BDNF and the brain: 12 weeks of cycling didn't lift resting levels, but changed the response.

BDNF is a protein that the brain produces in response to exercise. A whole cult of «neurotrophin from sport» has sprung up around it. A new study from University College London on 23 sedentary adults tested a simple promise. 12 weeks of cycling training were supposed to boost blood BDNF. The baseline resting level did not change. Something else changed – and it's more interesting than a sensational headline.

Let's break down what the numbers showed, why «more neurotrophin» doesn't equate to «a smarter brain,» and what to take away from this to the gym.

What did the study show about BDNF

The Flaminia Ronca team took 23 sedentary adults, of whom 7 were women. Participants were assigned to two groups. The first underwent a 12-week cycling training program. The second was a control group with no intervention.

Before and after, each participant donated blood for neurotrophin in two forms – serum and plasma. In parallel, participants underwent a VO₂max test and a battery of tasks for attention, inhibition, and memory. The function of the frontal cortex was monitored using fNIRS – an optical spectroscopy technique that measures blood flow in the brain during thinking.

The first result is disappointing for supplement proponents. Resting BDNF levels did not increase after 12 weeks. Neither in serum nor in plasma. In other words, the baseline «blood neurotrophin concentration», which is so often promised, did not rise at all from training.

And now, what has grown. By the 12th week, participants had an increased acute response to one type of exercise. After a specific cycling session, the release of serum neurotrophin was higher than at the start of the programme. And this increase correlated with the change in VO₂max. The better the aerobic fitness improved, the stronger the neurotrophin response to the episode.

It is worth distinguishing between the two forms that were measured. Serum reflects protein released from clotted blood, particularly from platelets. Plasma is closer to what circulates freely. The researchers took both precisely because they behave differently – and the result confirmed this: the shift affected the acute-phase serum reactants, not the static background of either form.

Why «more BDNF» isn't the same as a smarter brain

Marketing likes linear logic. You train, neurotrophin grows, the brain works better. Reality is more complex at every junction of this chain.

Firstly, a neurotrophin in the blood is not a neurotrophin in the brain. A blood test detects a protein that circulates throughout the body. How much of it actually reaches neurons and triggers plasticity is another question. A peripheral blood test does not resolve this.

Secondly, the link between neurotrophin and cognition in humans remains uncertain. The authors themselves write about this directly. The association between protein changes from exercise and cognitive improvement remains unexplained. Animal models show a clear effect. Human data comes out mosaic.

Thirdly, form matters. Dinoff's meta-analysis of 55 studies showed that after one bout of exercise, blood neurotrophin levels do indeed rise – moderately, but steadily. The effect is stronger in men and greater in plasma than in serum. This means that the acute spike is real. However, a consistently high baseline is not.

How did this affect the frontal cortex?

The most valuable part is not the number itself, but what it's linked to in the brain. A higher plasma level and sharp increase in a serum marker correlated with a decrease in workload in three areas of the prefrontal cortex. These are the frontopolar, dorsolateral, and orbitofrontal regions – those responsible for attention and control.

A decrease here means better. The brain was doing the same work at a lower «haemodynamic cost». It was allocating resources more efficiently during tasks. Figuratively, it's the same result at lower engine revs, not in the red zone.

But there was a clear boundary. This connection only appeared in tasks of attention and inhibition. It was not present in the memory task. The neurotrophin did not turn out to be a universal switch for the «mind». Its effect is visible in executive functions, not everywhere. This is precisely the detail that advertising posts erase.

Why these specific areas and not memory? The prefrontal cortex is the brain's «dispatcher». It inhibits unnecessary actions, maintains attention, and switches tasks. The acute episode of exercise and the associated release of neurotrophin appear to affect this dispatcher function first. The hippocampus, the memory centre, responds according to different logic and more slowly – this aligns with previous studies where acute exercise improved prefrontal rather than hippocampal function.

It is also important to consider exactly how the brain was measured. fNIRS does not show whether you have become «smarter or not». It shows how much oxygenated blood the brain uses to perform a task. Lower consumption for the same result is a sign of efficiency, not power. Therefore, the correct wording is not «exercise made the brain more powerful», but «fitness made the work of the frontal cortex more efficient in specific tasks».

Why did BDNF become such a big hit in the first place?

To understand where this craze comes from, it’s worth looking at the history of the issue. BDNF — brain-derived neurotrophic factor — is a protein that supports the survival of neurons, the growth of their axons and the formation of new connections. In animal models, it acts as «fertiliser for the brain». Exercise in mice increases its levels in the hippocampus, motor cortex and amygdala — almost everywhere.

