You may know that exercise is good for you, but have you ever wondered specifically what exercise is doing for you while you go through a workout? This will be a three part series discussing how exercise works your metabolic, skeletal, and cardiovascular systems! First up is the metabolic system. 

Metabolic System

Essentially, the metabolic system is how we turn calories into energy to fuel normal cellular processes and physical activities. Aging affects the metabolic system in a number of ways. A decline in physical activity may be due to experiencing painful symptoms or fear of injury. As activity levels decline, it can result in a subsequent decline in muscle mass which decreases the basal metabolic rate (BMR) and increases fat tissue (Shimokata & Kuzuya, 1993). Reduced activity can also lower insulin sensitivity and glucose tolerance (Ehrman et al., 2013). Finally, as fat accumulates in the abdominal area (either through lack of activity or overeating), this can lead to nonalcoholic fatty liver disease (NAFLD) which can reduce liver size and hinder blood flow through the portal vein (Rector & Thyfault, 2011). 

One’s BMR declines almost linearly as a result of the normal aging process (Shimokata & Kuzuya, 1993). However, regular physical activity has been shown to delay a decreasing BMR and can either help keep it stable or increase it depending on current activity levels. Oftentimes with exercise, the greater the intensity results in greater overall changes and less time before positive changes can start being seen. High-intensity interval training (HIIT) has been shown to be very effective at increasing BMR and reducing the risk for developing other conditions as a result of a low BMR due to a sedentary lifestyle and/or poor habits. One recent study found an increased VO2max, decreased body fat percentage, and increased skeletal muscle tissue and mitochondrial content as a result of HIIT (Chrøis et al., 2020). Because skeletal muscle tissue is more metabolically active than fat tissue, BMR will increase as fat is lost and muscle mass is gained. Another study found that HIIT caused a significantly increased glucose uptake after 12 weeks of HIIT in both young and elderly adults (Robinson et al., 2018). As glucose uptake is increased, this increases insulin sensitivity and glucose tolerance and helps prevent NAFLD and the subsequent reduced liver size and hindered blood flow associated with it. 

Always be sure to consult your physician when you're starting a new exercise routine. However, more often than not, they will encourage you to start getting active because of the numerous benefits. Each little step, like parking farther away at your destination or making sure to take the stairs, will add up over time. And if you're able to add in regular HIIT sessions, that's even better!

References

Chrøis, K. M., Dohlmann, T. L., Søgaard, D., Hansen, C. V., Dela, F., Helge, J. W., & Larsen, S. (2020). Mitochondrial adaptations to high intensity interval training in older females and males. European journal of sport science, 20(1), 135–145.

Ehrman, Gordon, Visich, & Keteyian. (2013). Clinical Exercise Physiology. Human Kinetics.

Rector, R. S., & Thyfault, J. P. (2011). Does physical inactivity cause nonalcoholic fatty liver disease?. Journal of applied physiology (Bethesda, Md. : 1985), 111(6), 1828–1835. https://doi.org/10.1152/japplphysiol.00384.2011 

Robinson, M. M., Lowe, V. J., & Nair, K. S. (2018). Increased Brain Glucose Uptake After 12 Weeks of Aerobic High-Intensity Interval Training in Young and Older Adults. The Journal of clinical endocrinology and metabolism, 103(1), 221–227. https://doi.org/10.1210/jc.2017-01571 

Shimokata, H., & Kuzuya, F. (1993). Nihon Ronen Igakkai zasshi. Japanese journal of geriatrics, 30(7), 572–576. https://doi.org/10.3143/geriatrics.30.572