The paper I'll point out today walks through the ways in which exercise is known to beneficially affect the Hallmarks of Aging. The Hallmarks are a list of the significant causes of aging that I disagree with about half of. The SENS catalog of root causes of aging, first published earnestly in the literature back in 2002, isn't cited anywhere near as much as the much later Hallmarks of Aging - which owes a great deal to its predecessor while failing to mention it in any way. There is some overlap between the two, but many of the Hallmarks are not causes of aging, but rather manifestations of aging, meaning secondary and later consequences of underlying molecular damage.
This question of whether not any specific manifestation of aging is or is not a root cause is important. The strategy adopted in the development of therapies to treat aging matters. Addressing root causes is far more effective than addressing downstream consequences. Near all medical technology employed to date to treat age-related diseases fails to touch on the root causes of aging, however, and this is why these therapies are only marginally effective at best. They modestly slow progression, or modestly ease suffering, but they cannot meaningfully turn back any aspect of the progression of aging. We can continue along that road, or we can choose to attempt a better strategy.
Traditionally, aging was not seen as an adaptation or genetically programmed phenomenon. More recently, biologic currents point to two main theories: the programmed aging and the damage or error-based theories. The first suggests an intrinsic biologic programmed deterioration of the structural and functional capacity of the human cells. The latter highlights the cumulative damage to living organisms leading to intrinsic aging. Nonetheless, a combination of these theories is usually preferred. In this sense, a state-of-the-art review, proposed nine cellular and molecular hallmarks that contribute to the process of aging, including (1) genomic instability, (2) telomere attrition, (3) epigenetic alterations, (4) loss of proteostasis, (5) deregulated nutrient sensing, (6) mitochondrial dysfunction, (7) cellular senescence, (8) stem cell exhaustion, and (9) altered intercellular communication. These hallmarks should be expressed during normal aging, with their experimental aggravation speeding up the aging process, and in contrast, their experimental amelioration retards the normal aging process, thus increasing a healthy life span.
Along with the nine cellular and molecular hallmarks stated above, aging is known to be correlated with several cardiovascular, cardiorespiratory, musculoskeletal, metabolic, and cognitive impairments. In this sense, regular physical activity in the older population - especially aerobic and resistance training - plays an important role at a multisystem level, preventing severe muscle atrophy, maintaining cardiorespiratory fitness and cognitive function, boosting metabolic activity, and improving or maintaining functional independence. In addition, physical exercise has a positive antiaging impact at the cellular level, and its specific role in each aging hallmark is described below.
In the face of genomic instability, the organism has developed a panoply of DNA repair mechanisms that skirmish altogether to overcome nuclear DNA damage. Exercise plays a role in maintaining genomic stability. In rodent models, aerobic exercise improves DNA repair mechanisms. It augments DNA repair and decreases the number of DNA adducts (up to 77%), related to aging and several risk factors for cardiovascular diseases. In addition, a six-month resistance training program in an institutionalized elderly population showed a tendency to reduce cell frequency with micronuclei (~15%) and the total number of micronuclei (~20%), leading to a higher resistance against genomic instability.
Telomere shortening is described during normal aging in human and mice cells. The fact that telomere length decreases with aging, contributing to the normal cell senescence process, suggested that this could be a potential marker for biological aging. Although the potential mechanism is unclear, exercise exhibits a favorable impact on telomere length, especially on a chronic pattern and particularly in older individuals antagonizing the typical age-induced decrements in telomere attrition. Several potential mechanisms have been discussed linking exercise and telomere length decrements to changes in telomerase activity, inflammation, oxidative stress, and decreased skeletal muscle satellite cell content.
The relationship between epigenetic regulation and aging is controversial and complex. A multiplicity of epigenetic modifications affects all tissues and cells throughout life. The literature clearly reveals that the epigenetic response is highly dynamic and influenced by different environmental and biological factors, such as aging, nutrient availability, and physical exercise. Regular aerobic exercise can change the human genome through DNA methylation. Thus, by using epigenetic mechanisms, aerobic exercise can induce the transcription of genes encoding telomere-stabilizing proteins and telomerase activity not only in animals but also in humans.
