Can an empty stomach help us learn?

In nature, the act of getting food is not as simple as just going to the fridge in order to get sufficient energy and nutrients. In the animal kingdom, searching for and stockpiling of enough food, defending it from enemies and consuming it at an appropriate time require some of the highest cognitive skills. Therefore, it is not at all surprising that numerous hormones controlling the energy balance of an organism also intervene in cognitive processes in a modulatory way, even partly influencing them positively. This also applies to ghrelin – at least as far as mice and rats are concerned.

For more than a decade, these facts have been repeatedly proven. Only little, however, was known about whether and how ghrelin influences cognitive performance in humans. For the first time, scientists in Prof. Axel Steiger’s research group at the Max Planck Institute of Psychiatry investigated the effects of ghrelin on spatial learning and on a series of other mental exercises in humans. To this end, 21 healthy men took a virtual walk: The scientists modified a freeware computer game for their purposes. The participants “walked” through a digital suburb while the activity of several brain areas was recorded in real-time using a state-of-the-art functional magnetic resonance imaging scanner (fMRI).

The participants were asked to memorize certain words that appeared on the screen during their virtual walk. First, ghrelin was administered, then, on a second date, a placebo. In this double-blind method, neither the participants nor the scientists involved knew which substance had been given – only the physicians acting in the background knew. The following day, the participants were asked which words they had remembered. The results of this investigation have recently been published in the prestigious journal NeuroImage: There was no difference at all between the effects of ghrelin and placebo.

What conclusions can be drawn from these results? This much seems certain: ghrelin does not seem to be a miracle drug that can turn us into memory geniuses. However, what about the long-term administration of the hormone? How are cognitive processes of emotional content influenced, e.g., the thought that a meal looks delicious or tastes bland? What effects do the endogenous amounts of ghrelin, i.e., the amounts that are naturally produced in humans, have on our cognitive performance? This research approach may help us understand the development of and provide therapy for diseases such as anorexia, diabetes and Parkinson’s disease as well as Alzheimer’s disease.

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