Quantification of “cholesterol lowering” soyasaponins in legumes and assessment of their bioaccessibility

Legumes, particularly lentils, are a primary dietary source of food saponins, which are bioactive compounds that have been demonstrated to possess multiple health-promoting properties, such as reduction of cholesterol levels, anticarcinogenic and antihepatotoxic properties, and antireplicative effects against HIV.1 There is evidence that dietary saponins (including soyasaponins) can lower plasma cholesterol values; this occurs directly by inhibiting absorption of cholesterol from the small intestine or, indirectly, by inhibiting the reabsorption of bile acids. Reduced entry of cholesterol or bile acids into the enterohepatic circulation results in the stimulation of cholesterol synthesis mainly by the liver.2 Legumes mainly contain soyasaponin I (soyasaponin βb) and soyasaponin βg (also called soyasaponin VI), both belonging to the B group soyasaponins. The aim of this work was the quantification of SS I and SS βg in raw and cooked legumes of different provenience and variety (60 samples) by a SPE-HPLC-MS method developed in our lab.3 Results of this study showed that the 60 raw legume samples analyzed had a content of soyasaponin I that ranged from 636 to 907 mg kg-1, while that of soyasaponin βg ranged from 647 to 1807 mg kg-1. Soybean contains the highest level of soyasaponins, and also pea and chickpea could be considered a great source of “cholesterol lowering” soyasaponins. Additionally, it is important to understand absorption and metabolic process of soyasaponins in the body. Hence, six legume samples were chosen for cooking studies for the investigation of the amount of soyasaponins that persists after cooking. Changes in the soyasaponins content were investigated for the first time in chickpea, lentil, bean, pea and chuckling after the seeds were soaked and cooked in distilled water. Cooking did not modify the total soyasaponins content; soyasaponin βg was partially degraded during cooking to soyasaponin I. Moreover, a small amount of soyasaponins leached into the cooking solution, and the percentage was related to the analyzed legume. Moreover, a study on the bioaccessibility of SS I and βg contained in lentils was assessed by using an in vitro digestion model, to understand the amount that can reach the duodenum and colon and that can explicate the cholesterol lowering activity. To our knowledge, no studies have been reported so far in the literature concerning the bioaccessibility of soyasaponins in food. Recently, in our laboratory a new, selective, and fast analytical procedure for the quantification of soyasaponins I and βg in lentils using HPLC-MS/MS triple quadrupole has been developed. With this method, the purification step (SPE) has been eliminated and, through the use of a triple quadrupole as mass analyzer, the specificity and sensitivity have been enhanced, compared to previous HPLC-MS published methods. This method will be very useful for future studies on soyasaponins in legumes. References 1. J. Hu, S. O. Lee, S. Hendrich, P. A. Murphy, J. Agric. Food Chem., 50, 2587−2594 (2002). 2. R. G. Ruiz, K. R. Price, A. E. Arthur, M. E. Rose, M. J. C. Rhodes, R. G. Fenwick, J. Agric. Food Chem., 44, 1526–1530, (1996). 3. G. Sagratini, Y. Zuo, G. Caprioli, G. Cristalli, D. Giardinà, F. Maggi, L. Molin, M. Ricciutelli, P. Traldi, S. Vittori, J. Agric. Food Chem., 57, 11226−11233, (2009).