The obvious explanation as to why you sometimes feel ravenous after exercise is that because you have just exerted energy, your body is literally craving fuel.
Biologically, we want to overcompensate by ingesting more calories than we burn. Not to mention, metabolism and adrenaline have just been given a boost.
Although ghrelin levels are lower post-workout, it is likely to surge back, says Barry Braun, director of the Energy Metabolism Laboratory at UMass. Furthermore, he found that leptin, the hormone that signals when you're full, is decreased after a workout. Another study concluded that ghrelin reduction occurs more with cardio than with weight training, so you're more likely to be hungry after weight lifting than after running.
Beyond simply feeding your body, filling depleted glycogen reserves and jump-starting the muscle recovery process, beyond hormone activity, recent studies suggest that exercise is capable of altering how the brain responds to the sight (and smell) of food.
Endurance trainer Matt Fitzgerald attributes increased appetite to "reward psychology." "You see this more with beginners, for whom exercise itself isn't rewarding," he says.
Although a number of studies have shown a reduced responsiveness to food following exercise, 41 percent of participants in a 2012 study had overactivity in their food-reward networks of the brain after working out—more than when they began the study.
University of Cal-Poly kinesiology professor Todd Hagobian asserts, "exercise has a definite impact on food-reward regions. But that impact may depend on the individual—gender, age, body type, fitness-level—and the type of exercise."
The trend—in both hormone and brain-focused studies— shows that the appetite-reducing effect is more likely to occur in those who are generally inactive. Thus, if you're fit as a fiddle, you may not experience that hunger-suppressant factor.
"Four or five years ago, it really looked like appetite hormones" primarily controlled what we eat, says Dr. Habogian. "But I'm more and more convinced that it's the brain. Hormones don't tell you to go eat. Your brain does."