A stomach hormone that stimulates appetite seems to promote the growth of new brain cells and protect them from the effects of ageing, and may explain why some people say that fasting makes them feel mentally sharper (New Scientist, 2017).

When ghrelin was first discovered, it became known as the hunger hormone. It is made by the stomach when it gets empty, and whenever we go a few hours without food its levels rise in our blood. But there is also evidence that ghrelin can enhance cognition. Animals that have reduced calorie diets have better mental abilities, and ghrelin might be part of the reason why. Injecting the hormone into mice improves their performance in learning and memory tests, and seems to boost the number of neuron connections in their brains.

Now Jeffrey Davies at Swansea University and his team have found further evidence that ghrelin can stimulate brain cells to divide and multiply, a process called neurogenesis. When they added the hormone to mouse brain cells grown in a dish, it switched on a gene known to trigger neurogenesis, called fibroblast growth factor.

If the same effect happens in animals, this could be how ghrelin exerts its effects on memory, said Jeffrey Davies, whose work was presented at the British Neuroscience Association conference this month.

Young brain cells are thought to enhance the ability to form new memories. This is because they are more excitable, so are more likely to be activated by new environments. “These neurons will fire more easily than old neurons, and they set in play a new memory” said Jeffrey Davies.

The work may also have implications for treating neurodegenerative conditions, such as Parkinson’s disease, which is caused by a loss of a type of brain cell. Previous research, including some by Jeffrey Davies’ team, has found that ghrelin can help protect animals from developing a form of Parkinson’s disease.

In further experiments, Jeffrey Davies’s team found that ghrelin protects brain cells in a dish from dying when they are encouraged to mimic Parkinson’s disease. And Jeffrey Davies’s colleague Amanda Hornsby found that, in a study of twenty eight volunteers, people with Parkinson’s dementia, cognitive impairment caused by Parkinson’s disease, have lower levels of ghrelin in their blood than people who don’t have the condition. This suggests that ghrelin, or other chemicals that act the same way, could be used as a treatment for Parkinson’s dementia, said Amanda Hornsby.

In humans, going on a permanent diet of about 25% fewer calories than the daily recommended amount has several benefits to health, such as better control of blood sugar levels. Some who try it have said it also improves their cognitive abilities, although this is controversial, with some studies have suggested it impairs people’s mental abilities.

In an effort to harness some of the health benefits of a calorie restricted diet, some people are turning to intermittent fasting. It’s likely, for example, that the 5-2 diet, where people eat normally for five days but stick to about five hundred calories a day for the other two, raises ghrelin levels.

Nicolas Kunath, of the Technical University of Munich, points out that new brain cells take a few days to weeks to start working, so people shouldn’t expect fasting to produce immediate effects on their brainpower in this way.