As dogs and cats get older, their energy needs and body composition change, influencing their nutritional requirements. Similar to humans, senior pets may also develop frailty. As companion animals are increasingly viewed as family members, veterinary care has evolved to prioritize not only lifespan but also enhancement of “healthspan”– the period of life spent in optimal health. This mirrors trends in human medicine, as pet owners seek the highest quality of life for their aging companions. Notably, Bobi, the oldest known dog, passed away in October 2023 at an estimated 31 years and 165 days, while “Crème Puff,” the oldest known cat, lived to be 38 years and three days, passing away in August 2005.1 These remarkable lifespans underscore the importance of nutrition strategies that support longevity and vitality during the senior years. Understanding the changes that occur with aging and the ability to intake nutrients will help in designing optimal nutrition plans for senior pets. Physiologic aging and the role of nutrition As dogs and cats age, changes in energy needs and body composition will occur, which can affect nutrient requirements. Similar to humans, older dogs and cats can be susceptible to frailty, a clinical syndrome marked by diminished physiological reserves. Dogs may also experience sarcopenia, a loss of lean body mass coinciding with increased fat mass, even without weight gain.1 This necessitates careful nutrition planning along with frequent reassessment to ensure the best care is provided to our senior patients. Unfortunately, despite the growing population of senior pets, formal nutrition guidelines for aging dogs and cats remain limited. Currently, the Association of American Feed Control Officials (AAFCO) only provides recommendations for “Growth and Reproduction” and “Adult Maintenance”. Individualized plans should account for lifestyle, physical condition, and co-morbidities, along with physiological changes that are likely to occur. The use of “therapeutic supplements”—nutrients provided beyond basic requirements for targeted health benefits—is also gaining traction. Photo courtesy Dr. Laura Gaylord Redefining “senior”: Life stage classification in dogs and cats An estimated 20-40 percent of dogs and cats in clinical practice are over 11 years of age.2,3 Definitions of “senior” have evolved over time, and recently, a new emphasis has been placed on using physiologic changes and disease risk related to different stages of aging rather than solely on chronological age. The American Association of Feline Practitioners (AAFP) updated feline life stages to describe cats as “mature” at 7-10 years, “senior” when over 10 years of age, while the term “geriatric” is no longer used. The last stage of life is described as “end of life” and can be quite variable in length.4 The American Animal Hospital Association (AAHA) has defined the senior life stage for dogs as the final 25 percent of expected lifespan, accounting for breed and size variability.5 Interestingly, more recent work has proposed life stage classification based on disease risk (Table 1, Table 2).6 Table 1: Feline Life Stages Kitten Birth up to 1 year Young adult 1-6 years Mature adult 7-10 years Senior >10 years End of Life Variable Source: 2021 AAHA/AAFP Feline Life Stage Guidelines Table 2: Canine and Feline Life Stages (based on Age Group Clusters6) Canine Toy breed (years) Small breed (years) Medium breed (years) Large breed (years) Youth 1-6 1-6 1-5 1-5 Midlife 7-11 7-11 6-9 6-9 Senior 12-13 12-13 12-13 12-13 Super-Senior >14 >14 >14 >12 Feline (years) Youth 1-4 Early midlife 5-9 Late midlife 10-11 Senior 12-13 Super-Senior >14 Gastrointestinal aging: Digestive changes in senior cats Age-related gastrointestinal changes observed in humans have prompted investigation into similar patterns in dogs and cats. While structural and functional differences between young and older animals have been documented within the digestive tract for both dogs and cats, the clinical impact of these changes is uncertain, as a vast functional reserve is typically present.7 In cats, appetite decline is common and may stem from dental issues or other co-morbidities.3 Digestibility of primarily fat but also protein decreases after age seven.8-10 This is somewhat influenced by dietary format, with cats consuming extruded dry foods maintaining weight longer.3 This may be due to higher caloric densities of dry foods compared to wet foods (average 850-1200 kcal/kg in canned/wet vs. >3800-4200 kcal/kg