What are Myokines? The latest research linking exercise to chronic illness prevention
We’ve all been told that keeping active is important to maintain health into old age. But how does this work? The answer may lie in your hormones.
• Alzheimer’s disease, cognitive decline, type 2 diabetes (T2DM), depression, cardiovascular disease (CVD), colon cancer and postmenopausal breast cancer are diseases associated with chronic inflammation.
• Exercise requires muscle contractions that release hormones, known as ‘myokines’.
• Myokines mediate inflammation, stimulate metabolic functions, regulate skeletal muscle regeneration and improve glucose uptake and fat oxidation, directly impacting the diseases linked to chronic inflammation.
Physical inactivity has been linked to Alzheimer’s disease, cognitive decline, type 2 diabetes (T2DM), depression, cardiovascular disease (CVD), colon cancer and postmenopausal breast cancer. These diseases, termed ‘the diseasome of physical inactivity’, are very different yet share important pathogenic mechanisms (Pedersen 2011). Physical inactivity is associated with an increase in abdominal fat. This increase in abdominal fat, which is independent to weight or BMI changes (Ross et al. 2004) results in chronic systemic inflammation. Visceral fat, or deep abdominal fat is more inflamed than subcutaneous fat (Pedersen 2011). Therefore body composition or where fat is stored may impact the stimulation of an inflammatory response.
Such inflammation leads to neuron degeneration, insulin resistance, atherosclerosis (hardening and narrowing of arteries) and tumour growth, thus contributing to the ‘diseasome of physical inactivity’ (Pedersen 2011).
Exercise can protect against diseases associated with chronic inflammation. This may be due to an anti-inflammatory effect of exercise, mediated through the endocrine system and hormones.
The endocrine system is the collection of glands of an organism that secrete hormones into the circulatory system where they are carried to distant organs. It is a means of regulating or communicating with distant organs through biochemical processes. This is similar to the nervous system, however the nervous system sends information quickly whereas the endocrine system sends information slowly but it is usually prolonged from a few hours to weeks.
The major endocrine glands include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland and adrenal glands.
So where does exercise come in?
Skeletal muscle attaches to the skeleton and therefore facilitates movement of the body by applying a force to bones and joints. Skeletal muscle serves many functions including generation of power, locomotion, posture, breathing, shivering to maintain body temperature and energy storage.
However, a more recently discovered function of skeletal muscle is their ability to produce hormones and therefore act as an endocrine organ. These hormones, termed myokines, are released in response to muscular contractions. Myokines, like other hormones, travel through the circulatory system to regulate other organs. Therefore, muscle contractions through exercise can affect whole body physiology.
What are the main myokines and how do they affect whole body physiology?
IL-6 is usually the first cytokine released during exercise. The level of circulating IL-6 increases dramatically in response to exercise and declines after exercise. IL-6 can act as an anti-inflammatory myokine. Additionally, when it is released in the skeletal muscle there is an increase in glucose uptake, insulin secretion and fat oxidation (Lightfood & Cooper 2016).
Exercise and in particular moderate-intensity resistance training, alters levels of IL-15. IL-15 is associated with muscle growth. It also plays a role in the metabolism of fatty acids and is therefore involved in ‘muscle-fat cross talk’, modulating abdominal fat mass (Pedersen 2011).
Brain-derived neurotrophic factor (BDNF)
It has been established that BDNF is low in people with major depression, Alzheimer’s disease, impaired cognitive function, CVD, type 2 diabetes and obesity.
A modest increase in BDNF in skeletal muscle was found following exercise (Pedersen 2011). BDNF increases fat oxidation in muscles and may also affect fat oxidation peripherally and therefore abdominal fat. BDNF may also lower blood glucose levels thus impacting energy balance and insulin signaling (Schnyder & Handschin 2015).
Myokines mediate anti-inflammatory effects, stimulate metabolic functions, regulate skeletal muscle regeneration and improve glucose uptake and fat oxidation, directly impacting the ‘diseasome of physical inactivity’.
As such, physical activity is important in the prevention of chronic disease.
It is clear that myokines form a conceptual basis for understanding inflammation and abdominal fat and in turn prevention of the ‘diseasome of physical inactivity’. However, more research needs to be done to provide clear links between myokines and these diseases.
Lightfood, A & Cooper, R 2016, ‘The role of myokines in muscle health and disease’, Current Opinion in Rheumatology, vol. 6, pp. 661-6.
Pedersen, B 2011, ‘Exercise-induced myokines and their role in chronic diseases’, Brain Behavior And Immunity, vol. 25, no. 5, pp. 811-816.
Ross, R., Janssen, I., Dawson, J., Kungl, A.M., Kuk, J.L., Wong, S.L., Nguyen-Duy, T.B., Lee, S., Kilpatrick, K., Hudson, R., 2004. Exercise-induced reduction in obesity and insulin resistance in women: a randomized controlled trial. Obes. Res. 12, 789–798.
Schnyder, S & Handschin, C 2015, ‘Skeletal muscle as an endocrine organ: PGC-1 alpha, myokines and exercise’, Bone, vol. 80, pp. 115-125.
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