Physicalexercise and Its Potential Influence on Mental Health

Physical exercise and mental health, two areas that can be investigated separated from each other, and yet it would only result in a fracture of the whole picture since these two are unequivocally correlated with each other. This was already known in ancient Rome; as the Roman poet Juvenal eloquently describes it: “Mens sana in corpore sano” (a healthy mind in a healthy body) (Juvenal, late 1st – early 2nd century A. D. ). And yet, for the sake of a better understanding, each will briefly be investigated individually.

Physical exercise or physical activity are both described by the World Health Organization (WHO) as health improvement activities. In fact, insufficient physical activity is one of the major causes of deaths worldwide (WHO, 2017 (b)). The WHO estimates that approximately 80 percent of adults globally are insufficiently physically active. This estimation is rather worrying since physical activity and its resulting health benefits could contribute towards a prevention of the development of non-communicable diseases (NCDs) (WHO, 2017 (b)). NCDs are responsible for up to 70 percent of all deaths worldwide and are estimated to kill around 40 million people each year (WHO, 2017 (a)). A statistics showing the immense importance of physical activity and exercise.

Now, continuing with mental health. A simple description for a rather complex concept. Mental health is defined by the World Health Organization (WHO) as “a state of well-being in which every individual realizes his or her own potential, can cope with the normal stresses of life, can work productively and fruitfully, and is able to make contribution to her or his community” (WHO, 2014). The Medilexicon’s medical dictionary even goes a step further in its definition; it describes mental health as “emotional, behavioural, and social maturity or normality; the absence of a mental or behavioural disorder; a state of psychological well-being in which one has achieved a satisfactory integration of one's instinctual drives acceptable to both oneself and one's social milieu; an appropriate balance of love, work, and leisure pursuits“ (Medilexicon, 2018).

Thus, mental health describes not only the absence of mental disabilities, or diseases, but includes the capacity of properly coping with daily challenges such as stress factors and, ideally, also the successful management and the pursuit of and balance between success in love, work and leisure adventures. Mental health is, therefore, a crucial component of human well being and will, thus, probably never lose its relevance for human health.

For centuries physical exercise has been theorized to have a positive impact on our mental health capacities, but only recent research is focusing scientifically on and showing the potential mental health benefits of physical exercise; its influence on the nervous and neurological systems. The relevance of physical activity for human health is, thus, undeniable. Which is why this paper investigates the potential impact of physical exercise on mental health, according to recent discoveries and with a focus on the influence of the brain-derived neurotrophic factor (BDNF).

Research question: What potential impact does physical exercise have on mental health?

Methods

Several databases were searched with the main key-words “physical exercise”, “physical activity”, “mental health” and “BDNF”. During the research process, the database of the National Center for Biotechnology Information (NCBI) proofed as most useful resulting in several suiting studies. These studies will be used to show the potential impact of physical exercise, producing BDNF and, thus, influencing mental capacities, and even its potential regarding mental disabilities.
Firstly, studies that are focusing on the influence of physical exercise on cognitive capabilities, or more precisely its effectiveness against the decline of cognitive abilities will be elaborated on. Secondly, the BDNF will be investigated, including its characteristics and general effect on the brain. Thirdly, physical exercise and its correlation with BDNF with regard to mental capacities will be explored. Lastly, it will be looked into the potential that BDNF has with regard to mental disabilities. All those findings will then be elaborated on in the discussion part.

Results

In 2018 Lee H. and his colleagues were conducting a study, that investigated the effects of intermittent intense exercise on cognitive deficit in chronically stressed mice. A group of forty mice was assigned into four subgroups. One group of mice was put under stress (restraint stress for prolonged periods of time on a daily basis), one was stressed and performing a moderate exercise, another stressed while being subjected to intense exercise. The last group was the control group. It could be observed, that the chronic stress caused decreased cognition and decreased the density of blood vessels in the hippocampus and newborn cell survival (Lee et al. , 2018). However, the regular intense exercise, as well as the moderate exercise could prevent the decrease of cognition and even improved the blood vessel density and newborn cell survival (all according to Lee et al. , 2018).

