Study: Moderate Exercise During Prolonged Fasting Impairs Learning and Attention

Twenty-two healthy adults entered a controlled laboratory at the Space Science and Technology Institute in Shenzhen, China, and stopped eating for fifteen days. Researchers wanted to know what happens to the brain when the body runs out of fuel—specifically, how attention, learning, and physical performance change when someone exercises while fasting completely.

The study divided participants into three groups. Seven ran at moderate intensity on a treadmill during the first, third, and fifth fasting days. Eight did light-intensity walking on alternating days. Seven did no exercise at all. All of them drank water freely and received continuous medical supervision, with heart monitors and glucose sensors tracking their bodies as the fast progressed. The experiment lasted twenty-eight days total: three days of baseline testing, fifteen days of complete fasting, five days of gradual refeeding, and five days of full recovery.

Within days, the body shifted its fuel source. Blood glucose dropped to about 3.2 millimoles per liter—dangerously low by normal standards—while ketone levels surged from 0.1 to more than 5 millimoles per liter. The brain was learning to run on ketones instead of glucose. But the cognitive effects were not straightforward. When researchers gave participants a simple attention task—press a button when you see a signal, don't press when you don't—reaction times followed an odd pattern. On day four, responses slowed from 0.415 seconds to 0.445 seconds. By day ten, performance partially recovered to 0.422 seconds. Then by day fourteen, it slowed again to 0.445 seconds. The body was adapting, but attention was not stable.

The learning picture was more troubling. Seventeen participants completed a task that measured implicit learning—the ability to pick up on patterns without conscious effort. The moderate-exercise group made significantly more errors than those who stayed sedentary. Their improvements in reaction time across repeated trials were less pronounced. Even after the running sessions ended on day five, the pattern persisted. The researchers suggested that moderate exercise during early fasting may have created competition for the body's limited energy, forcing the brain to choose between powering cognition and powering muscles. No one measured brain metabolism directly, so this remained a hypothesis.

Time perception, surprisingly, stayed relatively stable throughout the fast. Participants' sense of how much time had passed did not deteriorate the way attention and learning did. Mood shifted in smaller ways: people reported more sleepiness during fasting and less calmness at certain points, but exercise groups showed no broad differences in mental state.

The findings raise a practical question. In survival situations—shipwrecks, plane crashes, extreme food shortages—people often need to move while hungry. This study suggests that moderate physical exertion during prolonged fasting may actually impair the cognitive abilities needed to make good decisions or learn from mistakes. The authors propose that larger studies are needed to determine when and how hard someone should exercise during extended food deprivation. The current work was limited by its small sample size, more men than women, and lack of direct measurement of sleep and hydration. But it marks one of the first times researchers have systematically tracked what happens to cognition when exercise and complete fasting collide.