Eating, it’s something we all do. It’s an act we engage in driven by multiple signaling pathways but at its root necessity- to simply keep the lights on in terms of our bodies and to continue living our life. Each one of us has different nutritional practices around eating and how we go about getting the needed, or oftentimes unnecessary caloric energy & nutrients to power through our day. I wanted to discuss a topic that hits close to home & in a very personal way. It's our individual drives and internal mechanisms to how we regulate energy intake.
To get deeper into this, another subject I'd want to address is how exercise affects your hunger cues and eating habits. Do you find that exercising helps you eat less? Perhaps it helps to accelerate it. Do you think different types of exercise have an impact on your eating habits? Perhaps you go weightlifting before or after work, and this helps to keep your hunger in check. One day, go for a medium-to-long run and you'll find yourself in a completely hedonistic condition, ingesting literally anything in sight. This is something that fascinates me. What's interesting is that these findings and anecdotes differ dramatically from person to person.
The referenced study by Tobin & colleagues, (2021) (1) is open access, which means that anyone can read it for free. In the study, 24 individuals (12 males and 12 females) came to the lab three times: once to perform a 45-minute aerobic exercise program, once to complete a 45-minute resistance training protocol, and once to rest for 45 minutes. The day before each visit, they ate a standardized meal that was tailored to give the right amount of total energy for each person. Over the course of 180 minutes, researchers measured appetite, appetite-related indices, and cravings. After the baseline measurements, participants ate a standardized meal and then completed the allocated activity bout (or rest) after the 30-minute measures. Participants were placed in a private room for 30 minutes after the final measures were taken at 180 minutes to consume an ad libitum test meal. The term ad libitum refers to a food supply that is available without restriction and can be consumed at any time.
Female individuals reported less hunger than male participants, albeit this difference was more significant during the aerobic and resting visits than during the resistance exercise visit. These reduced subjective hunger ratings were not associated with any significant variations in hunger-related blood biomarkers, and they did not result in lower relative energy intake (scaled to estimated energy requirements) during the ad libitum meal. There's certainly some nuance to this problem that needs to be examined, and that's exactly what this study set out to do. The hypotheses (expected results from this study) (2) were not expressed clearly, but it appears that they expected females to have more appetite stimulation from exercise than males. Tobin's present research is a secondary analysis of a previously published study (3) by Halliday & colleagues, (2021).
Study Methods
Because of the various ways nutritional studies may be conducted and inferences drawn from them in general, the methodology section of this study is particularly intriguing. The Three-Factor Eating Questionnaire and a baseline body composition examination were required of participants (which assesses dietary restraint, disinhibition, and hedonic hunger). After that, the participants went through four familiarization sessions. They finished the aerobic exercise condition in two visits, which included a five-minute warmup, 35 minutes of walking at 65-70 percent of their age-predicted maximum heart rate, and a five-minute cooldown. They conducted the full-body resistance training condition in the other two, which consisted of one set of 12 separate exercises to "failure" (12-15 repetitions with an RPE of 9-10 on a 10-point scale) with 3 minutes of rest in between each exercise.
Participants did the three experimental visits (resistance exercise, aerobic activity, or rest) in a randomized order after the familiarization sessions. The day before each testing session, participants consumed a tailored diet (20% protein, 30% fat, and 50% carbohydrate), which was provided by the research team. Participants came to the lab fasted on the day of their testing visits (at least 10 hours). A blood sample and the completion of visual analogue scales to determine hunger, satiety, and future food consumption were part of the baseline assessments (how much people thought they could eat). The subjects next ate a standardized breakfast with the same macronutrient breakdown as the previous day's diet, which provided 25% of their daily energy requirements. Blood samples and appetite assessments were taken again 30, 90, 120, 150, and 180 minutes after the standardized breakfast was consumed. Following the 30-minute measures, participants did the exercise bout (or rested in the control condition) and completed the Food Craving Inventory questionnaire after the 90-minute measurements. After the 180-minute measures, participants were transported to a private room and given 30 minutes of ad libitum access to a test dinner consisting of lasagna, pound cake, and a variety of different side dishes.
Blood biomarkers for hunger and satiety were measurements of (ghrelin, peptide YY, and glucagon-like peptide-1), subjective questionnaire and visual analogue scale responses to hunger and cravings, and actual energy intake during the ad libitum test meal were all of interest.
Study Results
There were no significant sex (male or female) x condition (aerobic exercise, resistance exercise, or control) interactions, and no significant changes in hormone concentrations between sexes during the course of the study.