Hence the headlines. If exercise in mice boosts neurotrophin levels in the brain, and neurotrophin builds connections — then sport makes the brain smarter through this protein. The chain of reasoning sounds convincing. The problem is that each link, when applied to humans, loses its clarity.

We cannot take a brain biopsy from humans. We take blood samples – but blood reflects peripheral activity, not the brain’s core. A single bout of exercise reliably raises blood levels, as confirmed by dozens of studies. Chronic training, however, has a weak and inconsistent effect on baseline levels. A new UCL study adds another nail to this discrepancy: 12 weeks did not affect resting levels, but did affect the response.

That is precisely why the wording matters. «Exercise boosts BDNF» is true in the short term but only partly true in the long run. Whereas «exercise boosts BDNF, so you’ll become smarter» is a leap of faith into the unknown.

Why is the sample size critical here

Twenty-three people is a small number. Seven women in the sample is even fewer, and this is no trivial matter. Meta-analyses show that the neurotrophin response to exercise differs between the sexes. A sharp increase is more commonly observed in men. With such a sample size, it is difficult to study gender as a separate factor.

This study should therefore be read as an indication of a mechanism rather than as a set of instructions. It points the way forward. Fitness appears to train the brain not to maintain a constantly elevated baseline, but to produce a stronger response when needed. This direction will need to be tested on hundreds of people, not just twenty-three.

There is also a second, more subtle, limitation. The control group did nothing – this is not an active comparison. Therefore, part of the effect can theoretically be attributed to the very fact of regular visits, the attention of the researchers, and changes in routine. A clean design would compare cycling against some other activity, not against nothing. This does not invalidate the result, but it warrants caution in interpretation.

And thirdly, the duration. Twelve weeks is the average duration for such a protocol. It is possible that baseline neurotrophin levels shift later, at week 24 or 36, and the short observation window simply did not capture this. Meta-analyses suggest that a chronic effect on rest does exist, albeit a weak one. In other words, «did not increase in 12 weeks» does not equal «will never increase» — it simply means that there is no quick fix.

As we've already covered in the article about the brain-gut-muscle axis, In this realm of loud molecular connections, there are many, but few reliable ones. Neurotrophin is one of the strongest links. But «strongest» does not mean «proven to domestic advice».

What should I do with this at the gym?

The practical conclusion is simpler than the daunting terminology suggests. Don't chase «boosting baseline BDNF.» Research has shown that 12 weeks of it doesn't increase it. Instead, something else works – improve your aerobic fitness.

The chain of logic is as follows. A higher VO₂max leads to a stronger acute neurotrophin response to an episode of exertion. Specifically, the acute response, not the baseline level, correlated with more effective frontal cortex function. In other words, the goal isn't a one-off surge for the sake of a surge. The goal is the trained ability of the brain to produce neurotrophin when it's needed.

What does this mean in practice. Regular aerobic exercise that actually shifts VO₂max. The cycling in the study is 12 weeks of structured training, not random walks in the park. The second meta-analysis by Dinoff confirms this pattern more broadly. Aerobic interventions have an effect on background neurotrophin. Purely strength training has almost no effect, the effect is statistically insignificant.

That’s no reason to give up on hardware. It works through other channels — we wrote about one of them in materials about irisin. This is a reason to include cardio in the programme if the goal involves the brain and not just the muscles.

What to keep in mind

The sensational headline promised that «cycling boosts neurotrophin and gives your brain a workout». The actual data tells a more nuanced story. Baseline levels remained unchanged over the 12-week period. The acute response to exercise increased. And this — along with fitness — is linked to a more efficient prefrontal cortex in decision-making tasks, rather than in memory. All this is based on a sample of 23 people.

Exercise aerobically for your brain's ability to respond, not for the resting BDNF number that's so easy to sell.

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Sources

• Ronca F, Xu C, Kong E, et al. BDNF is associated with prefrontal cortex activity during physical exercise. Brain Research. 2026;1881:150253. DOI: 10.1016/j.brainres.2026.150253
• Dinoff A, Herrmann N, Swardfager W, et al. The effect of acute exercise on blood concentrations of brain-derived neurotrophic factor in healthy adults: a meta-analysis. European Journal of Neuroscience. 2017;46(1):1635-1646. DOI: 10.1111/ejn.13603
• Dinoff A, Herrmann N, Swardfager W, et al. The Effect of Exercise Training on Resting Concentrations of Peripheral Brain-Derived Neurotrophic Factor (BDNF): A Meta-Analysis. PLOS One. 2016;11(9):e0163037. DOI: 10.1371/journal.pone.0163037