Is aging a disease?

Holly Michele / stock.adobe.com

In nearly all parts of the world, humans are living longer.¹ Centuries of success combating causes of death in the young, such as infectious disease, malnutrition, and trauma, have allowed more of us to grow old. However, this demographic shift has led to an increase in chronic, age-associated diseases.

Many veterinarians believe that our patient populations are undergoing a similar demographic shift, though reliable data are difficult to come by. Periodic analyses of the medical record systems for the national veterinary organization Banfield Pet Hospital suggest that life expectancy is increasing in pets. In 2002, the average life expectancy for dogs was 10.5 years; by 2016, it was 11.8 years.^2,3 For cats, the average life expectancy in 2002 was 11 years; by 2016, it was 12.9 years.^2,3 Other sources of data also suggest increasing longevity, but there are no prospective studies designed to assess changes in life expectancy over time in cats or dogs.

Surveys indicate approximately 20% to 24% of cats and 15% to 17% of dogs are 11 years or older, qualifying as senior by common standards.^4,5 The same improvements in prevention of mortality in the young have taken place in veterinary medicine as in human health. Consequently, vets are increasingly having to manage age-related disease in older dogs and cats.

Doing so effectively means understanding the aging process and how it affects health. How we regard aging shapes our clinical practices, influencing how we monitor and treat age-related health problems and how we counsel our clients.

How often do we see patients with treatable conditions that owners do not recognize as problems—or choose not to treat because the patient is geriatric? We all have heard the rationalization, “Well, he’s just slowing down.” Or “He’s just getting old.” Many veterinarians may also routinely ignore mild symptoms or laboratory abnormalities in geriatric patients that they would investigate or treat in younger patients.

Understanding aging

There are several conceptual frameworks used to understand aging. One characterizes aging as a normal and inevitable aspect of the life cycle and distinguishes normal aging from the symptoms of age-related diseases. This approach emphasizes healthy aging, maintaining optimal function and quality of life, defined in terms relative to the age of the individual.^6,7

Proponents of this framework recognize that aging is associated with a progressive loss of function and resilience and an increased risk of disease and death.^6,7 However, they emphasize differentiating normal physical changes that occur with age from those that represent disease. In this approach, normal health is relative to age, and what may be seen as pathological or unacceptable in a young animal may be viewed as age-appropriate in an older dog or cat. Although a decline in activity level, a loss of lean body mass, or mild clinical laboratory abnormalities might be signs of disease in the young, these changes are considered normal in geriatric individuals and may not trigger diagnostic or treatment interventions.

Most veterinarians using this approach would not go so far as clients sometimes do in ignoring truly deleterious changes associated with age. Normal changes that adversely affect comfort and function are managed palliatively to preserve well-being as long as possible. Proponents recommend screening efforts and intervention when abnormalities cross the often-indistinct threshold between normal aging and disease. However, this view of aging does promote greater acceptance of the purportedly inevitable decline in function and resilience that comes with age, and it is predominantly reactive and palliative rather than preventative.

Another perspective is that aging itself is a disease, a constellation of biological processes and their clinical outcomes that should be diagnosed, treated, and, if possible, prevented like any other illness.^8,9 Proponents of this viewpoint out that treatment and prevention of individual age-associated diseases may not be as effective at extending life span or health span (the period of healthy life) as directly targeting aging.¹⁰ If there is an identifiable set of common mechanisms behind age-associated diseases, wouldn’t we be more successful at preserving health by addressing these rather than accepting some as symptoms of normal aging and treating others as pathological?

The advantage of this view is that it encourages investigating and manipulating the fundamental mechanisms of aging to mitigate their effect on health and well-being. The disadvantage is that we seldom have sufficient knowledge or tools to do this effectively in a clinical setting. There is also a risk of pathologizing minor changes in geriatric patients that can lead to overdiagnosis and overtreatment.¹¹

As both a clinician and someone involved in aging research, I lean toward a pragmatic middle ground between these views, understanding aging as the single greatest modifiable risk factor for many superficially dissimilar age-related diseases.¹² Aging is a ubiquitous process consisting of identifiable, evolutionarily conserved mechanisms that occur in all individuals. It is the root cause of many diseases, but it has a variable and often extended latency to clinical manifestations. The specific diseases that eventually emerge differ between populations and individuals due to a complex interaction of many factors.

