Do sex and age influence the metabolic and cardiovascular consequences of caloric restriction?
Thomas, Benjamin James
Cardiovascular disease (CVD) is the leading cause of death worldwide and metabolic diseases such as obesity and type 2 diabetes increasingly contribute to this burden. Caloric restriction (CR) increases glucose tolerance and insulin sensitivity, decreases fat mass, and improves diastolic function, and thus has emerged as a nutritional intervention to decrease the incidence of CVD and metabolic diseases. However, previous research from our laboratory suggests that, compared to males, the metabolic effects of CR are absent or blunted in female mice. As metabolic health status influences cardiovascular health, this raises the question of whether a sex difference in the metabolic effects of CR also extends to the cardiovascular effects. Further, studies in rodents and humans have shown that some effects of CR are influenced by age and sex, yet the extent and basis of this remains unknown. Thus, this project tested the hypotheses that 1) the metabolic and cardiovascular effects of CR are sexually dimorphic, 2) this sexual dimorphism disappears with age, and 3) that oestrogen (E2) signalling through oestrogen receptor alpha (ERα) in young females underlies observed sex differences. To test the hypothesis that the metabolic effects of CR are sex and age dependent, I investigated the effects of 30% CR initiated in male and female C57BL/6N mice from 9-15 or 78-84 weeks of age (young and old, respectively). CR robustly decreased fat mass in young males, old mice of both sexes, but young females resisted this effect. Instead, young females lost mass entirely through decreased lean mass. Further, young male mice increased glucose tolerance and insulin sensitivity in response to CR, but female mice increased glucose tolerance only, and to a lesser extent; both sexes of aged mice increased glucose tolerance, with males increasing insulin sensitivity to a greater extent than females. The time of feeding (morning vs evening) did not abolish this sexual dimorphism in young mice. These data confirm the effects of CR on body composition and glucose homeostasis are sexually dimorphic in young mice, and that the sexual dimorphism extends to insulin sensitivity but not body composition or glucose tolerance in old mice. To test the hypothesis that the cardiovascular effects of CR are sex and age dependent, I analysed the cardiovascular phenotype of the young and old mice described above. Echocardiographic analysis revealed that CR enhanced systolic and diastolic function, but in older mice only, regardless of sex. This was not accompanied by any detectable effect of CR on fibrosis, and although cardiomyocyte cross sectional area was reduced in old mice by CR, this also occurred in young mice and again there was no impact of sex. These data show that while CR impacted on cardiac function and cardiomyocyte size, sex or age did not consistently influence these effects. To test the hypothesis that E2 and ERα signalling underlie the resistance of young female mice to the metabolic effects of CR, 7-week-old C57BL/6N mice underwent ovariectomy and subcutaneous capsule implantation to administer either E2, the ERα-selective agonist propylpyrazoletriol (PPT), or a vehicle control. Mice then underwent 30% CR from 9-13 weeks of age and metabolic outcomes were assessed. Ovariectomy increased CR-induced fat and body mass loss, but did not influence the CR-induced increase in glucose tolerance, relative to intact (non-ovariectomised) females. Thus, distinct mechanisms underly the sex differences in adiposity and glucose homeostasis in response to CR. Delivery of E2 and/ or PPT could not be validated. In conclusion, my PhD research has revealed that 1) metabolic effects of CR are sex-dependent in young, but not old, mice; 2) CR improves systolic and diastolic function in older male and female mice; and 3) resistance to CR-induced fat loss in young female mice is dominantly influenced by ovarian factors, with distinct mechanisms underlying the CR-induced changes in body composition and glucose tolerance. This project mandates further investigations into the underlying basis of these sex differences.