Brain-derived neurotrophic factor (BDNF) is also a myokine, though BDNF produced by contracting muscle is not released into circulation. Rather, BDNF produced in skeletal muscle appears to enhance the oxidation of fat. Skeletal muscle activation through exercise also contributes to an increase in BDNF secretion in the brain. A beneficial effect of BDNF on neuronal function has been noted in multiple studies. Dr. Pedersen writes, "Neurotrophins are a family of structurally related growth factors, including brain-derived neurotrophic factor (BDNF), which exert many of their effects on neurons primarily through Trk receptor tyrosine kinases. Of these, BDNF and its receptor TrkB are most widely and abundantly expressed in the brain. However, recent studies show that BDNF is also expressed in non-neurogenic tissues, including skeletal muscle. BDNF has been shown to regulate neuronal development and to modulate synaptic plasticity. BDNF plays a key role in regulating survival, growth and maintenance of neurons, and BDNF has a bearing on learning and memory. However, BDNF has also been identified as a key component of the hypothalamic pathway that controls body mass and energy homeostasis.
With respect to studies of exercise and brain function, a 2010 report is of particular interest. Erickson et al. have shown that the volume of the anterior hippocampus increased by 2% in response to aerobic training in a randomized controlled trial with 120 older adults. The authors also summarize several previously-established research findings relating to exercise and brain function: (1) Aerobic exercise training increases grey and white matter volume in the prefrontal cortex of older adults and increases the functioning of key nodes in the executive control network. (2) Greater amounts of physical activity have been associated with sparing of prefrontal and temporal brain regions over a 9-y period, which reduces the risk for cognitive impairment. (3) Hippocampal and medial temporal lobe volumes are larger in higher-fit older adults (larger hippocampal volumes have been demonstrated to mediate improvements in spatial memory). (4) Exercise training increases cerebral blood volume and perfusion of the hippocampus.
Regarding the 2010 study, the authors conclude: "We also demonstrate that increased hippocampal volume is associated with greater serum levels of BDNF, a mediator of neurogenesis in the dentate gyrus. Hippocampal volume declined in the control group, but higher preintervention fitness partially attenuated the decline, suggesting that fitness protects against volume loss. Caudate nucleus and thalamus volumes were unaffected by the intervention. These theoretically important findings indicate that aerobic exercise training is effective at reversing hippocampal volume loss in late adulthood, which is accompanied by improved memory function."