Fasting after learning might sharpen some memories, but blur others, study suggests

When you’re hungry after learning something, your brain might prioritize remembering some kinds of information over others. A small preliminary study published in Neurobiology of Learning and Memory found that fasting after learning boosted memory for general knowledge and facts, but at the expense of memories for specific details like where and when events occurred. This indicates that being in a fasted state could shift the brain’s focus during memory consolidation.

Scientists have long been interested in how memories are strengthened, a process known as consolidation. Sleep is widely recognized as a key time for this process. However, emerging research
suggests that other states, like being hungry, might also influence how our brains solidify new information.

Previous studies in animals, and to some extent in humans, have hinted at a connection between fasting and memory. For example, research in fruit flies (Drosophila melanogaster) showed that starvation could enhance memory consolidation while the flies were awake. While these earlier findings were intriguing, it was not clear if fasting specifically affected the consolidation process itself, or if it influenced other aspects of memory, such as the initial learning or the later remembering of information.

“I am a memory researcher with a primary interest in how sleep consolidates memory,” said study author Jan Born, the head of the Institute of Medical Psychology and the Behavioral Neurobiology department at the University of Tübingen.

“My interest in hunger derived from a study in Drosophila (by Amita Seghal’s group) showing that fasting can enhance memory as well as sleep, although through different mechanisms. The study represents the first step in translating these findings in Drosophila to healthy humans.”

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To explore this question in humans, the researchers recruited healthy men for two separate experiments. In each experiment, participants took part in two conditions: one where they fasted, and another where they ate regular meals. To ensure a clear comparison, each participant experienced both the fasting and eating conditions at different times, with at least a four-week gap in between. This design allowed researchers to compare the effects of fasting versus eating within the same individuals.

Before the start of each condition, participants followed a regular diet for two days and avoided caffeine and alcohol. On the first day of each condition, participants arrived at the lab after having a standardized lunch and then began an 18.5-hour fasting period. The next morning, in a fasted state, they engaged in several memory tasks.

In Experiment 1, these tasks included learning pairs of words, completing a visual memory test using abstract shapes, and performing a finger tapping exercise to assess motor skill memory. In Experiment 2, the visual shape task was replaced with a more complex ‘What-Where-When’ memory test, which examined memory for objects, locations, and times. The word pair task and finger tapping task remained the same in both experiments.

After these learning tasks, the critical consolidation period began. In the ‘satiated’ condition, participants received standardized meals throughout the next ten hours, including breakfast, lunch, and snacks. In the ‘fasting’ condition, they continued to fast for the same ten-hour period, only receiving water and fruit tea. In both conditions, participants received a standardized dinner after this ten-hour consolidation period. The researchers carefully standardized the meals to ensure they were not overly appealing, to minimize any potential positive reinforcement from eating. The meals were also tailored to each participant’s individual calorie needs based on their height, weight, age, and activity level.

To assess memory recall, participants returned to the lab either 24 or 48 hours later, depending on the experiment. In Experiment 1, recall was tested after 48 hours, and participants were in a satiated state, having eaten before the test. In Experiment 2, recall was tested earlier, after 24 hours, and participants were in a fasted state, mirroring their state during the initial learning phase. During the recall sessions, participants were tested on the same memory tasks they had learned earlier. Throughout the study, researchers monitored blood glucose levels and asked participants to rate their hunger to confirm that the fasting manipulation was effective. They also used questionnaires to assess mood, fatigue, and sleepiness to account for these factors in their analysis.

The results of the first experiment showed that fasting during the 10-hour consolidation period improved memory for word pairs. Participants in the fasting condition recalled more word pairs correctly and responded faster when recalling them compared to when they were in the satiated condition. However, fasting impaired spatial memory in the visual Deese-Roediger-McDermott task. Participants in the fasting condition were worse at remembering the locations of the shapes. Procedural memory, as tested by the finger tapping task, was not affected by fasting.

In the second experiment, the researchers found that fasting enhanced recognition memory in the “What-Where-When” task. Participants in the fasting condition were better at recognizing previously seen images and were faster at this recognition. However, fasting again showed a negative effect on episodic context memory. Participants in the fasting condition performed worse on remembering the temporal context (when an image was presented) and the combined spatial and temporal context (where and when an image was presented). Similar to the first experiment, fasting had no impact on procedural memory in the finger tapping task.

These findings suggest that fasting has a selective impact on memory consolidation in humans. It appears to enhance the consolidation of semantic-like memories, which are more about general knowledge and recognition of items. This type of memory is thought to rely more on brain areas in the cortex, the outer layer of the brain. Conversely, fasting seems to weaken the consolidation of episodic context memories, which are more detailed and contextual and depend more on the hippocampus.

The researchers speculate that when we are hungry, our brains might prioritize solidifying basic, factual information while perhaps suppressing the encoding of detailed contextual information. They suggest that hunger might reduce interference from the hippocampus, which normally plays a strong role in episodic memory, allowing for more efficient consolidation of semantic-like representations in the cortex. But further research is needed to confirm and expand on these findings.

“This is basic research and, in my view, it is too early to infer any application from these findings,” Born told PsyPost. “Perhaps, one should be aware that (slight) starvation might be a condition to enhance (certain kinds of) memory.”

As an initial study in humans inspired by research in fruit flies, it opens up many avenues for future research. Future studies could use brain imaging to examine which brain networks are most active during memory consolidation in fasted and fed states. It would also be important to investigate the underlying biological mechanisms. For example, researchers could explore the role of hunger-related hormones and metabolic signals, such as neuropeptide Y, orexin, ghrelin, leptin, glucagon-like peptide 1, and blood glucose levels, to understand how they mediate the effects of fasting on memory consolidation. Understanding these mechanisms could provide new insights into how our nutritional state influences memory and brain function.

The study, “The effect of fasting on human memory consolidation,” was authored by Xuefeng Yang, Xiu Miao, Franziska Schweiggart, Sophia Großmann, Karsten Rauss, Manfred Hallschmid, Jan Born, and Nicolas D. Lutz.