Regulatory Behavior and Skin Temperature in Mid-Aged Male Rats on Three Different Isoflavone-Containing Diets

Soy isoflavones, the most abundant phytoestrogens, are known as endocrine modulators and appear to be an effective treatment in some women during perimenopause when symptoms such as hot flashes may be reduced. This study examined the effects of dietary soy isoflavones on regulatory behaviors such as body weight, food and water intake, and skin tail temperature by feeding male Long-Evans rats one of the three standard chows (Phyto-free, Phyto-200, and Phyto-600) containing approximately 10–15 ppm, 200 ppm, and 600 ppm of isoflavones, respectively. In an apparent dose-dependent manner, body weight was decreased as a function of increasing isoflavone levels in the diets. The average skin tail temperature of Phyto-600-fed rats (25.5°C) was significantly lower than Phyto-free (27.5°C) values by approximately 2°C. The rats on the Phyto-200 diet (26.6°C) displayed a temperature in between the Phyto-free and Phyto-600 values. Similar to the body weight results, skin tail temperature was decreased in an apparent dose-dependent manner as a function of increasing isoflavone concentrations in the diets. These results suggest that consumption of soy isoflavones alters regulatory behaviors (such as body weight and food and water intake) and skin temperature regulation that may help explain, in part, the beneficial effects of soy isoflavones on obesity and hot flashes in humans.