Feeding Obese Zucker Rats With Soy Protein Concentrate With High Isoflavones Compared to Low Isoflavones Leads to a Significant Reduction of Serum Tumor Necrosis Factor-α

OBJECTIVES: Soy isoflavones have been proposed to be the functional compounds that mediate the health benefits of soy-rich foods. This study was designed to investigate whether level of isoflavone content (high or low) of soy protein concentrate diet would affect serum TNF-α concentration in an obese Zucker rat model. METHODS: 42 six-week old lean (L, n = 21) and obese (O, n = 21) Zucker rats were randomly assigned to one of 3 dietary groups after one week of acclimation: casein diet (C = control), soy protein with low isoflavones (LIF), or soy protein with high isoflavones (HIF) for 18 weeks (n = 7 rats/group). Rats were weighed twice weekly. After 18 weeks, rats were sacrificed and serum samples were collected. Serum TNF-α concentrations were determined by ELISA and compared between groups. RESULTS: At the end of 18 weeks of dietary treatment, obese rats gained significantly more weight than lean rats in all three diet groups. In lean rats, there were no significant differences in body weight or serum TNF-α concentration between any diet groups (C, LIF, or HIF). In contrast, in obese rats, both LIF and HIF groups had significantly higher body weight compared to control group. While O-LIF and O-HIF groups did not differ in body weight, O-HIF group had significantly lower serum TNF-α concentration, suggesting a decreased level of systemic inflammation compared to O-LIF group. CONCLUSIONS: HIF soy protein diet significantly decreased serum TNF-α concentration in obese Zucker rats compared to LIF soy protein diet independent of body weight. Isoflavones may be responsible for the observed anti-inflammatory effect. Because systemic inflammation contributes to the etiology of a plethora of diseases, soy isoflavones may exert their many health benefits partially through downregulating systemic inflammation. FUNDING SOURCES: This project is support USDA #12,889,791 to RH.