BDNF

What is BDNF?

BDNF, short for brain-derived neurotrophic factor, is a signaling protein in the neurotrophin family. It helps regulate neural development and the formation and modification of synapses. Its expression, release, and effects interact with neural activity. Park and Poo review these mechanisms in Neurotrophin Regulation of Neural Circuit Development and Function (2013).

BDNF, learning, and memory

Learning involves changes in connections between neurons, a process called synaptic plasticity. BDNF participates in that process. This biological role is why researchers study its relationship with learning and memory.

What can BDNF levels tell us?

BDNF measured in blood is not a simple measure of brain health or learning ability. A 2020 systematic review and meta-analysis by McPhee, Downey, and Stough found a weak, statistically nonsignificant pooled relationship between BDNF and cognitive outcomes, with substantial differences between studies.

BDNF's role in neural plasticity does not mean that raising a blood measurement necessarily improves cognition.

BDNF and neurological or mood disorders

Researchers also examine BDNF in neurological and mood disorders. An association with illness does not, by itself, show that reduced BDNF causes the illness or explains a treatment’s benefits. The mechanism, the measurement, and the clinical outcome are separate questions.

Serum BDNF in a depression study

For example, Shimizu and colleagues (2003) measured BDNF in blood serum from 16 patients with major depression who had not taken antidepressants, 17 treated patients, and 50 controls. The untreated group had lower serum BDNF than either comparison group. Across the patients, lower levels were also associated with higher depression ratings.

What the comparison can show

The finding concerned serum, not a direct measurement inside the brain. The authors proposed a role for BDNF in depression, informed by earlier animal research, but this group comparison did not establish that low BDNF caused the illness. A preliminary follow-up of three patients found increased levels after treatment; it did not establish that the increase caused clinical recovery. A biological association, a proposed mechanism, and an effective treatment are different findings.