Dietary soy protein reverses obesity-induced liver steatosis and alters fecal microbial composition independent of isoflavone level

Non-alcoholic fatty liver disease (NAFLD) is a major public health concern that is exacerbated by the obesity pandemic. Dietary interventions have the potential to alleviate obesity-associated NAFLD through variable mechanisms, including optimizing the gut microbiota. Previously, we reported that soy protein concentrate (SPC) with low or high levels of isoflavone (LIF or HIF) protected young obese Zucker rats from developing liver steatosis. The current study was designed to test whether SPC-LIF and SPC-HIF diets would reverse liver steatosis and alter fecal microbial composition in adult obese Zucker rats with existing steatosis. Compared to CAS diet, feeding SPC-LIF and SPC-HIF diets resulted in significantly lower liver weight, liver steatosis score, and liver microvesicular score, but did not lead to difference in body weight, liver macrovesicular score, serum ALT, or serum AST. Isoflavone levels (e.g. LIF vs. HIF) did not affect any of these measurements except the SPC-HIF group had an additional decrease in liver weight compared to the SPC-LIF group. SPC-HIF group also had significantly higher levels of all aglycone forms of and total daidzein, genistein, and equol compared to SPC-LIF or CAS diet fed rats. When taken together, dietary soy protein can reverse liver steatosis in adult Zucker rats, and the reversal of steatosis is accompanied by alterations in gut microbial composition.