in dry extruded foods). Morphologic changes in the intestinal tract for older cats reflect altered permeability and absorption capacities, and while total average transit time was similar, senior cats had a greater variability in total transit times compared to younger cats.11 Additional age-related changes include altered bile composition, reduced pancreatic enzyme output, delayed gastric emptying, and decreased enteric transit. Reduced colonic motility may be a factor contributing to chronic constipation that commonly occurs in aging cats.3 Digestive efficiency and energy needs in aging dogs In dogs, intestinal morphology changes have been noted with age, but nutrient processing remains mostly unaffected.12 Studies consistently show aging dogs maintain sufficient digestibility of macronutrients or minerals, such as calcium, phosphorus, magnesium, zinc, copper, iron, potassium, and sodium.13-15 Energy requirements, however, will decline, often by 20 percent, and in some cases, up to 50 percent, primarily due to a decrease in lean body mass.16-18 Mobility impairing disease will result in a loss of lean muscle mass, resulting in a lower basal metabolic rate.7 Ingredient choice is important to consider due to differences in nutrient digestibility beyond just the macronutrient breakdown of a particular food.15 While more research is needed, food format and composition—extruded, retorted, fresh or raw, etc.—also affects nutrient availability.7,1 (Table 3) Table 3: Changes in nutrient intakes for senior dogs and cats Nutrient of concern Change Comment Energy Dogs Decrease or increase Lower caloric density foods indicated if overweight/obese; increased caloric density if underweight or co-morbidity expected to cause weight loss Cats Increase, unless with co-morbidity needing reduction Increased caloric density is needed for most cats, as body weight tends to decline in the oldest age categories; if overweight, lower caloric density is appropriate Protein Dogs Increase, unless with co-morbidity needing reduction Higher protein will support lean body mass and immune function, which often declines with age; Lower protein is indicated if co-morbidity is present that requires protein reduction Cats Increase, unless co-morbidity needing reduction Higher protein will support lean body mass and immune function, which often declines with age; Lower protein is indicated if co-morbidity is present that requires protein reduction Fat Dogs Decrease or increase Fat digestion capacity is maintained, but the amount should be adjusted to maintain the best body condition/weight Cats Increase or decrease Fat digestion may decline, and weight loss is common in older cats, thus, increasing fat content is often indicated; reduce fat intake if overweight/obese Essential minerals: Phosphorus and sodium in senior diets Other essential nutrients to consider in senior pet diets include phosphorus and sodium. While there is a lack of evidence that phosphorus reduction will prevent the development of chronic kidney disease, reduced intake can slow progression of the disease state once established in both dogs and cats.19,20 Routine screening is thus a critical part of senior health care (Table 4). With unrestricted access to water, healthy dogs and cats can tolerate wide ranges of sodium intake, and no specific intake level is defined for senior pets without disease conditions. In cases of hypertension or chronic renal disease, avoiding excessive sodium intake is recommended, although this intake level is not clearly defined. Sodium restriction is not warranted as blood pressure and sodium intake level have not been correlated in experimental studies.21 For any diet, “restricted” is defined as falling below the National Research Council Recommended Allowance guideline.22 Table 4: Recommended diagnostic tests for senior dogs and cats Diagnostic Test Canine Feline Fecal testing for parasites 1-4x/year lifestyle dependent and owner history or preventative use 1-2x/year depending on health, lifestyle factors Tick-borne disease testing Annually N/A Heartworm Annually See: American Heartworm Society guidelines* Comprehensive CBC (includes cell morphology, differentials, and pathologist review if abnormalities) Every 6-12 months Chemistry (Total protein, albumin, ALT, glucose, BUN, Creatinine, sodium, potassium, calcium, SDMA) Every 6-12 months Urinalysis (Urine concentration, sediment, glucose, ketones, bilirubin, protein, blood) Every 6-12 months Imaging (Radiographs, whole body; Abdominal