Page 129 - Biobehavioral Resilence to Stress

106 Biobehavioral Resilience to Stress

For example, physical exertion and mental stress are known to cause robust
activation of the HPA axis. However, this response tends to be dampened
in exercise-trained, aerobically fit individuals. The result is reduced cortisol

release, less brain exposure to cortisol, and possibly less neuronal damage.

Additionally, it is well known that aerobically fit individuals tend to have
reduced heart rate and blood pressure and that these parameters return more

rapidly to baseline after physical exertion in individuals who are aerobically

fit (Dienstbier, 1991).
Exercise also appears to reduce allostatic load by enhancing neurogenesis
and increasing a number of nerve growth factors such as BDNF. BDNF is a
protein that is known to enhance the growth of brain cells, cell survival (Barde,
1994), formation of connections between neurons based on activation patterns,
and learning and repair of damaged nerve cells (Cotman & Berchtold, 2002).

Unlike chronic stress, which reduces the effectiveness of neurotrophic fac-
tors, aerobic exercise increases the production of these factors by “turning on”
relevant genes. Increased levels of neurotrophic factors (e.g., BDNF) have been

detected in rodent brains after several days to several weeks of wheel running.
It is also likely that chronic stress impairs neurogenesis in human subjects,
which may, in turn, contribute to depression and poor cognitive functioning.
Further, it is likely that vigorous aerobic exercise increases neurogenesis in
human subjects, which may help to improve cognition and reduce symptoms
of depression (Smith, Makino, Kvetnansky & Post, 1995).
Finally, aerobic exercise may serve to protect against the negative eff ects
of future stress. Rats that are kept stationary for a week prior to severe
experimentally induced stress experience greater reductions in hippocam-
pal BDNF and tolerate stress well lesser than do rats that have engaged in
voluntary wheel running for the same 1-week period prior to induced stress
(Russo-Neustadt, Alejandre, Garcia, Ivy & Chen, 2004). Analogously, in
a prospective study of adult human subjects, Harris and colleagues found
that exercise predicted less concurrent depression over a period of 10 years
(Harris, Cronkite & Moos, 2006). It is important to note, however, that not
all studies have observed that physical activity confers protective eff ects. For
additional consideration of exercise and protective effects against stress and

trauma, see Friedl and Penetar (this volume).

Social Support

Social support has been extensively studied in relation to stress and resil-
ience. Clinical studies of traumatic stress clearly demonstrate the benefi cial
effects of social support on mental health. For example, Boscarino (1995)

found that after controlling for trauma exposure, military veterans with high

levels of social support were 180% less likely to develop PTSD compared with
those with low levels of social support.

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