Exercise, stress, immunity and the microbiome
Over the last 100 years humans have tended to adopt a more sedentary
lifestyle, and this appears to have been accelerated with
westernization. Appropriate levels of exercise are associated with
multiple benefits and appear to cause positive changes in the gut
microbiome[96]. Even minimal amounts of exercise have been shown to
be beneficial in helping maintain physical function and avoiding falls
in the frail elderly[97]. Exercise has also been associated with
reduced cardiovascular disease and all-cause mortality, indicating
benefits even at only one third the recommended level[98,99].
However, excessive exercise, as in the overtraining syndrome, can
increase stress and cause a decrease in sleep quality and immune
function[100]. Thus, care must be taken to maintain an appropriate
level of exercise for maximum health benefits. At least 150 minutes of
moderate-intensity exercise per week has been recommended to be
beneficial for the average person and even greater benefits were found
at 3 to 5 times the recommended level[99]. However, the type and
amount of exercise must be tailored to the health status of the
individual.
Unfortunately, many people in westernized societies do not achieve the
optimal level of exercise. There may be many reasons for this, including
lack of energy and motivation. These problems might be ameliorated by a
diet that is less associated with inflammation. Inflammation appears to
have negative effects on brain areas associated with motivation[101]
and has the potential to diminish energy production by
mitochondria[102]. Neuroinflammation and impaired mitochondrial
function have been linked to a highly processed high-fat diet fed to
mice[103]. Thus, diets such as those discussed above that emphasize
whole plant foods and appear to have anti-inflammatory effects may
enhance the ability and motivation to engage in adequate exercise.
Thus, overall, there is evidence that air pollution and a westernized
diet and lifestyle contribute to multiple common chronic diseases and
their associated elevations in inflammation. It has been suggested that
both air pollution and a westernized diet contribute to dysbiotic human
microbiotas, both indirectly and directly, through post-hunter-gatherer
era microbes and various substances present in polluted air and a
westernized diet[12]. It might be postulated that the immune
reactions to these pathogenic/dysbiotic microbes may be kept to a
relatively low-grade inflammation because immune system signaling only
indicates the presence of a slow-growing chronic microbial
colonization/infection rather than a rapidly increasing microbial
threat. However, this immune system signaling could change in the
context of an acute infection, as discussed further below.