Effects of soy protein supplemented with methionine on blood lipids and adiposity of rats

OBJECTIVE: The effects of soy protein isolate (SPI) versus casein on blood lipids and adiposity were investigated in rats fed methionine-equivalent diets. METHODS: Twenty-eight male Sprague-Dawley rats (230 to 250 g) were assigned in equal numbers to groups consuming SPI- or casein-based diets (20%) supplemented with L-methionine. After 28 d, blood was collected for triacylglycerol, total cholesterol, and high-density lipoprotein cholesterol assessment and epididymal fat pads were weighed. RESULTS: Food intake (519 +/- 90 versus 490 +/- 115 g), weight gain (144 +/- 35 versus 133 +/- 28 g), food efficiency ratio (0.29 +/- 0.09 versus 0.28 +/- 0.06), epididymal fat pad weights (5.409 +/- 2.076 versus 4.768 +/- 1.867 g), and serum concentrations of triacylglycerol (96.3 +/- 41.8 versus 93.4 +/- 37.4 mg/dL) and high-density lipoprotein cholesterol (32.6 +/- 7.4 versus 33.8 +/- 4.4 mg/dL) were similar between the casein and SPI groups, respectively. However, total cholesterol (73.8 +/- 17.8 versus 59.3 +/- 11.9 mg/dL) concentration was higher for the casein-fed rats than for the SPI-fed rats, respectively (P < 0.05). CONCLUSIONS: These results suggest that methionine supplementation may eliminate the decreased fat deposition previously ascribed to soy protein; however, methionine did not abolish the commonly observed hypocholesterolemic effects of soy.