The greatest improvements in health and longevity will come from understanding and manipulating the core mechanisms of aging well before clinical disease develops. However, while we develop the necessary tools to accomplish this, we must strike a balance between palliating the impacts of aging on comfort and function and not subjecting our geriatric patients to excessive or unproven interventions. The concept of healthy aging is clinically useful, but we should not overlook the importance of treating the root causes of age-associated physical decline and ultimately preventing many of the clinical problems we now view as either normal aging or as age-associated disease.

Traditional medical training emphasizes detection and treatment of extant disease more strongly than prevention. Even when we act prophylactically, we often focus our efforts narrowly on specific diseases, overlooking the underlying mechanistic connections between them. As research evidence elucidating the mechanisms of aging grows, we must shift our practices away from a reactive and narrow focus on age-associated diseases and toward a broader emphasis on preventing the global biological processes that lead to age-related degeneration.

Although there is much work to do to develop clinical therapies directly targeting aging, there are interventions veterinarians can recommend now that can slow aging changes and mitigate their effect on health. The most significant of these is weight management.

How obesity compounds the effects of aging

There is ample evidence in humans, lab animals, and companion dogs that obesity shortens life and accelerates aging, producing many of the same metabolic abnormalities and diseases associated with age.^13-15 The metabolic dysfunction associated with obesity is remarkably similar to that which develops in normal aging, and the obesity phenotype may effectively be a form of accelerated aging.¹⁶ There is an epidemic of obesity among companion animals, which contributes to significant morbidity and mortality, especially in geriatric populations.^15,17

Fortunately, there is also clear evidence that caloric restriction can be an effective longevity-promoting intervention. Long-term studies in many species, including dogs, have demonstrated that reducing caloric intake and maintaining a healthy body condition slows aging and significantly extends both health span and life span.¹⁸ Although drastic caloric restriction is not a reasonable option for most dog owners, avoiding overfeeding is a practical and effective way to reduce age-associated disease and prolong the health and life span of dogs.

Using physical activity to prevent age-associated conditions

Another well-demonstrated method of slowing the damage done by aging is increased physical exercise. Exercise influences the metabolic dysfunction that accompanies aging in many beneficial ways, extending health span and reducing the risk of death from age-associated conditions.¹⁹ We lack evidence for the optimal type and amount of exercise most beneficial to dogs and cats, but we do know that many lead excessively sedentary lives and would benefit from increased activity. This is especially true for older pets, for whom owners and vets may assume that negligible physical activity is just a manifestation of normal aging. Older humans clearly benefit from physical activity, and the same is almost certainly true for older veterinary patients.

There is evidence that veterinarians can successfully support owners in reducing weight and increasing activity for their pets.20 The perspective that aging is a modifiable risk factor for disease encourages veterinarians and pet owners to employ preventative strategies throughout the lifecycle to slow aging and mitigate the negative impact it can have on health and well-being.

Efforts such as the Golden Retriever Lifetime Study by the Morris Animal Foundation and the Dog Aging Project at the University of Washington illustrate how understanding the need to investigate aging and age-related disease can generate both significant enthusiasm among veterinarians and pet owners and useful scientific evidence.

Conclusion

The semantic and philosophical debates over whether aging should be called a disease are unlikely to be resolved any time soon. However, the pragmatic view that aging is the most important risk factor for many diseases allows both prudent clinical management of age-related changes in health and the development and implementation of interventions directly targeting aging. Veterinarians can use this to promote healthy lifestyle interventions such as optimal weight management and increased physical exercise, as well as encourage pet owners to think proactively about aging and not to give up on useful diagnostic and treatment interventions in senior pets. We can have the best of both perspectives, maximining function and comfort in geriatric patients now and preparing for a future in which many of their ailments can be prevented.

Brennen McKenzie, VMD, MSc, MA, is an associate at Adobe Animal Hospital in Los Altos, California, and director of veterinary medicine at Loyal in San Francisco.

