Feeding Soy Protein Concentrates with Low and High Isoflavones Alters 9 and 18 Weeks Serum Isoflavones and Inflammatory Protein Levels in Lean and Obese Zucker Rats

Soy's anti-inflammatory properties contribute to the health benefits of soy foods. This study was designed to investigate the bioavailability of soy isoflavones and whether the isoflavone content of soy protein concentrate diet would affect serum inflammatory proteins in an obese (fa/fa) Zucker rat model. Six-week-old male lean (L) and obese (O) Zucker rats were fed a casein control diet (C), soy protein concentrate with low isoflavones (SPC-LIF), or soy protein concentrate with high isoflavones (SPC-HIF) (7 rats/dietary group) before being killed at 9 and 18 weeks. Serum samples were analyzed for isoflavones and inflammatory proteins. At both time points, serum total (aglycone + conjugates) genistein, daidzein, and equol concentrations were significantly higher in L-SPC-HIF and O-SPC-HIF groups compared with L-SPC-LIF and O-SPC-LIF groups, respectively, and were not detectable in either L-C or O-C groups. At week 9, serum C-reactive protein (CRP) concentration was significantly lower in O-SPC-HIF group compared with O-C and O-SPC-LIF group, whereas proteins tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) levels did not differ between any groups. At week 18, serum CRP levels in both O-SPC-HIF and O-SPC-LIF groups were significantly lower compared with the O-C group. TNF-alpha level was higher in the O-SPC-LIF group compared with both O-C and O-SPC-HIF groups, whereas IL-6 levels were not different between any groups. Taken together, feeding Zucker rats SPC-LIF and SPC-HIF diets led to different serum isoflavone concentrations in both L and O Zucker rats and altered CRP and TNF-alpha levels in obese Zucker rats compared with controls.