Whether you're fasting for a religious observance, weight loss or other personal reasons, you can expect your fast to affect your metabolism -- and, potentially, your weight. How much it affects your metabolism depends on how long and how often you fast. You should expect a bit of water weight gain after breaking a fast, and you might also gain some fat.
Fasting, Weight and Your Metabolism
You've probably heard the adage "food is fuel." That's true in the literal sense -- food provides energy you need to survive, but food also fuels your metabolism. Part of your total calorie burn comes from digesting food, so the less you eat, the less you'll burn through digestion.
Abstaining from food -- or eating significantly less than you need to maintain your weight -- also encourages your body to slow your metabolism as a survival mechanism. Fasting also encourages your body to start breaking down muscle tissue.
Although fasting once, for a day or two, may not cause serious damage to your metabolism, longer fasts can alter your calorie-burning potential and put you on track for eventual weight gain when you start eating normally again.
Short-Term Weight Gain
It's normal to experience some short-term weight gain when you break a fast. When you start eating again, you'll see the physical weight of those meals show up on the scale, at least temporarily. Resuming a normal meal plan also triggers some water weight gain. Carbohydrates stored in your body as glycogen, for example, come heavily "hydrated" -- this means they're bound to water. When you fast, your body uses the glycogen for energy. When you break the fast and eat carbs again you'll gain water weight as your glycogen stores are replenished.
The sodium in foods also triggers water retention, which can make a difference on the scale. Just 400 milligrams of sodium -- the amount in about 4 ounces of cottage cheese -- is enough to trigger 2 pounds of water weight gain. This almost immediate weight gain isn't excess fat, though -- it's just part of your body getting back to normal after a fast.
Fighting Long-Term Weight Gain
Although you're more likely to notice harmless water weight gain, fasting could make you gain body fat. If you've fasted long enough to slow your metabolism and lose muscle tissue, you'll burn significantly fewer calories during the day. This means that if you go back to your old diet, you'll likely gain fat.
For example, if you used to burn 2,000 calories a day and fasted so that now you're burning around 1,800 calories daily, going back to a 2,000-calorie diet means you're eating about 200 extra calories each day. That's enough excess calories to gain just shy of half a pound a week.
The best way to minimize fat gain after a fast is to go back to a healthful diet. Resist the urge to binge on any food you see -- including fatty fast or processed foods -- and instead follow a balanced diet that includes lots of veggies, fruits, whole grains, legumes and pulses, lean protein and healthy fats. You might still notice a small amount of weight gain, but it's less likely to pack on the pounds than a diet high in processed foods. As your body gets used to your healthy eating plan, you'll increase your metabolism and get back to "normal."
The Flipside: Using Fasting for Weight Loss
Fasting isn't generally a good way to lose weight because it has the potential to lower your metabolism. But intermittent fasting -- where you fast for short periods, such as for 16 to 24 hours -- has become a diet trend, particularly among Paleo dieters. Intermittent fasting has the same muscle-wasting and metabolism-lowering effects as total fasting, and it can be harmful if you have type-1 diabetes, have a history of disordered eating or are pregnant.
Fasting for as little as 16 hours can offer some weight-loss benefits and increase your fat burn, explained a paper published in the Proceedings of the National Academy of Sciences in 2014. If you want to try intermittent fasting, time your "fast" at night so you're asleep for most of the fasting period -- this mimics your natural eating cycle anyway, so you're less likely to feel too hungry. If you typically eat breakfast at 7 a.m., for example, stop eating at 3 p.m. the day before for a 16-hour fast.