References

1. World Population Ageing 2019: Highlights. United Nations Department of Economic and Social Affairs. Accessed February 21, 2022. https://bit.ly/3BEFV9K
2. State of Pet Health 2013 Report. Banfield Pet Hospital. Accessed February 21, 2022. https://bit.ly/3H6r613
3. State of Pet Health 2016 Report. Banfield Pet Hospital; 2016. Accessed February 21, 2022. https://www.banfield.com/Home/pet-health/State-of-pet-health
4. Creevy KE, Grady J, Little SE, et al. 2019 AAHA Canine Life Stage Guidelines. J Am Anim Hosp Assoc. 2019;55:267-290. doi:10.5326/jaaha-ms-6999
5. Quimby J, Gowland S, Carney HC, DePorter T, Plummer P, Westropp J. 2021 AAHA/AAFP Feline Life Stage Guidelines. J Feline Med Surg. 2021;23(3):211-233. doi:10.1177/1098612X21993657
6. Bellows J, Colitz CMH, Daristotle L, et al. Defining healthy aging in older dogs and differentiating healthy aging from disease. J Am Vet Med Assoc. 2015;246(1):77-89. doi:10.2460/javma.246.1.77
7. Bellows J, Center S, Daristotle L, et al. Evaluating aging in cats: how to determine what is healthy and what is disease. J Feline Med Surg. 2016;18(7):551-570. doi:10.1177/1098612X16649525
8. Zhavoronkov A, Bhullar B. Classifying aging as a disease in the context of ICD-11. Front Genet. Published online November 4, 2015. doi:10.3389/fgene.2015.00326
9. Bulterijs S, Hull RS, Björk VCE, Roy AG. It is time to classify biological aging as a disease. Front Genet. Published June 18, 2015. doi:10.3389/fgene.2015.00205
10. Olshansky SJ. Articulating the case for the longevity dividend. Cold Spring Harb Perspect Med. 2016;6(2):a025940. doi:10.1101/cshperspect.A025940
11. McKenzie BA. Overdiagnosis. J Am Vet Med Assoc. 2016;249(8):884-889. doi:10.2460/javma.249.8.884
12. Franceschi C, Garagnani P, Morsiani C, et al. The continuum of aging and age-related diseases: common mechanisms but different rates. Front Med (Lausanne). 2018;5:61. doi:10.3389/fmed.2018.00061
13. Kanasaki K, Koya D. Biology of obesity: lessons from animal models of obesity. J Biomed Biotechnol. 2011;2011:197636. doi:10.1155/2011/197636
14. Kitahara CM, Flint AJ, Berrington de Gonzalez A, et al. Association between class III obesity (BMI of 40-59 kg/m2) and mortality: a pooled analysis of 20 prospective studies. PLoS Med. 2014;11(7): e1001673. doi:10.1371/journal.pmed.1001673
15. Salt C, Morris PJ, Wilson D, Lund EM, German AJ. Association between life span and body condition in neutered client-owned dogs. J Vet Intern Med. 2019;33(1):89-99. doi:10.1111/jvim.15367
16. Tam BT, Morais JA, Santosa S. Obesity and ageing: two sides of the same coin. Obes Rev. 2020;21(4):e12991. doi:10.1111/obr.12991
17. State of Pet Health 2017 Report: Spotlight on Obesity. Banfield Pet Hospital. Accessed February 21, 2022. https://www.banfield.com/en/pet-health/State-of-pet-health
18. Lawler DF, Larson BT, Ballam JM, et al. Diet restriction and ageing in the dog: major observations over two decades. Br J Nutr. 2008;99(4):793-805. doi:10.1017/S0007114507871686
19. Gremeaux V, Gayda M, Lepers R, Sosner P, Juneau M, Nigam A. Exercise and longevity. Maturitas. 2012;73(4):312-317. doi:10.1016/j.maturitas.2012.09.012
20. Niese JR, Mepham T, Nielen M, et al. Evaluating the potential benefit of a combined weight loss program in dogs and their owners. Front Vet Sci. 2021;8:653920. doi:10.3389/fvets.2021.653920