If this is true, experiments in mice may offer an explanation. In these animals, enforced fasting has been found to cause changes in the brain that are likely to give neurons more energy, and enable them to grow more connections.
Mark Mattson of the National Institute on Aging in Maryland and his team looked at 40 mice that consumed the same total calories, but either ate normally every day or ate nothing every other day.
The team found that fasting caused a 50 per cent increase in a brain chemical called BDNF. Previous studies have shown that such a rise 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.
The finding makes sense from an evolutionary perspective, as animals that are hungry would benefit from more intellectual resources to find food, he says. "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 DC last month.
However, results in mice don't always translate to people. Other studies have found that living on a permanent low-calorie diet can allow mice to live as much as 50 per cent longer, although the effect seems smaller in primates.
Mattson is currently testing the 5:2 diet in obese older people in a randomized trial to see if it really does make them mentally sharper.
Comment: More on the benefits of fasting: