Your body needs energy to fuel a workout. It first draws on sugar (glucose) that’s readily available from recently eaten food, pulling it from the bloodstream into the muscles.
If you’ve fasted and there’s no glucose available, the liver steps in. “Your liver acts like a glucose savings account. It stores sugar as glycogen and releases it when needed, especially during exercise,” says Dr. Koutnik. “When you exercise without eating, your liver releases that glucose to keep blood sugar stable.”
Some people experience the opposite, though, where blood sugar levels rise during exercise. This can happen for a few reasons:
- High-intensity workouts, such as weight lifting and sprinting, can trigger the body to produce stress hormones, such as adrenaline, prompting the liver to release glucose into the bloodstream.
- Low levels of insulin — the hormone that normally blocks glucose release — can encourage the liver to release glucose into the bloodstream more easily.
A number of other factors can also affect whether blood sugar levels drop or rise during a fasted workout, including the length of the exercise session, the time spent fasting, and your overall health, says Ricardo R. Correa, MD, an endocrinologist and adjunct assistant clinical professor of internal medicine at the University of Arizona College of Medicine in Phoenix.
“This is where people get tripped up. The problem isn’t the ‘fasting,’ per se. It’s the release of stress hormones,” he says. “Intense and prolonged exercise while fasted increases cortisol and adrenaline, which can raise your blood sugar.”
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