ultrasound) Imaging of the region of interest, if clinically indicated. Breed-specific screening may be appropriate Electrocardiogram (ECG) Perform an ECG annually in breeds with high risk of cardiac disease (boxers, Dobermans), or in other patients when clinically indicated or if an arrhythmia is detected on examination. Urine protein:creatinine ratio (UPC) If proteinuria is discovered or microalbuminuria is persistent. Thyroxine (T4) Recommend annually Strongly recommend annually Blood pressure Recommend annually Strongly recommend annually Retroviral testing N/A Consider based on lifestyle/ risk NT-proBNP Annually; strongly consider prior to anesthesia Source: 2023 AAHA Senior Guidelines; page 5 *American Heartworm Society. Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Cats. American Heartworm Society, 2020. Functional nutrients: Antioxidants, omega-3s, and MCTs Additional nutrients of interest in senior pet diets include antioxidants, omega-3 fatty acids, and medium chain triglycerides (MCTs) due to their ability to support cognition, immune function, and musculoskeletal health. Antioxidant supplementation has been reported to improve owner and veterinarian-perceived clinical scores for dogs with cognitive dysfunction in combination with diet interventions.23 A blend including antioxidants, vitamin C, and vitamin E, carnitine, lipoic acid, glutathione, along with fruits and vegetables, improved learning, task performance, agility, and recognition skills in dogs compared to a control diet. Increased neutrophil function was also observed, suggesting improved immune status.24 Although dogs can synthesize vitamin C endogenously (and it is thus not an essential nutrient), this antioxidant vitamin may become conditionally essential with age due to increased demands. Supplementation of omega-3 fatty acids provides the biologically active molecules eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which can support brain health, cognition, joint function, cardiovascular integrity, and skin condition for senior pets.25 MCTs offer alternative energy substrates for the brain, which can improve mitochondrial function, reduce amyloid deposits, and improve redox status in older dogs.26,27 Personalized and proactive nutrition for senior pets As our pets live longer, the way we think about nutrition in their later years needs to evolve as well. Understanding the age-related changes in body composition, digestive efficiency, and nutrient needs will help facilitate more personalized dietary strategies for both dogs and cats. Guidelines for senior pets are still evolving; however, current data on select nutrients and supplements may make meaningful changes in senior pet quality of life by supporting mobility, immune function, cognition, and overall vitality. Ultimately, optimizing nutrition for senior pets is not just about meeting minimum requirements; it is about anticipating physiologic shifts and tailoring interventions that truly support the healthspan. Table 5: Resources on nutritional management of senior dogs and cats American Animal Hospital Association (AAHA) 2023 Senior Care Guidelines for Dogs and Cats. American Association of Feline Practitioners (AAFP) FEDIAF (The European Pet Food Industry) FEDIAF Scientific Advisory Board Statement Nutrition of senior dogs Senior nutritional requirements for cats and dogs Article: Nutritional needs and health outcomes of ageing cats and dogs: is it time for updated nutrient guidelines? Laura B. Gaylord, DVM, DACVIM (Nutrition), is an independent consultant and the owner/founder of Whole Pet Provisions, PLLC, a nutrition consulting company established in 2016, which offers veterinary nutrition consulting to pet owners, veterinarians, the pet food industry, and pet supplement companies. Dr. Gaylord offers homemade diet recipe formulation and commercial diet consultations through her business for pet parents and their veterinary team. References 1. Bermingham, E. N., Patterson, K. A., Shoveller, A. K., Fraser, K., Butowski, C. F. & Thomas, D. G. 2024. Nutritional needs and health outcomes of ageing cats and dogs: is it time for updated nutrient guidelines? Anim Front, 14, 5–16. Bellows, J., Colitz, C. M., Daristotle, L., Ingram, D. K., Lepine, A., Marks, S. L., Sanderson, S. L., Tomlinson, J. & Zhang, J. 2015. Common physical and functional changes associated with aging in dogs. J Am Vet Med Assoc, 246, 67–75. Bellows, J., Center, S., Daristotle, L., Estrada, A. 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