of carbohydrates and thereby promote weight loss.

Therapeutic Uses

Technically, starch blockers are amylase inhibitors . Amylase is one of the main enzymes the body uses to digest starch. In theory, when amylase is blocked, starch can pass through the body undigested. If the body is unable to digest a part of the meal the calorie intake will be lower which can aid in weight loss.

However, theory is one thing, reality another and clinical studies of amylase inhibitors have not established definitive benefits. 1-3 For example, one published study failed to find that use of a phaseolus product reduced the usual blood sugar rise that follows a meal. 7 Several possible reasons for this discrepancy have been proposed, such as that the amylase inhibitor may be broken down and made ineffective in the stomach, the product may supply enough of its own amylase to counteract any benefit, and that another enzyme, glucoamylase, may be able to take over when amylase can’t do the job. However, according to the manufacturer of a current product, more concentrated extracts of Phaseolus vulgaris, taken in higher doses, do work. 4-6 Follow up research conducted primarily by the manufacturer have had mixed results.

A randomized trial of 39 obese participants compared phaseolus vulgaris extract supplement vs. placebo before 2 meals everyday. Participants were instructed to concentrate their carbohydrates to the two meals associated with the supplement. There was no significant difference in weight loss at 8 weeks. 11 However, a second randomized trial with 25 overweight participants took phaseolus vulgaris extract supplement vs. placebo as part of comprehensive weight loss program that included behavioral change, nutritional counseling, calorie restriction plan, and provided meals.12 After 4 weeks, the supplement group had a weight loss of 6 lbs (2.7 kg) vs 4.7 lbs (2.1 kg) with placebo which was statistically different. Those that had the highest carbohydrate intake in the supplement group had largest weight loss. A third randomized trial of 101 participants compared phaseolus vulgaris extract supplement vs. placebo.13 After 60 days the average weight loss with supplement was 4.1 lbs (1.9 kg) vs 0.88 lbs (0.4 kg) with placebo which was also statistically different.

Additional trials were completed with supplement mixes that included a phaseolus extract and at least one other ingredient. A double-blind, placebo-controlled study with 60 slightly overweight people were given either placebo or a phaseolus extract once daily 30 minutes prior to a main meal rich in carbohydrates. Over the thirty days of the study, the results indicated that the supplement was associated with a significantly greater reduction of body weight and and improvement of lean/fat ratio as compared to placebo.8 Two additional trials also showed higher weight loss in supplement group but the results were not statistically compared so it is unclear if the mixed supplement was significantly effective.9,10 In addition to statistical shortcomings these 3 trials only compared the mixed supplement to placebo so it is unclear what specific element or combination of elements were actually responsible for weight loss.

A systematic review and meta-analysis including the above randomized trials that was published in 2011 could not draw firm conclusions about the benefits of phaseolus vulgaris for weight loss.16 The conclusions of the review found that all of the trials had serious flaws in trial design or interpretation and the meta-analysis found no statistically significant difference in weight loss between the supplements and placebo. However, the supplement was associated with a statistically significant reduction in body fat compared to placebo. There were limits with both meta-analysis because of heterogeneity of results included. A randomized trial completed after the systematic review included 124 participants who were randomized to phaseolus vulgaris supplement or placebo for 12 weeks in addition to lower calorie diet plan. At the end of 12 weeks, there was an average weight loss of 6.41 lbs (2.91 kg) for those with the supplement compared to an average of 2 lbs (0.92 kg) for those in placebo. There were no adverse events reported.17

Independent confirmation from larger rigorously designed trials will be necessary before phaseolus can be considered a proven weight-loss product and who it is most beneficial for.


The recommended dose of amylase inhibitors varies among products. Follow label instructions.

Safety Issues

On the basis of their widespread presence in commonly consumed foods (beans), amylase inhibitors are believed to be quite safe. One side effect, however, is to be expected: flatulence. It is the amylase inhibitors in beans that are responsible for their notorious gassiness.

Maximum safe doses in pregnant or nursing women, young children, or individuals with severe hepatic or renal disease have not been established.