The researchers did find a primary influence of sex in the aerobic exercise and control situations, with males having significantly higher hunger levels than females, whereas the difference between sexes was smaller (and non-significant) in the resistance exercise condition.
The sex-condition interaction effect, as well as the main effects of sex and condition, were not statistically significant for satiety responses.
Post-hoc analyses (4) revealed that in the aerobic exercise and control conditions, prospective food intake values were substantially greater in males than females, whereas the difference between sexes was less (and non-significant) in the resistance exercise condition.
The researchers stated that there were no statistical differences between men and women, as well as no statistical differences among circumstances, when it came to the results of the Food Craving Inventory questionnaire.
Men ingested higher total calories during the ad libitum test meal. This is to be expected, given that the male participants' energy requirements were larger than the female participants' (in absolute terms). Energy consumption was not substantially different between male and female participants in relative terms (scaled to estimated energy requirements).
Interpreting the Findings
Before we get into the specifics, let's define "appetite" in terms of its operational definition (5). Beaulieu & colleagues, (2018) does a good job of examining what this phrase actually means and how it's influenced by various elements in his study. Appetite is affected by a combination of psychological and physiological factors related to hunger (which drives feeding), satiation (which stops feeding), and satiety (which makes you feel full) (which is the post-meal suppression of hunger). Episodic signals (meal-to-meal and diurnal changes throughout the day that affect meal initiation, termination, and fullness) and tonic signals control appetite and calorie intake (which stem from body tissues and cells that convey information about cumulative, long-term aspects of energy availability). Following that, an article (6) by Carreiro & colleagues, (2016) examines macronutrient intake and subsequent energy intake.
While physiological variables play a significant role, psychological and behavioral factors also play a significant role. The hedonic impulse to eat for the pleasure of wonderful food, behavioral features or habits that influence our feeding behaviors, obesogenic feeding environment, and possible linkages between emotional reactions and feeding behaviors are just a few examples. In summary, a combination of homeostatic (primarily driven by physiological parameters) and non-homeostatic systems influence hunger and energy intake (largely driven by psychological and behavioral factors).
We're interested in appetite because we believe there's a link between it and energy consumption. We can observe that there were no variations in ad libitum energy intake or appetite ratings across the conditions (resistance exercise, aerobic exercise, or control) in the first study published from this dataset (3). While this appears to show no appetite suppression despite completing an energy-draining exercise session, you could argue (as the authors did) that similar appetite and energy intake reflects some degree of appetite reduction because the calories burned during exercise were not replaced or compensated for. Females reported lower subjective hunger and projected food intake than males in the current study, but they did not consume fewer calories (relative to their energy demands) in the ad libitum meal. Although this disconnect between perceived hunger and energy intake appears contradictory, it appears to be a fairly regular occurrence. According to two separate studies (7, 8), there is substantial experimental support for the hypothesis that exercise acutely decreases hunger as long as the intensity is high enough, but that this appetite suppression does not reliably alter calorie intake in meals ingested after the exercise bout.
There could be a variety of reasons for this disparity, but I believe that ad libitum test meals in laboratories are just less realistic of real-world eating situations than researchers would want. As stated at the outset of this Interpretation section, both homeostatic and non-homeostatic factors influence energy intake, and psychological and behavioral elements of eating can be highly important. It goes without saying that being locked in a room with a bunch of food while being observed by researchers isn't reflective of an usual meal (or the psychological state that accompanies a more typical, unsupervised eating experience). As a result, a disparity between subjective hunger ratings and ad libitum eating in experimental circumstances is not surprising.
While it's worth noting that energy intake in the lab may not be completely representative of real-world intakes, we shouldn't get too caught up in the nitty gritty of immediate post-exercise appetite and energy intake reactions anyhow. Understanding them has some use, but the most essential applications of hunger and energy intake reactions are in the context of long-term energy balance. If you soon develop accustomed to this response or compensate at subsequent meals, suppressing your hunger for 30-60 minutes won't make much of a difference, therefore the chronic impacts of exercise on appetite are far more significant than any acute effects. Will regular exercise, for example, result in significant weight loss by suppressing appetite while raising energy expenditure? Will an endurance athlete's appetite be permanently suppressed, making it difficult to eat correctly for training?
The independent effect of exercise on weight loss is unimpressive, as addressed in an article (9) by Steele & colleagues, (2021). This may appear odd, given that exercise increases total daily energy expenditure and normally appears to have a negative or neutral influence on appetite ratings and energy intake. Longitudinal studies, however, reveal that compensatory adaptations begin to take effect, resulting in less weight reduction than we might predict. Some of these adaptations are related to downregulation of non-exercise components of energy expenditure (10), as (Broskey & colleagues) excellently discusses, but compensatory increases in appetite and energy intake have also been recorded (11). As you read this, you may be wondering why exercise leads to unimpressive weight loss results due (in part) to compensatory increases in energy intake, when multiple articles (12, 13, 9) have acknowledged that exercise has a significant positive effect on maintaining a stable body weight and preventing weight regain after weight loss?
The answer can be found in the previously mentioned study (5): Exercise does not always cause an increase or decrease in appetite. Rather, it reconnects hunger and satiety cues with energy expenditure, allowing homeostatic considerations to have a greater impact on energy intake. To put it another way, a sedentary individual may be more prone to positive energy balance because they live in a "non-regulated" zone where appetite is disproportionately impacted by non-homeostatic factors such as psychological, behavioral, and hedonic eating urges. They are more prone to drift toward neutral energy balance when their activity level rises, as homeostatic mechanisms such as energy expenditure and body composition take over a bigger share of appetite regulation.
I indicated in the introduction that anecdotal accounts of the association between exercise habits and hunger differ significantly. I believe one of the main reasons for this is that exercise appears to re-couple energy intake and appetite to energy expenditure rather than indiscriminately boosting or decreasing hunger. As a result, for someone who is sedentary and has a positive energy balance, increasing exercise may passively nudge them toward lower energy intake and higher post-meal satiety levels. In contrast, we would expect an increase in physical activity or exercise volume to increase hunger in someone who is already quite active and in a neutral energy balance; their appetite and energy intake are already quite tightly linked to energy expenditure, and the additional activity is pushing energy expenditure even higher. In this setting, exercise is a stimulator of energy intake rather than a suppressor. More activity may not be the answer if you're trying to lose weight and curb your appetite; homeostatic forces, even though closely linked to appetite, would likely move you toward hunger rather than fullness while you're in a negative energy balance. However, if you're trying to maintain a lower body weight after weight loss but aren't getting enough exercise, some extra activity may help you get closer to the "regulated" zone, which is largely influenced by homeostatic factors reflecting energy balance and may help you regulate your appetite and maintain a neutral energy balance. This may give the impression that we know everything there is to know about exercise and appetite control, but this is far from the reality. Researchers largely agree that appetite and energy intake responses to exercise differ from person to person, but they disagree on what causes this variation. This has sparked an understandable curiosity in figuring out which variables can predict a person's appetite response to exercise.
To answer some of the lingering uncertainties about exercise and appetite, we'll need a lot more research. It's still unclear how much of the inter-individual variability seen in prior studies is due to regular day-to-day fluctuations or statistical "noise," and how much is due to real individual differences. Some researchers believe the true magnitude is small (7), while others believe it is enormous (8), but there appears to be widespread agreement that it is not inconsequential. Beyond that, we're not sure what factors might be causing any "real" variations between people. It appears that habitual physical activity level is one relevant component, but additional study is needed to better understand how outcomes are influenced by factors such as energy balance, exercise bout characteristics, and individual characteristics. For the time being, the best we can do is recognize that as physical activity levels rise, the relationship between energy expenditure and appetite tightens, use this knowledge to set context-dependent expectations for how an increase or decrease in exercise might affect appetite, and be prepared for the possibility that some people will respond very differently than others.
Applications & Takeaways
As long as the intensity level is high enough, exercise tends to decrease appetite acutely and transiently. More importantly, regular physical activity (which includes but is not limited to organized exercise) has an effect on hunger and satiety management, making appetite and energy intake more closely connected to energy expenditure. As a result, the effect of a shift in activity level on hunger is context-dependent; you may notice an unintentional decrease in calorie intake while transitioning from sedentary to somewhat active, but an unintentional increase in energy intake when transitioning from active to very active. Furthermore, the effect of exercise on appetite appears to be subject to a significant degree of inter-individual variation, but studies have yet to definitively identify the particular traits and contextual elements that cause these apparent differences.
