walnut on spoon
© Gergely Kishonthy / Alamy Stock PhotoFasting diets may make you smarter.
Could regular fasting make you smarter? People following regimes like the popular 5:2 diet usually do so for weight loss, but some who try it say it makes them mentally sharper too.

If this is true, experiments in mice may have explained why. In these animals, fasting has been found to cause changes in the brain that likely give neurons more energy, and enable them to grow more connections.

Mark Mattson of the National Institute on Aging in Bethesda, Maryland and his team looked at 40 mice, which were given regimes in which they either ate nothing every other day, or ate normally - but consumed the same total calories as the fasting mice.

The team found that fasting was linked to a 50 per cent increase in a brain chemical called BDNF. Previous studies have shown that such an increase is likely to boost the number of mitochondria - which provide a cell's energy - inside neurons by 20 per cent.

BDNF also promotes the growth of new connections - or synapses - between brain cells, which helps in learning and memory, says Mattson.

Brainpower boost

The finding makes sense from an evolutionary perspective, as animals that are hungry would need more intellectual resources to find food, says Mattson. "If human ancestors hadn't been able to find food, they had better be able to function at a high level to chase down some prey."

The team's results were presented at the Society for Neuroscience conference in Washington D.C. last month. However, results in mice do not always translate to people. For instance, other studies have found that living on a permanent low-calorie diet can lengthen the lifespan of mice by as much as 50 per cent, but in primates the effect seems smaller.

In the 5:2 diet, people stick to 500 to 600 calories on two days of the week and eat normally on other days. In another version, every day people cram all their meals into a six-hour window. The theory is that when blood sugar levels fall, the body gets energy from fat, which triggers metabolic changes said to be beneficial in various ways.

Matteson is currently testing the 5:2 diet in obese older people in a randomised trial to see if it really does makes people mentally sharper.