How Should You Exercise While You're Intermittent Fasting? Doctors Weigh In
Intermittent fasting has picked up quite a bit of steam in the health world, as people who experiment with it have seen impressive results: improved energy levels, stable blood sugar, weight loss, and more. In a world where we reach for a snack every two hours, it makes sense that our bodies welcome the natural fasting patterns of our ancestors.
But what does exercise look like during a fast? That depends on a variety of factors, from the fast you choose—some people fast for 16 hours and eat for eight daily, while others eat between 500 and 600 calories on two nonconsecutive days of the week—to how your body responds to it. "It’s important to listen to your body," Vincent Pedre, M.D. of Pedre Integrative Health tells mbg. "If you feel too weak to work out from fasting, then you should take care of your nutrition and work out later."
While safety should always come first, a variety of workouts are excellent complements to IF. Here's what you need to know.
Plan your meals around your workouts.
Vincent recommends cardio on an empty stomach, so booking that early morning spin class or going for a jog works well if you're fasting. But choosing the right foods the night before is crucial.
"Knowing you're going to exercise, you should be thinking about what to eat the day before, depending on the intensity of the workout. For example, you may want to build your glycogen stores with complex carbs for dinner the night before so that you have readily available energy for a cardio workout," he says. "You never want to do cardio on a full stomach, as the sudden demand for blood flow from the muscles will steal vital blood flow needed by the digestive system for digestion and assimilation of nutrients. The key is to plan ahead so your nutrition can meet the demands required by the intensity of your workout, even when it's the next morning."
Which workouts should you choose?
Unless you find yourself getting lightheaded during a fast, exercise to your heart's content—cardio, weightlifting, the works. "Several elite-level strength athletes have told me that their strength peaks after a 16- or 20-hour fast," Dr. Dominic D'Agostino, PhD, BS at USF health says. "Cognitively, people tend to feel more lucid and focused. The more frequently you fast the easier it gets and the more benefits you'll derive from it."
"If you have a carb-centered diet for fuel, then you have to be careful about intense exercise (like CrossFit), especially toward the end of a fasting period, because you may run out of fuel and feel pretty horrible (dizzy, lightheaded, nauseous, and weak)," Vincent adds. "This happens when glycogen stores are depleted, which is more likely the longer you have fasted. The good news is, with less intense exercise during an intermittent fast, the body will turn to burning fat for fuel. This is great for anyone looking to trim a few inches around the waist."
When to back off
As we noted above, when it comes to fasting and exercise, there is nothing more important than listening to your body. "One risk is really low blood sugar," Albert Matheny, MS, RD, CSCS, and adviser to Promix Nutrition, says. "So if new for someone then I would not recommend a high-intensity class where you are working at a high-percentage heart rate. Blood sugar will drop rapidly; you may feel faint, could possibly pass out."
Sounds a little scary, right? Vincent goes into more detail, saying a little planning goes a long way.
"The most important thing to consider for people who do intermittent fasting, whether it is 14 hours or 16 hours from dinner until the first meal the next day, is what the first meal of the day is and how that fits into your exercise schedule," he says, adding that It is important to eat protein, complex carbs, healthy fats, and plant-rich fibers during the eating window to maintain a healthy fast. "More complex carbs are needed for workout days. More protein, plant fibers, and fats are needed on rest days."
So, there you have it. Go easy or hard, plan your meals accordingly, and most importantly: Listen to your body.