4. Discussion
nutritional ketosis, depressed, appetite
INSULIN SENSITIVITY
Recent research indicates that fasting may lead to weight loss but may also increase concentrations of ghrelin while reducing concentrations of GLP-1, PYY, and PP throughout the fast [We found that completing a bout of vigorous exercise at the beginning of a fast depressed concentrations of ghrelin by 17% while raising concentrations of GLP-1 by 13%, an effect independent of biological sex. In contrast, PYY, PP, leptin, and insulin were not altered with exercise. These results help describe the impact of acute exercise on appetite and generally agree with the notion that exercise may be a mild to moderate anorexigenic stimulus [Previous studies have examined how different exercise intensities and durations can affect GLP-1 by examining post-exercise GLP-1 concentrations. For example, Martins et al. reported that total plasma GLP-1 concentration increased between 16–70% an hour after exercising at 60% of max heart rate [Because blood was taken every 12 h, our study was not designed to capture the acute effects of exercise. In contrast to the acute post-exercise studies described previously, the current study demonstrated that GLP-1 was suppressed in both conditions 12 to 24 h into the fast. However, the findings from this study agree with previous research in that exercise elevated GLP-1 by comparison to the control condition. One of the unique contributions of this study is that we observed the elevation of GLP-1 beyond the 2 h post-exercise seen in other research, demonstrating that the impact of exercise on GLP-1 lasted the duration of the fast. Interestingly, between 24 and 36 h of fasting, GLP-1 rose above baseline in both conditions, suggesting that the impact of fasting on GLP-1 is not linear, though the mechanism describing the rise in GLP-1 after 24 h of fasting remains uncertain [Similar to GLP-1, multiple studies have evaluated how moderate to intense exercise influences ghrelin. However, the findings from these studies are less consistent [We demonstrated that exercise mitigates the rise in ghrelin typically observed with fasting, as there was no change in baseline ghrelin concentrations for the first 24 h of the fast in the exercise condition. Our study agrees with most of the current research because, relative to the control, adding exercise to the fast reduced ghrelin concentrations. Though there are significant differences between the experimental conditions during the first 24 h of the fast, our results indicate that the impact of exercise on ghrelin during a fast disappears by 36 h.While additional work is needed to describe the mechanisms responsible for changes in ghrelin in response to acute exercise [Several studies have also shown that aerobic exercise can alter plasma concentrations of PYY [The results from this study show that exercise also did not alter insulin concentrations following 12 h of fasting. Physical activity is considered a means of improving insulin sensitivity, which stems from increased membrane permeability and glucose transport at the skeletal muscle independent of insulin [A host of evidence suggests that hunger increases over the course of a fast. This is based on subjective ratings of hunger as well as alterations in hormones such as ghrelin, GLP-1, PYY, PP, GIP, and leptin [After 24 h, GLP-1 rebounded to concentrations higher than baseline, and ghrelin concentrations dropped below baseline. The exact impact of these two hormones on hunger and the drive to consume food is complicated since other appetite-suppressing hormone concentrations remained low throughout the duration of the fast. However, the changes in GLP-1 and ghrelin may partially mitigate the drive to consume food and could at least partially explain why hunger is highest during the first 24 h of a fast [Results from our lab recently found that a state of nutritional ketosis could be reached approximately 3.5 h sooner when a fast was initiated with exercise [