A build-up of “lactic acid” is often wrongly blamed for making muscles feel heavy and tired after an intense workout. Scientists have now proved that the substance formed during exercise is a friend, not a foe, which improves the health of the brain.

A study even suggests that we could give our brains a similar boost by injecting it, skipping the exercise entirely.

During exertion the body breaks down glucose to release energy. For a more gentle workout, oxygen from breathing is used as the main fuel to help break down the glucose.

During more intense exercise, the body cannot meet the demand for oxygen from the muscles. The body has to break down glucose without it, in a process known as anaerobic glycolysis.

One by-product from this is lactate.

Some blame a “build-up of lactic acid” for muscle fatigue, but this is incorrect in two ways. First, it is levels of lactate that increase during vigorous exercise, not lactic acid. Second, lactate can actually help to combat fatigue, not exacerbate it. Now scientists in Sweden have found it also helps the brain.

Research has previously shown that exercise can increase the levels of a protein called brain-derived neurotrophic factor or BDNF, which helps maintain the healthy growth of neurons and the “plasticity” of message-sending cells, improving learning and memory.

For the study, published in the journal Frontiers in Cellular Neuroscience, 12 healthy volunteers aged between 20 and 40 received an hour-long infusion of lactate and another of saline solution a few weeks apart. Blood samples were taken every ten minutes during the treatment and at intervals afterwards.

The researchers found BDNF levels increased after lactate infusion and stayed high for two hours, proving that lactate is the mechanism which triggers increased levels of the brain-boosting hormone. This could be delivered intravenously, though the team cautioned against skipping exercise.

Dr Marcus Moberg, of the Swedish School of Sport and Health Sciences, recommended intense exercise twice a week “for healthy neurological ageing”.