Loss of Proteostasis
Aging and some aging-related diseases are linked to impaired protein homeostasis, also known as proteostasis. The array of quality control is guaranteed through distinct quality control mechanisms that prevent the aggregation of damage components and ensure the continuous renewal of intracellular proteins, degrading altered proteins. Aerobic exercise induces autophagy, thus preventing the loss of strength and muscle mass through the modulation of signaling pathways. Chaperone associated functions, such as folding and protein stability, are impaired in aging. In animal models, the upregulation of co-chaperones of the heat-shock proteins (HSPs) was associated with prolonged life-span phenotypes. Despite limited comparison studies, evidence supports that acute endurance- and resistance-type exercise protocols are associated with increased HSPs transcription not only during activity but also immediately postexercise or several hours following exercise, which points out the possible favorable impact of physical activity on proteostasis.
Deregulated Nutrient Sensing
Exercise plays an important role in not only the glucose-sensing GH / IGF-1 somatotrophic axis but also other nutrient-sensing systems, promoting a beneficial anabolic cellular state. The effect of exercise on glucose metabolism through increased glucose transporter type 4 production is another well-known mechanism of improved insulin sensitivity associated with physical activity. Additionally, exercise-induced GH and IGF-1 levels are influenced by exercise intensity, duration, and type (higher in intense interval protocols and resistance exercise).
The clear causal relationship between mitochondrial dysfunction and aging has long been a target of great discussion. With increasing age comes a decline in mitochondrial integrity and biogenesis because of alterations in mitochondrial dynamics and mitophagy inhibition, impairing dysfunctional mitochondria removal. The regular practice of physical exercise has a positive impact in mitochondrial function. In this sense, endurance-trained humans presented higher levels of mitochondrial proteins expression. Regular physical exercise may maintain a pool of bioenergetically functional mitochondria that, by improving the systemic mitochondrial function, contribute to morbidity and mortality risk reduction throughout one's life span.
Senescent cell accumulation in different tissues seems to be dependent, in one hand, on an increased rate of senescent cell generation and, in other hand, on a decreased rate of clearance. Exercise, specifically aerobic, induces the secretion of antitumorigenic myokines and greater natural killer cell activity, contributing to a decreased incidence of oncologic disease and improved cancer prognosis. This may also impact clearance of senescent cells. Aerobic exercise has been inversely correlated with p16INK4a mRNA levels in peripheral blood T lymphocytes, which might promote protective outcomes against age-dependent alterations. Aerobic exercise suppresses liver senescence markers and downregulates inflammatory mediators.
Stem Cell Exhaustion
For the long-term maintenance of the organism, the deficient proliferation of stem and progenitor cells is harmful, but an excessive proliferation can also be deleterious by speeding up the exhaustion of stem cell niches. Within this line, physical exercise is one of the most potent stimuli for the migration/proliferation of the stem cell subsets from their home tissue to impaired tissues for later engraftment and regeneration. In this sense, regular physical exercise attenuates age-associated reduction in the endothelium reparative capacity of endothelial progenitor cells. In addition, exercise activates pluripotent cells' progenitors, including mesenchymal and neural stem cells, which improve brain regenerative capacity and cognitive ability.
Altered Intercellular Communication
The physiological aging process implicates several alterations on intracellular communication mechanisms, namely, in neuroendocrine, endocrine, and neuronal levels. Inflammation plays a central role in this age-related alteration. Muscle contraction is traditionally associated with myokine secretion (proteins, growth factors, cytokines, or metallopeptidases) elevated during and after exercise. Interestingly, the muscle-released IL-6 creates a healthy influence, inducing the production of anti-inflammatory cytokines. Within these lines, several authors associated lifelong aerobic exercise training with lower inflammatory levels, particularly in advanced decades of life.