Introduction: Overweight and obesity are major risk factors for the development of metabolic diseases as well as cancer and mainly affect the elderly population over 55 years of age. Besides a decrease in physical activity and an increased intake of energy-dense foods, an age-related decline in sexual hormones can contribute to the development of obesity late in life.
Epidemiologic as well as in vitro and in vivo experimental evidence suggests that the intake of soy isoflavones (ISO) can have beneficial effects on adipose tissue and fat metabolism. Moreover, animal studies in rodents indicate that ISO act gender specific, showing anti-adipogenic effects in females but adipogenic effects in males. Due to their spatial configuration, ISO binds to the estrogen receptor (ER) and can exert estrogenic but also anti- estrogenic effects on several target tissues. Furthermore, ISO are proved to exert non-ER-mediated effects through numerous other signaling pathways involved in adipose deposition and lipid metabolism.
This study investigates the effects of lifelong dietary ISO intake on fat metabolism and development of obesity in male and female Wistar rats. Therefore, obesity was induced by adipogenic factors such as western diet (WD) and inhibition of sexual hormone production by gonadectomy (ovarectomy = OVX; orchiectomy = ORX). In particular, the study focuses on the comparison of effects in male and female animals.
Methods: Animals were exposed to ISO starting in utero via exposure of the dams, continued during lactation period by mother’s milk and maintained to adolescence by dietary intake up to postnatal day (PND) 170/173 (f/m). During the first experimental period, animals grew up on ISO free control diet (CON) or ISO enriched control diet (CON ISO; 467 mg/kg diet). On PND 83/85, animals were divided into seven groups. Two intact control groups received CON diet with or without ISO up to study termination (CON intact and CON ISO intact). Four groups switched to a western diet rich in sugar and fat with ISO (WD ISO; 431 mg/kg diet) or without ISO on PND 85/87 after two of these groups were gonadectomized (WD OVX/ORX and WD ISO OVX/ORX) and two were left intact (WD intact and WD ISO intact). One further group of OVX/ORX animals, which was nursed ISO free, switched to ISO enriched WD on PND 85/87. This group was intended to mimic short-term exposure in andro- /menopause. The study was terminated on PND 170/173 and animals were subjected to necropsy where body weight, weight of visceral adipose tissue, sexual organs, selected skeletal muscles and adipocyte sizes in perirenal adipose tissue were determined. In blood samples, ISO concentration and respective ISO metabolites as well as leptin and lipid levels
were analyzed. Levels of mRNA gene expression of several markers associated to lipogenic and inflammation processes were determined in liver (females only) and adipose tissue. Results: ISO plasma concentrations of 330 – 1400 nM and 814 – 3641 nM were achieved in females and males, respectively. Due to a lower food intake, lowest plasma levels were obtained in WD fed animals and highest levels in intact CON animals. In females, glucuronides were the main phase II ISO metabolites in plasma, whereas in males, disulfates and sulfoglucuronides were the main metabolites and the portion of sulfate conjugates was diminished by ORX. Furthermore, females showed higher amounts of ISO aglycones in plasma than males.
In females, lifelong but not short-term ISO exposure resulted in reduced body weight, reduced visceral adipose tissue, adipocyte sizes and serum leptin levels. This effect was more pronounced in lean CON intact than in obese WD OVX females. ISO did not affect increased lipid levels induced by WD.
In males, ISO slightly diminished adipose deposition, leptin and triglyceride levels only in intact WD animals without affecting adipocyte sizes. In addition, ISO increased weight gain after ORX induced weight loss without affecting visceral fat mass.
OVX induced muscle weight loss of M. soleus was antagonized by short-term and lifelong ISO exposure in females, but no effect was observed in muscle mass of ORX males.
Hepatic gene expression of lipogenic markers SREBP 1c, FAS and ACC was down- regulated by lifelong ISO exposure in CON intact and WD OVX females. On the contrary, SREBP 1c, FAS, ACC and PPARγ gene expression was up-regulated in adipose tissue of intact CON females. In adipose tissue of CON intact males, expression of SREBP 1c and ACC was down-regulated by ISO. Furthermore, a WD induced decrease of SREBP 1c, FAS and ACC expression was antagonized by ISO in males.
ISO decreased the expression of inflammatory marker genes IL6 and MCP1 in adipose tissue of obese intact and OVX females fed WD and increased the expression of anti- inflammatory marker IL10 in WD OVX females. The effect of WD on expression of inflammatory marker genes was less pronounced in males than in females and no statistically significant differences in expression levels could be obtained by ISO exposure. Furthermore, no biologically relevant weight changes in uterus, testes, seminal vesicles or prostate were observed after ISO exposure.
Discussion: Main results obtained within this study suggest that a lifelong ISO exposure affects adipose tissue in a gender specific way, while reducing adipose deposition in females but only to a less extend in males. Increased weight gain and adipose deposition as described for male mice in the literature were not observed. Further, results indicate that body composition was affected in both sexes, while a gonadectomy induced loss of muscle
mass was shown to be antagonized by ISO in females only. XV
The less pronounced effects on adipose tissue in males are assumed to mainly result from a gender specific phase II metabolism of ISO, lower plasma amounts of biologically active aglycones or a reduced bioavailability of ISO in target tissues. Moreover, effects of ISO appear to depend on plasma concentrations achieved by dietary intake. Substantial effects were attained at ISO plasma levels of 1000 – 2400 nM, which can similarly be achieved by soy-rich diets in East Asian human populations. At very low ISO plasma levels of < 1000 nM in females and very high levels of 3641 nM in males less pronounced or no effects were obtained on adipose tissue.
In the absence of effects on sexual organ weights, no indications for severe acute endocrine disrupting effects were observed on sexual organs at achieved plasma concentrations.
ISO appeared to modulate mRNA gene expression of signaling pathways involved in lipid metabolism of liver and adipose tissue and adipose inflammatory processes. A possible hypothesis is that ISO improves dynamics of fat cell turnover in lean females, while it appears that in males, regulation of de novo lipogenesis plays a minor role and improved utilization of fatty acids and triglycerides might most be responsible for slight reductions in adipose deposition and triglyceride levels of intact animals.
Conclusion: The results of this study suggest that a lifelong dietary ISO exposure resulting in plasma levels that can similarly be achieved in human diminishes visceral adipose deposition and hypertrophy in female, but not in male rats. A balanced adipose tissue homeostasis can be beneficial in the prevention of obesity, associated metabolic diseases and cancer. Human relevance needs to be addressed in further investigations.