Sometimes you're ravenous. Other times the sight of food makes you sick.
It’s a paradox: After a blazing high-intensity interval training class that left you in a puddle on the floor and spent hundreds of calories, you’re not the least bit hungry. However, you’re ravenous after an hour and a half of yoga.
Culturally, we kind of panic when we feel hungry: it's a major culprit of overeating, after all. So it doesn't hurt to try to reframe hunger as merely a sign that your body's using food the way it's supposed to, says Lindsea Burns, a clinical nutritionist and exercise physiologist in Encino, California. "We naturally assume hunger is the enemy, but what this really means is that your body is working so efficiently that it's metabolizing and storing your food as fuel."
Still, there are certain workouts that tend to deplete your energy stores more than others. Hunger is dependent on a lot of individual factors such as metabolism and researchers are still trying to nail down the physiology, but here’s where we’re at.
All-out effort: Not hungry
Put simply, hunger is your body’s demand for fuel. When the stomach is empty, research suggests that a hormone called ghrelin sends signals to your brain to eat. But your body reacts differently in the hour or so after a bout of tough exercise. Researchers in the UK compared a high intensity workout (treadmill running) withweight lifting. Ghrelin levels appeared to drop more in the aerobic runners, and they were subsequently less likely to be hungry than a group of weightlifters were after their sessions. This starts to explain why even a hardcore HIIT session doesn’t spur an immediate desire to refuel.
Part of the reason is that, during a taxing exercise session, your body redirects all its blood flow and energy to areas (lungs, muscles). It doesn’t want you to eat because it’s temporarily shut down your digestive system.
“The more intensely you exercise the less ghrelin your body produces, generally,” says Isabel Smith, a registered dietitian in New York City.
Long, low-key workouts: Hungry
The length of your workout also can affect how hungry you get, so even if you barely get out of breath, 90 minutes of asanas can make you weak and hungry after yoga. Your body might perceive a long workout as a famine and get hungrier, says Nancy Clark, MS, a Boston-based sports nutritionist. And as you might imagine, prolonged, intense exercise such as endurance running can make you really hungry, even if it’s not right away. Some people actually gain weight while training for marathons because they overeat, even though they’re burning 2,000 calories some day, Clark says.
Weight Lifting: Hungry
Pumping iron causes small tears in muscle tissue; as those tissues repair and heal, you build stronger muscles. That process requires nutritional refueling and replenishing, Clark explains. “We know that muscle drives hunger,” Clark says. “It’s very active tissue, so the more muscle you have, the hungrier you get, generally.”
A very sweaty workout: Not Hungry
Body temperature is thought to play a role in hunger suppression after a tough workout as well. “Sometimes the core body temperature is raised so much during intense exercise, and blood flow is redistributed so much to the muscles and skin, that digestion is essentially shut down until blood flow and body temperature normalize,” Burns says. “During a short window after exercising, you won’t be hungry because the body knows digestion would be ineffective.”
Another factor is hydration, which affects body temperature and recovery (which includes refueling). “Inadequate hydration can trigger sugar and salt cravings and make you feel hungry when you’re not,” Smith says. “Most people don’t get enough fluids to begin with, and we lose sodium in our sweat when we work out.” So if you get back from a summer run and want to devour a three-course meal, try drinking water first so you can better gauge whether you're actually hungry.
Body temperature starts to explain why swimming leaves you so hungry. The activity can be intense cardio exercise yet tends to spark appetite in many people, however, likely because the body stays cool in the water despite the workout the heart is getting, Clark says. You also use all your muscles when you swim as opposed to mostly just using your legs during a run.