This study shows the remarkable potential that physical exercise has on our cognitive capabilities. And in a world where constant stress seems to be an integrative factor of many peoples daily lives, its potential positive societal impact seems to be ever increasing. The study of Lee H. is only one among many confirming this. Another study of 2017, performed by Feng X. , for example, investigated the correlation between exercise and postoperative neuroinflammation as well as cognitive decline.

Postoperative cognitive decline (PCD) is a phenomenon, that can occur among surgical patients. Certain risk factors do increase the likelihood of experiencing PCD, among them, are metabolic conditions like obesity and insulin resistance, as well as advanced age and perioperative infections (Feng X. et al. , 2017). In an attempt to define underlying pathophysiologic processes and the effect of preoperative exercise on the development of PCD, an experiment with two groups of rats was performed. The first group descended from high capacity runner rats, and were themselves high capacity runners, while the other group of rats descended from, and were themselves, low capacity runner rats. Both groups were then randomly assigned into four sub-groups, of which two were subjected to surgery and two to a sham-surgery.

Additionally, two groups (one sham-surgery and one surgery group) were placed on a stationary treadmill (no exercise), while the other two groups were subjected to 10km on a treadmill over the course of 6 weeks before the surgery. Low capacity runner rats displayed an increased cognitive decline and suffered from excessive postoperative neuro-inflammation. These effects, however, could be normalized by the preoperative exercise (all according to Feng X. et al., 2017).

After elaborating on the general observations that can be made when investigating the correlation between physical exercise and its influence on cognitive capabilities, it is time to look into possible factors that are underlying and causing these positive impacts. This paper specifically focuses on the brain-derived neurotrophic factor, but what exactly is it

The brain-derived neurotrophic factor (BDNF) is a growth factor of the neurotrophin family. More specifically: it is a basic, homodimeric, secretory protein. BDNF can be found in the peripheral nervous system (PNS) and in the central nervous system (CNS). It is most prevalent in still-developing brains but is still expressed in the adult brain, even though at lower levels (Mc Allister, 2002). The list of, so far known, parts of the NS that are influenced by BDNF is long and does include axon pathfinding, neuronal migration and proliferation, synapse formation and maintenance, dendritic growth, neuronal excitability, long-term plasticity and neuronal survival. It is a crucial factor for neuronal health and development. Mice with low or no BDNF suffer from severe neuronal deficits and early postnatal death (all according to Mc Allister, 2002).

This shows the immense importance of BDNF and its potential with regard to cognitive capabilities and, thus, mental health. But what is its relation to physical exercise?
Yong Kyun Jeon and Chang Ho Ha performed a study in 2007 that investigated the effectiveness of exercise varying in intensity on BDNF levels [. . ] and memory of adolescents. For this purpose, forty male students with no physical illness or other disabilities were randomly assigned to a control group, which did only stretching, and into high, middle, or low-intensity exercise groups. The participants were followed over a 12 week period and performed their exercises 4 times per week. In comparison to the control group and low and middle-intensity exercise groups, the high-intensity exercise group experienced a significant increase in cortisol, concentration level of insulin-like growth factor 1, BDNF resting levels, and in working memory (all according to Jeon & Ho Ha, 2017). In addition to these findings, a study was performed by Freitas D. A. and his colleagues. In this study, the effects of six weeks of high-intensity interval training (HIIT), specifically its influence on the hippocampal redox state, were investigated among a population of young, male Wistar rats. After the six weeks of HIIT, a reduction of oxidative damage and an increase in non-enzymatic and enzymatic activity could be observed in the hippocampus. Furthermore, the HIIT did cause a decrease in cytokine content, as well as an increase in hippocampal BDNF levels (all according to Freitas et al. , 2018). These studies seem to confirm, that there is indeed a correlation between physical exercise, BDNF production and expression, as well as resulting positive cognitive outcomes for people, improving their mental health. After seeing these positive impact on healthy minds, the question stirs whether BDNF could potentially also have a positive impact on people already suffering from mental diseases.

Martina Parrini and her colleagues were wondering the same and conducted a study in 2017, investigating the effects of aerobic exercise and a BDNF-mimetic therapy on mouses affected by the Down syndrome. Down syndrome (DS) originates from the triplication of the chromosome 21 and is a major genetic cause of intellectual disabilities (Martina Parrini et al. , 2017). Patients with DS show lower levels of hippocampal BDNF levels, thus, the positive effects of BDNF are less expressed. To evaluate the effects of aerobic exercise on BDNF levels and the related stimulation of brain neuroplasticity mechanisms, the trisomic mice were exposed to physical exercise on running wheels over a period of 4 weeks, while also having a control group of sedentary mice who were not exposed to physical exercise. After 4 weeks, the group of mice performing the aerobic exercise showed reduced learning and memory deficits, a restoration of functional hippocampal adult neurogenesis, and an increase of hippocampal BDNF levels (Martina Parrini et al. , 2017). Due to the positive effects of the increased BDNF levels, caused by the exercise, the researchers went on and tested the effects of the BDNF-mimetic drug 7,8-dihydroxyflavone. The drug resulted in observable, but not significant, improvement of synaptic plasticity and reduced cognitive deficits among the trisomic mice (all according to Martina Parrini et al. , 2017).

Another study focusing on the correlation between BDNF and mental diseases was performed by Bun- Hee Lee and Yon-Ku Kim in 2010. They were interested in the role of BDNF in the pathophysiology of major depression and in antidepressant treatments. Major depression disorder (MDD) is associated with lower levels and functioning of BDNF, and impaired neuronal plasticity (a similar phenomenon can be observed among animals affected by constant stress). Severely impaired neuronal plasticity in the brain can lead to suicidal behaviour (Lee & Kim, 2010). The reduction of BDNF expression and the reduced activity in the hippocampus can be decreased by a treatment with antidepressant drugs. An antidepressant treatment over a course of four weeks promotes increased levels of BDNF, which in turn is related to improved neuronal plasticity, neurogenesis, neuronal maturation, and synaptogenesis. It, furthermore, improves the modulation of neuronal networks, which is related to a positive change in mood, or even the recovery from depressed moods (all according to Lee & Kim, 2010).

Discussion

When investigating the results, it seems undeniable that physical exercise does have a positive impact on mental capacities. Increased physical activity is apparently followed by an increased expression of BDNF levels. Thus, it seems there is a correlation between BDNF, with its remarkable positive characteristics, and intellectual and mental capabilities. The results do, furthermore, suggest that the absence of BDNF is related to reduced intellectual capacities since a lower expression of BDNF can be observed in people affected by mental disabilities. However, when increasing BDNF expression by physical exercise, an improvement of mental capacities can be detected.

There are already drugs mimicking BDNF which do seem to have a positive, but not significant influence on mental capacities. Therefore, physical exercise seems to be superior when it comes to increasing BDNF expression and improving the mental health of people.

These outcomes suggest that the potential positive impact of physical exercise on health is huge. Especially in a world where people are affected by stress and the increasing prevalence of NCDs on a daily basis. Physical exercise could be one factor decreasing their burdens.

Concluding, physical exercise and its capacity to increase the BDNF levels, which seemingly improves the intellectual capabilities, can mean a reduction of the burden that is caused by mental diseases and increases the mental health of every human being. Thus, even though it took almost 2000 years to scientifically understand and proof the underlying mechanisms behind it, it seems Juvenal was right all along when he said: “Mens sana in corpore sano” (Juvenal, late 1st – early 2nd century A.D.).