It was the late 1880s in the city of Rajkot, India. The meeting was to take place on the banks of the local river – and discretion was essential. Mahatma Gandhi, who was just a teenager at the time, hadn’t told his parents where he was going; if they had found out, they would have been shocked to death.

As it happens, Gandhi was having a picnic. And on this occasion, India’s future national hero – and one of the most famous vegetarians in history – wasn’t planning to dine on cucumber sandwiches. No, for the first time in his life, he was going to eat meat.

As he later wrote in his biography, Gandhi was raised as a strict Vaishnava Hindu, so he had never even seen meat before this fateful day. But his picnic companion was a shady character with an unusual obsession – the idea that meat held the key to being physically and mentally strong.

In the end, Gandhi braved the meat. It was as tough as leather.

More stories on The Vegan Factor from BBC Good Food

The idea that avoiding meat is bad for our brains makes some intuitive sense; anthropologists have been arguing about what our ancestors ate for decades, but many scientists think that there was a lot of bone-crunching and brain-slurping on the road to evolving these remarkable 1.4kg (3lb) organs. Some have even gone so far as to say that meat made us human.

One reason is that intelligence is expensive – the brain devours about 20% of our daily calories, though it accounts for just 2% of our body weight – and what better way to find the enormous array of fats, amino acids, vitamins and minerals these fastidious organs require, than by feasting on animals which have already painstakingly collected or made them.