References
1.) Tobin, S. Y., Cornier, M. A., White, M. H., Hild, A. K., Simonsen, S. E., Melanson, E. L., & Halliday, T. M. (2021). The effects of acute exercise on appetite and energy intake in men and women. Physiology & behavior, 241, 113562. https://doi.org/10.1016/j.physbeh.2021.113562
2.) Wilkinson M. (2013). Testing the null hypothesis: the forgotten legacy of Karl Popper?. Journal of sports sciences, 31(9), 919–920. https://doi.org/10.1080/02640414.2012.753636
3.) Halliday, T. M., White, M. H., Hild, A. K., Conroy, M. B., Melanson, E. L., & Cornier, M. A. (2021). Appetite and Energy Intake Regulation in Response to Acute Exercise. Medicine and science in sports and exercise, 53(10), 2173–2181. https://doi.org/10.1249/MSS.0000000000002678
4.) Zhang, Y., Hedo, R., Rivera, A., Rull, R., Richardson, S., & Tu, X. M. (2019). Post hoc power analysis: is it an informative and meaningful analysis?. General psychiatry, 32(4), e100069. https://doi.org/10.1136/gpsych-2019-100069
5.) Beaulieu, K., Hopkins, M., Blundell, J., & Finlayson, G. (2018). Homeostatic and non-homeostatic appetite control along the spectrum of physical activity levels: An updated perspective. Physiology & Behavior, 192, 23–29.
6.) Carreiro, A. L., Dhillon, J., Gordon, S., Higgins, K. A., Jacobs, A. G., McArthur, B. M., Redan, B. W., Rivera, R. L., Schmidt, L. R., & Mattes, R. D. (2016). The Macronutrients, Appetite, and Energy Intake. Annual review of nutrition, 36, 73–103. https://doi.org/10.1146/annurev-nutr-121415-112624
7.) King, J. A., Deighton, K., Broom, D. R., Wasse, L. K., Douglas, J. A., Burns, S. F., Cordery, P. A., Petherick, E. S., Batterham, R. L., Goltz, F. R., Thackray, A. E., Yates, T., & Stensel, D. J. (2017). Individual Variation in Hunger, Energy Intake, and Ghrelin Responses to Acute Exercise. Medicine and science in sports and exercise, 49(6), 1219–1228. https://doi.org/10.1249/MSS.0000000000001220
8.) Dorling, J., Broom, D. R., Burns, S. F., Clayton, D. J., Deighton, K., James, L. J., King, J. A., Miyashita, M., Thackray, A. E., Batterham, R. L., & Stensel, D. J. (2018). Acute and Chronic Effects of Exercise on Appetite, Energy Intake, and Appetite-Related Hormones: The Modulating Effect of Adiposity, Sex, and Habitual Physical Activity. Nutrients, 10(9). https://doi.org/10.3390/nu10091140
9.) Steele, J., Plotkin, D., Van Every, D., Rosa, A., Zambrano, H., Mendelovits, B., Carrasquillo-Mercado, M., Grgic, J., & Schoenfeld, B. J. (2021). Slow and Steady, or Hard and Fast? A Systematic Review and Meta-Analysis of Studies Comparing Body Composition Changes between Interval Training and Moderate Intensity Continuous Training. Sports (Basel, Switzerland), 9(11), 155. https://doi.org/10.3390/sports9110155
10.) Broskey, N. T., Martin, C. K., Burton, J. H., Church, T. S., Ravussin, E., & Redman, L. M. (2021). Effect of Aerobic Exercise-induced Weight Loss on the Components of Daily Energy Expenditure. Medicine and Science in Sports and Exercise, 53(10), 2164–2172.
11.) Martin, C. K., Johnson, W. D., Myers, C. A., Apolzan, J. W., Earnest, C. P., Thomas, D. M., Rood, J. C., Johannsen, N. M., Tudor-Locke, C., Harris, M., Hsia, D. S., & Church, T. S. (2019). Effect of different doses of supervised exercise on food intake, metabolism, and non-exercise physical activity: The E-MECHANIC randomized controlled trial. The American Journal of Clinical Nutrition, 110(3), 583–592.
12.) Dahle, J. H., Ostendorf, D. M., Zaman, A., Pan, Z., Melanson, E. L., & Catenacci, V. A. (2021). Underreporting of energy intake in weight loss maintainers. The American journal of clinical nutrition, 114(1), 257–266. https://doi.org/10.1093/ajcn/nqab012
13.) Hall K. D. (2022). Energy compensation and metabolic adaptation: "The Biggest Loser" study reinterpreted. Obesity (Silver Spring, Md.), 30(1), 11–13. https://doi.org/10.1002/oby.23308