Importance of Maintaining and Improving Immune Function by Physical Activity During the COVID-19 Pandemic
Gen Li1, Laikang Yu1,2*
1Department of Strength and Conditioning Training, Beijing Sport University, Beijing, China.
2Key Laboratory of Physical Fitness and Exercise, Ministry of Education, Beijing Sport University, Beijing, China
Received Date: 09/02/2022; Published Date: 11/04/2022.
*Corresponding author: Laikang Yu. Key Laboratory of Physical Fitness and Exercise, Ministry of Education, Beijing Sport University, Beijing, China
In early December 2019, a novel coronavirus disease, designated as COVID-19, came into light in Wuhan, Hubei Province of China. With the first pneumonia cases of unknown origin being identified, it has become a pandemic worldwide. The World Health Organization (WHO) has declared it as a potential threat to the world population, and a public health emergency of international concern on January 30, 2020 . Many countries and regions have advised people to reduce traveling and stay at home to avoid human-to-human transmission of the virus. As a result, most work, physical and recreational activities were suspended. However, staying at home can reduce the physical activity of the general population. Sedentary behaviors such as watching television and movies, browsing the Internet, using smartphones, and playing video games are associated with an increased risk of obesity [2-4], hypertension [5, 6], cardiovascular disease [4, 7], and type 2 diabetes mellitus [4, 7, 8]. Evidence showed that individuals with weak immunosuppression or immune defense mechanisms are more susceptible to severe disease [9-11]. However, there are several research suggested that physical activities such as moderate-intensity continuous training (MICT), resistance training (RT), and high-intensity interval training (HIIT) have a significant effect on modulating the immune system. Physical activity-induced immunoregulation has been recognized for more than 30 years, and there are about 5000 peer-reviewed original and review papers in the PubMed database .
Effects of MICT has been reported in many studies, i.e. MICT can improve cognitive function in patients with mild Alzheimer's disease , Parkinson's Disease , and schizophrenia , provide comparable reductions in resting blood pressure in adults with pre- to established hypertension , induce modest body composition improvements in overweight and obese individuals , and improve cardiorespiratory fitness in participants of cardiac rehabilitation . MICT is also considered a preventive measure to bring down the further incidence of COVID-19 . Evidence showed that even a slight increase in baseline maximal oxygen uptake (VO2max) may convert a significant proportion of high-risk patients to low-risk patients . A randomized controlled trial assessed preventive effects of exercise on acute respiratory infection (ARI) illness, the results showed that 8 weeks of MICT contributed to the reduction in ARI illness, and the magnitude of observed benefit was similar to that from accepted medical interventions, such as influenza vaccination . As shown in Table 1, a large number of randomized controlled trials have investigated the relationship between MICT and immune system function. Previous studies suggested that 12 weeks , 15 weeks [23, 24], or 12 months  of MICT had significant effects on the prevention of upper respiratory tract infections (URI), and the mechanisms were related to the improvement of vaccination responses, T-cell proliferative capacity, neutrophil phagocytic activity, NK cell cytotoxic activity, and leukocyte telomere lengths, and the depression of numbers of exhausted/senescent T cells, circulatory levels of inflammatory cytokines, and inflammatory response to bacterial challenge [26-31].
Table 1: Research on the relationship between MICT and immune function
RT, a form of exercise that actively activates specific skeletal muscle groups against external resistance, has become a popular form of exercise and is recommended by the American Heart Association , the American College of Sports Medicine (ACSM) [32, 33], and the American Geriatrics Society . RT has been proven to be an important strategy to improve muscle mass, muscle strength, and power output, as well as functional capacity . And a growing number of studies strongly support the beneficial effects of RT on various aspects of cognitive performance, such as attention , memory , executive control , and mini-mental state examination (MMSE) score . For example, Cassilhas et al. found that 24 weeks of moderate- and high-intensity RT had equally beneficial effects on cognitive functions in the elderly , and the mechanism was related to the increase of insulin-like growth factor-1 (IGF-1) . In addition, RT has been shown to have significant effects on immune system function. Table 2 summarizes published evidence from randomized controlled trials on the relationship between RT and immune system function. Previous studies suggested that 10 weeks [42-44], 12 weeks , and 16 weeks  of RT had significant effects on improving the immunity of elderly people, and the mechanisms were related to the improvement of the resting natural killer cell activity , and monocyte and T-cell-mediated immunity , and the reduction of the inflammatory reactivity and overall inflammatory milieu [43, 44], and C-reactive protein (CRP) levels .
Table 2: Research on the relationship between RT and immune function.
HIIT can be described as “a short interval of vigorous activity interspersed with periods of low activity or rest”, which can cause a strong acute physiological response . A growing number of studies suggested that HIIT had effects on promoting cognitive function [48, 49], glucose and lipid metabolism in skeletal muscle , the expression of genes related to endogenous antioxidant enzyme activity and inflammation , the insulin sensitivity , the cardiopulmonary function , the aerobic capacity [54, 55], anaerobic capacity [56, 57], and the vascular endothelial function , and on reducing the oxidative stress level . In addition, HIIT can provide comparable reductions in resting blood pressure in adults with pre- to established hypertension , induce modest body composition improvements in overweight and obese individuals , and improve cardiorespiratory fitness in participants of cardiac rehabilitation . In addition to MICT and RT, HIIT has also been shown to have significant effects on immune system function. Table 3 summarizes published evidence from randomized or controlled trials on the relationship between HIIT and immune system function. Previous studies suggested that one single session [60, 61], 3 weeks , and 10 weeks [63, 64] of HIIT had significant effects on improving the immune function, and the mechanisms were related to the improvement of the potential anti-inflammatory benefits , the number of antimicrobial proteins in the saliva , and the innate immune functions , and the reduction of monocyte  and IFN-γ/IL-4 ratio .
Table 3: Research on the relationship between HIIT and immune function
In conclusion, different types of physical activity have significant effects on immune function, and maintaining physical activity levels during the current situation initiated by the COVID-19 pandemic will have significant physical health benefits.
Gen Li was supported by the Graduate Students’ Innovative Scientific Research Program of Beijing Sport University (20212011); Laikang Yu was supported by the Chinese Universities Scientific Fund (2021QN001).
Conflict of interest
The authors declare no conflict of interest.
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