Loss of appetite can follow a vigorous workout, and can be a sign that you're training too much. Keeping your workouts under 60 minutes per session and getting plenty of sleep at night are just two ways to reduce the odds of overdoing it at the gym. The primary reason you may experience a loss in appetite following a workout has to do with your body's hormones.
There are two important hormones involved in provoking appetite: ghrelin and peptide YY. Ghrelin is a hormone that stimulates hunger while peptide YY suppresses it. Aerobic and anaerobic exercise, such as weight training, both stimulate an increase in the peptide YY hormone. This is the primary reason healthy people have a feeling of fullness post-workout.
Resistance is Futile
Resistance training and aerobic training both increase levels of peptide YY, but aerobic exercise actually decreases levels of ghrelin while resistance training does not. This makes aerobic training more effective for suppressing appetite, so it could make it a more beneficial choice in terms of weight loss.
In extreme cases, a loss of appetite caused by exercise can be harmful. The American Council on Exercise lists "loss of appetite" as one of the top indicators of overtraining your body. The cause is an unhealthy increase in the release of epinephrine and norepinephrine, which are both appetite suppressant hormones. Monitor your daily calorie intake to make sure it's well within the Department of Health and Human Services recommendations of at least 1,000 calories per day for women and 1,200 calories for men. Also, listen to your body for signs of overtraining, including chronic muscle soreness, frequent illness and restless sleep.
What to Do
Follow the Centers for Disease Control and Prevention recommendations to exercise at the right intensity and take advantage of the positive effects of exercise rather than the potential negative aspects. The CDC recommends you perform at least 150 minutes of moderately intense aerobic exercise each week. As you become more physically fit, you can increase this figure to five hours per week total, or the equivalent of five 60-minute workouts. This workout regimen will allow you to get the appetite suppressant benefits that aerobic exercise offers without overtraining your body.
- American Journal of Physiology: Influence of Resistance and Aerobic Exercise on Hunger, Circulating Levels of Acylated Ghrelin, and Peptide YY in Healthy Males; David R. Broom, et al.
- American Council on Exercise: Top 10 Signs You're Overtraining
- U.S. Department of Health and Human Services: Aim for a Healthy Weight
- Centers for Disease Control and Prevention: How Much Physical Activity do Adults Need?