The independent effects of fasting8, 10, 13, 16, 26
and diet-induced obesity17, 27-32
on cardiovascular and metabolic variables in rodents are well known. To our knowledge, specific comparisons of the tempo of fasting-induced reductions in cardiovascular and/or metabolic variables between lean and obese mice have not been made. We hypothesized that fasting would induce a smaller and/or delayed reduction in HR, BP, VO2
and heat production in obese vs. lean mice because the absence of food
would pose less of a stress in the former group of animals. Biotelemetry and metabolic chamber studies confirmed that HR and BP were higher, while oxygen consumption, heat production, and locomotion were lower, in obese vs. lean mice in the fed condition. Further, each of these variables decreased in response to a 16 h fast at 22°C. In contrast to our hypothesis, the overall severity of the respective decreases was similar between groups. These data are important for investigators who use murine models of diet-induced obesity because they indicate that the degree of metabolic and cardiovascular adaptations to fasting were similar after 16 h in lean and obese mice.
Cardiovascular and metabolic data were collected at ~ 15-min intervals and averaged hourly. These procedures allowed us to determine the time required for the onset of fasting-induced reductions to become evident, and whether differences existed between groups. Compared to the fed condition, fasting-induced reductions in HR and BP occurred ~ 4 h earlier in obese vs. lean mice. Likewise, VO2 and heat production became statistically lower ~ 7 h earlier in HF vs. CON animals. While the mechanisms responsible for these unexpected findings are unclear, several possible explanations including differences in gender, differential activation of the autonomic nervous system, and altered leptin signaling will be discussed.
First, differences in sex may modulate metabolic responses to fasting. For example, Swoap et al. reported that female mice are more likely to enter a state of fasting-induced torpor than male mice.12
However, gender cannot contribute to our findings because all animals were male. A second possible explanation involves the generalized fasting-induced depression of the sympathetic limb of the autonomic nervous system thought to exist in rodents.13, 33-37
We assessed overall activation of the sympathetic nervous system by measuring urinary epinephrine and norepinephrine following the fed and fasting periods. Fasting-induced reductions of urinary catecholamines occurred in HF but not lean animals. While these data are consistent with the notion that overall sympathetic withdrawal existed to a greater degree in HF vs. CON mice, the exact time when sympathetic activity became lower in HF vs. CON animals cannot be discerned from our data. To accomplish this would have required serial blood sampling throughout the fasting period (rather than urine collection), which is difficult to do in an organism with a small blood volume, i.e., 4% of body weight38
without influencing baroreceptor function. As such, without accurate serial assessment of sympathetic and/or parasympathetic activity, we cannot state with certainty whether differential activation of the two limbs of the autonomic nervous system contributed to earlier reductions in cardiovascular and metabolic variables in obese vs. lean mice.
A third possibility that merits consideration for why reductions in cardiovascular and metabolic variables occurred sooner in HF vs. CON mice is that central and/or peripheral leptin resistance might have developed in the former group of animals. The hormone leptin is secreted in proportion to fat stores. Decreased food intake (e.g., fasting or starvation) results in decreased serum leptin concentration and subsequent decreases in energy expenditure.39
This effect is mediated primarily via leptin signaling in the hypothalamus (i.e., centrally).40
We do not believe central leptin resistance existed in HF vs. CON mice in the present study. Support for this statement is our observation that HF mice, while continuing to gain weight, consumed fewer calories (; ). These observations are similar to those reported by Lin et al.25
In that study even though less energy was consumed by fat-fed vs. lean mice after 4 weeks of feeding, body mass and adipose tissue mass continued to increase in the former group for an additional 11 weeks. The authors concluded that hypophagia in the high-fat-fed group likely resulted from an attempt to regulate the rate of adipose tissue accumulation via intact hypothalamic/central leptin signaling. Accelerated decreases in HR and BP in HF vs. CON mice could also be the result of an attempt to regulate energy expenditure via leptin signaling, and thus also suggests intact leptin sensitivity centrally and/or peripherally. However, because neither central nor peripheral leptin signaling was assessed directly in the present study, we cannot state with certainty that the tempo of reductions in VO2
, heat production, BP, and HR in obese mice was hastened by factors related to leptin resistance. The ability to make accurate comparisons between lean and obese mice after they have been fasted might be jeopardized if fasting
poses a greater/lesser stress on one group vs. another. Our findings indicate that the severity of cardiovascular and metabolic stress evoked by a 16 h fast at 22°C is similar among lean and obese mice. These data indicate that a 16-hour fast is a suitable period after which to study vascular function in models of diet-induced obesity. We also observed that the tempo of fasting-evoked reductions in cardiovascular and metabolic variables is hastened in obese vs. lean animals, although shifts in substrate usage (i.e., from carbohydrate to fat) was not. As such, shorter-term periods of fasting could be confounded by differential rates of metabolic and vascular adaptations, and should be taken into consideration when designing experimental protocols.