A build-up of lactic acid is often wrongly blamed for making our muscles feel heavy and tired after an intense workout. Scientists have now proved that it is actually a friend, not a foe, that improves the health of the brain when we exercise.
A study even suggests that we could give our brains a similar boost by injecting the substance created by our muscles without having to exercise at all.
During exercise the body breaks down glucose to release energy. During a more gentle workout, oxygen from breathing is used as the main fuel, helping to break down the glucose in a process known as aerobic glycolysis.
During more intense exercise, the body can no longer get enough oxygen to keep up with demand from the muscles. At this point, the body has to start breaking down glucose without oxygen, in a process known as anaerobic glycolysis. One by-product from this process is called lactate.
It is common to hear amateur athletes and even sports commentators blame a “build-up of lactic acid” for muscle fatigue, but this is incorrect in two ways.
Lactate is often described as lactic acid, though this is not quite correct as lactic acid loses hydrogen to become lactate. It is levels of lactate, not lactic acid, that increase during vigorous exercise.
Lactate is a by-product of heavy exercise
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Second, lactate actually acts as a form of fuel that can be turned back into glucose and can help to combat rather than exacerbate fatigue.
Scientists have now proved, however, that lactate has another important function that makes gains for the brain.
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Research has previously shown that an exercise session can increase the levels in the body of a crucial protein called brain-derived neurotrophic factor or BDNF. This protein helps to maintain the healthy growth of neurons in the brain and also increases the “plasticity” of our brain’s message-sending cells, which helps improve learning and memory.
Scientists from Sweden said it was not previously clear how exercise boosts the levels of this protein. They set out to find the missing piece of the puzzle, aiming to identify the mechanism that drives “the link between exercise and brain health”. They hypothesised that the lactate produced by muscle and red blood cells during intense exercise may trigger the production of BDNF.
For the study, published in the journal Frontiers in Cellular Neuroscience, 12 healthy volunteers aged between 20 and 40 were asked to fast. They then received an hour-long infusion into their blood of lactate followed a few weeks later by an hour-long infusion of saline solution. Some participants had this process in reverse, with blood samples taken every 10 minutes during the treatment and at intervals afterwards.
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Levels of lactate in their blood increased as expected, but the results showed something else.
“Importantly, the concentration of the prohormone pro-BDNF increased 15 minutes after the end of lactate infusion and stayed high for two hours afterwards.”
This proves that lactate is the mechanism which triggers increased levels of the brain-boosting hormone and that it can do so even when simply delivered intravenously rather than through exercise.
Asked if this could lead to a form of treatment in future that would allow people to enjoy the brain gains of exercise without lifting a finger, the researchers said they “caution against skipping the exercise step and relying on lactate infusion alone”, especially as exercise benefits the body in many other ways.
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“The intensity of exercise is important if brain health is the desired goal,” said Dr Marcus Moberg of the Swedish School of Sport and Health Sciences in Stockholm. “High-intensity exercise a couple of times per week, thus exposing the brain to high levels of lactate, should be recommended for healthy neurological aging.”