Dietary Soy Isoflavone–Aglycone Lowers Food Intake in Female Rats With and Without Ovariectomy

Abstract

Objective: Estrogens downregulate eating behavior, and soy isoflavones are known to be estrogenic agents. We aimed to examine whether the estrogenic property of soy isoflavones can affect food intake and body weight. Methods and Procedures: Seven-week-old male, female, and ovariectomized (OVX) Sprague-Dawley rats were given free access to a diet containing 100-300 mg total isoflavone/kg diet, or to a control diet, either with or without concurrent administration of estradiol by subcutaneous implantation. Results: Dietary soy isoflavone was shown to lower food intake in female rats, whether or not the animals had undergone ovariectomy. Administration of estradiol lowered the food intake in male rats and in OVX female rats. The decrease in weekly food intake in female rats led to a reduction in their weekly gain in body weight. Dietary soy isoflavone significantly increased the concentration of serum isoflavones, especially equol (a metabolite of daidzein), regardless of gender or ovariectomy. Dietary soy isoflavone did not affect either serum estradiol concentration or uterine and didymus weights, but estradiol administration improved the uterine atrophy in OVX rats, and decreased the didymus weight in male rats. Discussion: Soy isoflavone lowers the food intake in female rats, but not in the male animals. Contrary to the hypothesis currently in vogue, the reduction in food intake caused by soy isoflavone may not be a purely estrogenic effect. This follows from the finding that the effects of soy isoflavones on food intake and on the reproductive organs differ from the corresponding effects produced by estrogen.

Dynamics of appetite-mediated gene expression in daidzein-fed female rats in the meal-feeding method

“…However, we have also observed that daidzein feeding and estradiol treatment significantly decreased food intake in both female LongEvans Tokushima Otsuka and Otsuka Long-Evans Tokushima Fatty (CCK1R-deficient) rats, suggesting that CCK signaling is not essential for the anorectic effects of daidzein and estradiol. 41) In rats given food on the MF3 schedule as well as rats fed ad libitum, 11) daidzein feeding significantly decreased food intake 6 days after initiation of the experiment (Fig. 2(A)), leading us to speculate that dietary daidzein as it is not able to suppress food intake by directly inducing the expression of CCK mRNA in intestinal endocrine cells.…”

“…11) Further, estradiol treatment in OVX rats recovered uterine weight to levels similar to those seen in sham rats, but daidzein feeding did not. 11) These observations suggest that anorectic effect of daidzein might be mediated by a different mechanism than estradiol. However, daidzein appears to function as selective estrogen receptor modulators 14) which are a group of estrogen receptor agonist/antagonist with tissue selective effects.…”

“…9) In ERα knockout mice, administration of estradiol did not change food intake, suggesting that the effect of estrogen on food intake is mediated via ERα. 10) We 11) and others 12) found that dietary daidzein reduced food intake in female rats, while genistein feeding did not change food intake. Guo Y et al also reported that daidzein feeding induced an anorectic effect in obese mice fed high-fat diets.…”

“…We showed that 17β-estradiol treatment reduced food intake in both male and female rats. 11) Meanwhile, feeding with an isoflavone aglycone-rich fermented soybean extract or dietary daidzein decreased food intake in female rats but not in male rats. 11) Further, estradiol treatment in OVX rats recovered uterine weight to levels similar to those seen in sham rats, but daidzein feeding did not.…”

“…11) Meanwhile, feeding with an isoflavone aglycone-rich fermented soybean extract or dietary daidzein decreased food intake in female rats but not in male rats. 11) Further, estradiol treatment in OVX rats recovered uterine weight to levels similar to those seen in sham rats, but daidzein feeding did not. 11) These observations suggest that anorectic effect of daidzein might be mediated by a different mechanism than estradiol.…”