Supplements made from white kidney beans, Phaseolus vulgaris , are widely promoted as a type of “starch blockers,” a weight-loss aid said to work by interfering with the digestion of starch. Phaseolus contains a substance that inhibits the enzyme amylase, one of the primary enzymes the body uses to digest carbohydrates. In theory, if the enzyme amylase cannot function properly, some proportion of ingested starch might pass through the body undigested, reducing effective calorie intake.
However, theory is one thing, reality another. Up until recently, published studies of amylase inhibitors from Phaseolus and other sources have generally failed to find them effective.
Several possible reasons for this failure have been proposed. For example, the amylase inhibitor contained in the supplement may break down in the stomach. Some sources of amylase inhibitors also supply amylase, perhaps counteracting their own benefits. Finally, the body possesses an alternative starch digester known as glucoamylase, which may be able to take over when amylase can’t do the job. Whatever the cause, in these older studies, use of amylase inhibitors did not in fact block the digestion of starch nor aid weight reduction.
A few studies did report benefit with a concentrated amylase inhibitor product, but because these trials were never published in complete form they failed to make any scientific impact. The situation changed in 2007 when the full text of a relevant trial was published in an international journal of some (if modest) repute.
In this double-blind, placebo-controlled study, 60 slightly overweight people were given either placebo or a highly concentrated phaseolus extract once daily. This single daily dose was given a ½ hour prior to a large meal rich in carbohydrates.
Over the 30 days of the study, use of Phaseolus led to a significantly greater reduction of body weight as compared to placebo. Relatively greater benefits were also seen in measurements of waist, hip and thigh circumferences, as well as body mass index and fat mass.
These results are quite promising. However, keep in mind that only independent confirmation in large trials can actually prove Phaseolus effective.