“He is haunted by a demon, a demon against which he feels powerless, because in its first manifestation it has no face, no name, nothing; and the words, the poem he makes, are a kind of exorcism of this demon.”
~ T. S. Eliot
Hunger can be a demon of sorts, a faceless urge that compels us into, at times, unwanted behaviors. But hunger is a demon whose name we know, and names have power over the uninvited, especially when we know both their name and their physiology. The name of hunger is ghrelin.
Ghrelin was discovered in 1999 and identified as an endogenous ligand to the growth hormone secretagogue receptor 1a (GHSR1a). The hunger-ghrelin link was first discovered during early studies that used a supraphysiological ghrelin infusion in humans and reported hunger as a side effect, a finding that many studies have since confirmed. Studies have also demonstrated a pleiotropic range of biological effects beyond appetite, including energy balance, glucose homeostasis, immune function, sleep, and memory.
Ghrelin exists in two forms, acylated (AG) and deacylated (DAG), with the majority, approximately 80%, of bioavailable gremlin circulating as DAG. This is important because “while AG has stimulated appetite in human and animal models, DAG has been shown to either have no effect or to suppress appetite.” The variables in our food-health relationship depend not only on what we eat but also how much. How much we eat is often directly related to the hunger we experience. While the inclusion of an exercise program is a given for any prescription for health and wellness, an additional benefit of exercise could be appetite suppression.
This week’s study sought to investigate the effect of exercise intensity on ghrelin levels and appetite in humans.
- The study involved individuals between the ages of 18 and 55 years with a BMI of 18.5 to 24.9 kg/m².
- The group consisted of eight males with a mean age of 43.1 years and a BMI of 22.2 kg/m² and six females with a mean age of 32.2 years and a BMI of 22.7 kg/m².
- The participants were subjected to three levels of exercise intensity: control (CON—no exercise), moderate intensity (MOD—a moderate exercise level), and high intensity (HIGH—a high-intensity exercise level).
- Following exercise, blood levels of lactate (a measure of muscle work during exercise), total ghrelin (TG), acylated ghrelin (AG), and deacylated ghrelin (DAG) were obtained.
- Participants also underwent an assessment of hunger following each exercise, which was calculated as a total appetite score (APP).
- Females had higher baseline percent body fat, DAG, and TG levels compared to males at baseline.
- All participants exhibited significantly reduced DAG levels following high-intensity exercise compared to moderate-intensity exercise and control.
- Only females exhibited significantly reduced AG following high-intensity exercise.
- Hunger scores were increased following moderate exercise compared to control.
The Caveat:
An exercise program is a foundational piece of any health and wellness plan. There are a plethora of positive benefits associated with exercise. While any level of exercise is helpful in our increasingly sedentary population, this study suggests that in relation to helping us curb our copious portions, high-intensity exercise may offer additional advantages.
This study suggests that high-intensity exercise – but not moderate-intensity exercise – “suppresses total ghrelin (TG), acylated ghrelin (AG), and deacylated ghrelin (DAC) levels.” It is essential to note in this study that only women had significantly reduced levels of acylated ghrelin (AG) following intense exercise. This translates into less hunger following high-intensity exercise compared to moderate-intensity exercise.
A possible mechanism suggested by the research is that there is a threshold level of lactate or muscle work that triggers a response that results in a decrease of ghrelin. However, as the study authors point out, “[I]t does not appear that lactate is the primary driver of suppressed ghrelin.” Regardless, this study highlights additional benefits of exercise programs in any health and wellness endeavor, particularly including high-intensity exercise, as it may attenuate the hunger gremlin and thus aid with weight loss and maintenance.
While the study is encouraging in that the results align with our current understanding of hunger and the drivers of hunger, there are several caveats. The study included only 14 people and suggested different responses based on biological sex. Future studies may need to examine larger groups of both males and females. Additionally, other research has shown that ghrelin levels are entrained to follow mealtimes, for example, decreasing after breakfast time regardless of whether food is consumed or not.[1] It is also likely that there are other contributors to hunger, including gut hormones such as polypeptide YY, glucagon-like peptide-1, and pancreatic polypeptide. There are likely additional variations in response due to age, weight, and gut microbiome contributions. Previous research has shown certain short-chain fatty acid production by gut bacteria can act to suppress the hunger drive.
Nonetheless, in our current society, where it seems almost everyone is looking for a magic weight loss pill, it may be helpful to imagine a trip to the gym as less than a form of medieval torture. Perhaps as the study’s lead researcher, Kara Anderson, from the University of Virginia School of Medicine, suggests, “Exercise should be thought of as a ‘drug,’ where the ‘dose’ should be customized based on an individual’s personal goals.” Now, that’s a prescription we can all get behind!
[1] (Natalucci, 2005)
The Study:
Additional resources: