If you need yet another reason to exercise as part of your health and longevity strategy, then check out this study, which suggests that aging of the immune system can be slowed by exercise.
The problem with our modern lives is that we are, in general, much more sedentary than our ancient ancestors, who hunted and engaged in intense physical activity in order to survive. They often covered great distances in their hunt for food, and their world was one of constant movement and activity. In contrast, we spend more time behind a desk or sitting on the sofa than we do hunting bison on the Great Plains.
To make matters worse, as we age, we become even less physically active, which is seriously bad news for the body and contributes to muscle loss, bone thinning, and even decline of the immune system.
What confounds human studies of immunosenescence is that physical activity is not taken into account in either cross-sectional or longitudinal studies of immune aging. The majority of older adults are largely sedentary and fail to meet the recommended guidelines for physical activity of 150 min of aerobic exercise per week. Regular physical activity in older adults has been associated with lower levels of pro-inflammatory cytokines such as IL-6, TNFα, improved neutrophil chemotaxis and NK cell cytotoxicity, increased T-cell proliferation and improved vaccination responses. Thus, the current literature on immunosenescence is not able to determine which aspects of age-related immune change are driven by extrinsic factors and which may be the consequence of a constitutive aging programme.
Here, we studied several aspects of the adaptive immune system in highly physically active older individuals (master cyclists) in which we have shown the maintenance of a range of physiological functions previously reported to decline with age. We show that compared with more sedentary older adults, the cyclists show reduced evidence of a decline in thymic output, inflammaging and increased Th17 cell responses, although accumulation of senescent T cells still occurred. We reveal high serum levels of IL-7 and IL-15 and low IL-6, which would together provide a environment protective of the thymus and also help to maintain naïve T cells in the periphery. We conclude that maintained physical activity into middle and old age protects against many aspects of immune aging which are in large part lifestyle driven.
The decline of the immune system
As we age, the thymus, the organ that produces the majority of T cells, starts to shrink in a process known as involution. During this process, the T cell-producing tissue changes to fat and the production rate of T cells steadily falls.
The first major drop in thymic output occurs towards the end of childhood; prior to this, we produce T cells at a furious rate, which may also somewhat explain why children are so resilient and can heal injuries faster.
Ultimately, the loss of thymic tissue and the decline of T cell production leads to the failure of the immune system, leaving us wide open to infections and microbial invasion. The immune system is also responsible for clearing senescent cells, and, as it declines, more and more of these problem cells build up, leading to chronic inflammation and increasingly poor tissue repair.
Exercise keeps the thymus young
The new study by Janet Lord, Professor of Immune Cell Biology at the University of Birmingham in the UK, looked at 125 male and female cyclists between 55 to 79 years old who had bicycled heavily during their adult lives. They found that these people did not suffer from the typical loss of muscle mass (sarcopenia) seen during aging, nor did their bones become significantly thinner as is often observed in normal aging.
The research also showed that the age-related decline of T cell production in the thymus was negligible in older people who have maintained high physical activity throughout their lives compared to people who did not exercise regularly. The study results showed that active older people have a similar level of T cell production as people in their 20s.
The cyclists had high levels of the hormone interleukin 7 present in their blood, which helps to slow down the shrinking of the thymus. The hormone is produced by various cells in the body, including muscle cells; the researchers believe that the more active the muscles are, the more hormone is produced, which keeps the thymus functionally younger.
While the results are significant and are yet another reason to exercise, it would be interesting to see the outcome if the thymus could be restored to the production level that we all enjoy as children. Certainly, there are researchers working on rejuvenating the thymus, and the initial results have been positive.
Until that therapy arrives, the best we can do right now is to maintain physical activity in order to try to slow down aging of the immune system as much as possible.
 Duggal NA, Pollock RD, Lazarus NR, Harridge S, Lord JM. Major features of immunosenescence, including reduced thymic output, are ameliorated by high levels of physical activity in adulthood. Aging Cell. 2018;e12750. https://doi.org/10.1111/acel.12750