Many forms of mild cellular stress produce benefits to health because they trigger the more efficient operation of cellular maintenance processes such as autophagy. That in turn causes better cell and tissue function, and thus improved health. This stress response and benefit is known as hormesis, and has been robustly proven to take place for calorie restriction, heat, cold, low dose ionizing radiation, and numerous other environmental circumstances. When it comes to slowing aging, the benefits of hormesis to life span are much larger in short-lived species. The short-term changes to metabolism are very similar, however, regardless of species longevity. Reconciling this apparent paradox will require a far greater understanding of metabolism and aging at the detail level than presently exists. Meanwhile, we should not expect the application of hormetic therapies to produce effects that are all that much better than regular exercise or the practice of calorie restriction.
Hormesis is any kind of biphasic dose-response when low doses of some agents are beneficial while higher doses are detrimental. Radiation hormesis is the most thoroughly investigated among all hormesis-like phenomena, in particular in biogerontology. In this review, we aim to summarize research evidence supporting hormesis through exposure to low-dose ionizing radiation (LDIR). Radiation-induced longevity hormesis has been repeatedly reported in invertebrate models such as C. elegans, Drosophila, and flour beetles and in vertebrate models including guinea pigs, mice, and rabbits. On the contrary, suppressing natural background radiation was repeatedly found to cause detrimental effects in protozoa, bacteria, and flies.
We also discuss here the possibility of clinical use of LDIR, predominantly for age-related disorders, e.g., Alzheimer's disease, for which no remedies are available. There is accumulating evidence that LDIR, such as those commonly used in X-ray imaging including computer tomography, might act as a hormetin. Of course, caution should be exercised when introducing new medical practices, and LDIR therapy is no exception. However, due to the low average residual life expectancy in old patients, the short-term benefits of such interventions (e.g., potential therapeutic effect against dementia) may outweigh their hypothetical delayed risks (e.g., cancer). We argue here that assessment and clinical trials of LDIR treatments should be given priority bearing in mind the enormous economic, social, and ethical implications of potentially-treatable, age-related disorders.