CHARLOTTESVILLE, Va. -- If you've ever noticed that an intense workout kills your appetite while a moderate jog leaves you heading straight for the kitchen, you're not imagining things. Scientists have now uncovered biological evidence explaining this phenomenon, and it centers around a hormone called ghrelin, otherwise known as the "hunger hormone."

Scientists at the University of Virginia conducted a study, published in the Journal of the Endocrine Society, examining how different exercise intensities affect levels of ghrelin in the body and subsequent feelings of hunger. Their findings suggest that high-intensity workouts may be particularly effective at reducing appetite compared to moderate exercise or no exercise at all.

"Exercise should be thought of as a 'drug,' where the 'dose' should be customized based on an individual's personal goals," says lead author Kara Anderson, in a statement. "Our research suggests that high-intensity exercise may be important for appetite suppression, which can be particularly useful as part of a weight loss program."

Inside our bodies, ghrelin exists in two forms: acylated ghrelin (AG) and deacylated ghrelin (DAG). Think of AG as the form that tells your brain "I'm hungry," making up about 22% of total ghrelin. DAG, which makes up the remaining 78%, either suppresses appetite or has no effect on it. Beyond just controlling appetite, ghrelin plays surprisingly diverse roles in the body, influencing energy balance, blood sugar control, immune function, sleep, and even memory. Understanding how exercise impacts these different forms of ghrelin could help develop more effective exercise strategies for weight management.

Led by Anderson and her research team, the study included 14 adults who weren't regular exercisers but were otherwise healthy. Eight men (average age 43) and six women (average age 32) made up the sample. All participants had healthy body weights according to standard body mass index measurements. Each person completed three different experimental sessions: one where they rested without exercising, another with moderate-intensity cycling, and a third with high-intensity cycling.

To ensure the exercise intensities were appropriate for each person, researchers used something called the lactate threshold. This is the point during exercise where you start breathing harder and the workout feels notably more challenging. For moderate exercise, participants cycled at this threshold. For high intensity, they cycled 75% harder than this threshold, making it a genuinely strenuous workout.

"We found that high intensity exercise suppressed ghrelin levels more than moderate intensity exercise," adds Anderson. "In addition, we found that individuals felt 'less hungry' after high intensity exercise compared to moderate intensity exercise."

Some fascinating differences emerged between men and women. Women naturally had higher levels of ghrelin before exercise compared to men. During high-intensity exercise, both men and women showed reduced levels of the appetite-neutral form of ghrelin (DAG), but only women showed significant decreases in the hunger-stimulating form (AG). This suggests that intense exercise might affect appetite differently in women versus men.

"We found that moderate intensity either did not change ghrelin levels or led to a net increase," study authors write. This suggests that exercising above the lactate threshold "may be necessary to elicit a suppression in ghrelin."

When asked about their hunger levels, people actually reported feeling hungrier after moderate exercise compared to when they didn't exercise at all. However, high-intensity exercise seemed to keep this increased hunger in check. Men reported stronger hunger after moderate exercise compared to intense exercise, while women felt hungrier during moderate exercise compared to rest.

While everyone in this study was at a healthy weight and not regularly exercising, the results open up interesting questions about how these effects might differ in athletes or people trying to lose weight. The researchers acknowledged that more work is needed to determine exactly how and why exercise effects differ between men and women. It's also important to note that all participants exercised on an empty stomach in the morning, which might not match how most people typically work out.

The study's results point to a clear connection between exercise intensity and appetite control through ghrelin regulation, especially for women. Future research will need to explore these effects in more diverse populations and determine the optimal "dose" of exercise for appetite control in different groups. However, results reveal that when it comes to managing post-exercise appetite, intensity matters.