Exercise usually helps the brain. During a 15-day fast, the story was different

By Hugo Francisco de SouzaReviewed by Susha Cheriyedath, M.Sc.Sep 15 2026

A closely monitored 15-day fast reveals how attention, learning, mood, and exercise interact as the body adapts to a severe energy shortage.

Study: 15-day fasting and aerobic exercise impair cognitive abilities and mental states. Image Credit: FeriDhaniHasri / Shutterstock

In a recent study published in the journal Scientific Reports, researchers investigated the physiological, cognitive, and psychological outcomes of prolonged complete fasting combined with aerobic exercise. The study comprised a 28-day clinical trial that tracked 22 healthy adults through a 15-day complete food-deprivation protocol to examine how severe energy restriction affects attention and inhibitory control, internal time estimation, and implicit learning.

Study findings revealed that fasting produced a rapid metabolic shift, reflected by lower blood glucose and higher ketone levels, while its cognitive effects were selective rather than uniform. Performance on the Go/No-go task followed a non-linear "double-hump" trajectory, with responses slowing on day 4, partially recovering by day 10, and slowing again by day 14. The authors proposed that metabolic adaptation through ketosis may have temporarily supported cognitive performance, but the study did not directly measure brain fuel use.

The study found that the moderate-intensity exercise group also showed poorer implicit learning on some measures, with more errors than sedentary controls and less pronounced improvement in reaction times over repeated testing. They suggested that competition for limited energy during fasting could help explain this pattern, but brain energy allocation was not measured. These findings indicate that physical exertion during severe food deprivation may offset some of the cognitive benefits typically associated with exercise, with potential relevance to survival settings and prolonged fasting protocols.

Background

Fasting has attracted growing public and research interest, but controlled food deprivation is by no means a modern concept. Historical records indicate that fasting has been practiced across cultures for millennia for religious and cultural reasons.

Research has linked some fasting approaches to potential metabolic and neurological benefits, but their effects vary by protocol and population. Long-term complete fasting (total food deprivation lasting 4 or more days) produces a major metabolic shift from glucose toward ketone use and can affect mood and cognition.

Regular physical activity can support brain health and cognitive function, but far less is known about exercise during severe food deprivation. Earlier studies examined shorter periods of calorie deprivation combined with exercise, leaving prolonged complete fasting combined with aerobic activity largely untested.

About the study

The present study examined this gap by systematically tracking the impacts of complete fasting and routine physical activity in 22 healthy participants (7 females and 15 males; mean age = 41 years; mean body mass index [BMI] = 24.84 kg/m2) through a 28-day supervised experimental protocol at the Space Science and Technology Institute in Shenzhen, China.

Study participants were randomly assigned to one of three cohorts: 1. Moderate-intensity running (Group EM; treadmill running at 5 kilometers per hour [km/h] on fasting days 1, 3, and 5; n = 7), 2. Light-intensity exercise (Group EL; 3 km/h on fasting days 2, 4, 6, 8, 10, and 12; n = 8), and 3. Sedentary controls (Group CN; no running; n = 7).

The study’s experimental protocol comprised four distinct phases: 1. A 3-day baseline (BL) evaluation of participants’ physiological, cognitive, and psychological measures, 2. A 15-day complete fasting (CF) period during which participants could drink water freely and received intensive medical supervision, with ECG and continuous glucose monitoring used during aerobic exercise, 3. A 5-day gradual calorie restriction (CR) refeeding phase, and 4. A 5-day full recovery (FR) phase.

Researchers assessed participants’ cognitive performance using three primary behavioral tasks: 1. A Go/No-go task, 2. A Time Perception task, and 3. An Auditory Serial Reaction Time (ASRT) task.

Study findings

Physiological assays revealed that fasting lowered participants’ blood glucose concentrations to about 3.2 millimoles per liter (mmol/L) early in the fasting phase alongside significant increases in their blood ketone concentrations from about 0.1 mmol/L at baseline to more than 5 mmol/L later in fasting. Both measures quickly returned to baseline after food was reintroduced.

Starvation-induced metabolic transformations were associated with slower responses on Go trials. Mean reaction time (RT) increased from about 0.415 seconds at baseline to 0.445 seconds on day 4, partially recovered to 0.422 seconds on day 10, and increased again to about 0.445 seconds by day 14. The authors interpreted this pattern as reduced attention maintenance during fasting. Time-perception performance showed an overall time effect, but comparisons between individual study phases were not significant, and the authors described temporal processing as stable. Psychological ratings were generally stable, though participants reported greater sleepiness during fasting and reduced calmness at some time points; exercise groups showed no broad differences in mental state.

Among the 17 participants included in the final ASRT analysis, after two did not complete later assessments and three were excluded because of high error rates, physical activity evaluations revealed that combining running with fasting was associated with less favorable performance on some implicit sequence-learning measures in the moderate-exercise group. The moderate-exercise cohort (Group EM) committed significantly more errors than sedentary controls. Participants across the experiment still showed general and sequence-specific learning, but improvements in the moderate-exercise group were less pronounced across testing. The pattern persisted after the 5 km/h running sessions ended on fasting day 5, which the authors said may indicate a lingering effect of early physical stress during metabolic adaptation.

Conclusions

This study is among the first to examine prolonged complete fasting combined with exercise under controlled conditions. The findings suggest that fasting selectively affected attention and learning, while time perception remained relatively stable, and that moderate physical exertion was associated with poorer performance on some learning measures.

The authors propose energy-conservation and reticular-activating hypofrontality models as possible explanations, including competition for limited energy between higher-order cognition and motor, sensory, and autonomic demands. The study did not measure regional brain metabolism, so this proposed mechanism was not directly tested.

These findings may help inform future survival protocols in extreme food-shortage environments, particularly the timing and intensity of exercise during prolonged fasting. The authors state that larger studies are needed to identify appropriate exercise patterns. Interpretation is also limited by the small sample size, uneven sex distribution, lack of objective measures of hydration and sleep, potential motivational effects, and the exclusive focus on aerobic exercise.