White kidney bean extract - what is its structure and is it possible to get it just by eating kidney beans?

[Alex K Chen]

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Alpha-amylase (α-amylase) is an enzyme that has a major role in carbohydrate metabolism [22], catalysing the hydrolysis of the α-(1,4) glycosidic linkages in starch and other oligosaccharides [23]. This may represent a potential mechanistic target for interventions seeking to inhibit carbohydrate uptake in the body. Indeed, bioactive ingredients that inhibit catabolic enzymes (i.e., amylase and glucosidase) are becoming increasingly popular [24], with potential applications for both weight loss and metabolic health.

White kidney beans (Phaseolus vulgaris L.) possess three isoforms of α-amylase inhibitors—α-AI1, α-AI2, α-AIL [25], with the α-AI1 isoform shown to inhibit amylase activity in mammals [26]. The structure of α-AI1 is a classic lectin fold, containing no α-helices and a plethora of antiparallel β-sheets [23]. α-AI1 has no carbohydrate-binding abilities due to the complete absence of surface carbohydrate-binding loops on its three-dimensional structure [26]. Functioning optimally within a pH range of 4.5–5.5 and a temperature range of 22–37 °C [26]. α-AI1 has been suggested to interact with pancreatic α -amylase at its active site, competitively inhibiting its binding with carbohydrate [26]. α-AI1 can block the enzyme’s substrate-reducing end and obstruct the non-substrate-reducing end through a steric hindrance process, targeting all catalytically-competent components of the enzyme [22]. Several commercially-available white kidney bean extracts (WKBEs) now exist which reportedly contain high quantities of white kidney bean-derived α-amylase inhibitors, typically available in powder form, allowing their encapsulation or incorporation into drinks and foods (for further information on the preparation of these extracts, see [26]). These supplements claim to reduce carbohydrate absorption and digestion via their α-amylase inhibitory properties, with attendant beneficial effects on body weight and metabolic health. Whilst this has been the subject of two meta-analyses, one was published almost ten years ago [27] and the other focussed on a specific brand (Phase 2) [28], and other forms of WKBE might be differentially effective. Therefore, in the following section, the evidence for and against these claims is reviewed.