An 11-year-old cat still needs adult-level protein: roughly 6–8 g of crude protein per kilogram of body weight per day, which is 27–36 g for a 4.5 kg cat. The common habit of moving every older cat onto a lower-protein senior formula is the wrong default. Switch food slowly, weigh the cat weekly, and change the diet only when a diagnosis or a measured weight loss gives you a reason.
Why protein needs stay flat after 11
Age reduces a cat's ability to digest and use protein more than it reduces the need for it. Digestibility of fat and protein falls by roughly 5–10% in geriatric cats, and lean body mass declines by about 1–2% a year after middle age. The result is a cat that must eat slightly more protein to hold the same muscle. Restricting protein in a healthy old cat accelerates the loss you are trying to prevent.
What does change is reserve. An older cat has less capacity to survive 3 days of poor intake, and hepatic lipidosis risk rises fast in any cat that was overweight before it stopped eating. That is why the transition plan below is built around intake, not around the label.
Reading the label before you buy
Senior has no legal definition in most markets. What matters is the guaranteed analysis converted to dry matter: a typical adult maintenance dry food sits at 30–38% protein, many senior 7+ formulas at 28–34%, and renal diets at 26–32% with phosphorus restricted to roughly 0.3–0.6% dry matter. Two bags labelled senior can differ by 8 percentage points of protein and by 400 kcal per kilogram.
Compare carbohydrate too, because senior formulas often cut protein and add starch to keep the kibble cheap and crunchy. Dry-matter carbohydrate above 35% is high for a cat. Check carb content in cat food for the conversion maths, and confirm the taurine guarantee is at least 0.2% on a dry-matter basis using taurine in cat food.
The 7-day switch, in grams
Work out the cat's current daily intake in grams first by weighing a scoop for 3 days. Then split the new food in at 25%, 50%, 75% and 100%. Never change more than a quarter of the bowl per 48 hours, and keep the total daily weight identical so the only variable is composition.
| Day | Old food | New food | For a 40 g/day cat | For a 60 g/day cat |
|---|---|---|---|---|
| 1–2 | 75% | 25% | 30 g + 10 g | 45 g + 15 g |
| 3–4 | 50% | 50% | 20 g + 20 g | 30 g + 30 g |
| 5–6 | 25% | 75% | 10 g + 30 g | 15 g + 45 g |
| 7 | 0% | 100% | 40 g | 60 g |
Mix the two foods with 30–40 mL of warm water at 37°C and stir until the kibble softens. A softened bowl is easier on worn teeth and slows eating, which reduces vomiting after a fast meal.
Three fallbacks when appetite drops
First, temperature and texture. Food at 37°C releases more aroma than food at 8°C straight from the fridge, and a pâté warmed and mashed draws more interest than a cold block. Offer 2 tablespoons, not a full bowl, and take it away after 20 minutes.
Second, frequency and position. Four to six small meals a day beat two large ones in a cat with a small stomach capacity, and raising the bowl 5–10 cm helps a cat with arthritis in the neck. Feed in a quiet room away from a dog or a competing cat.
Third, medical appetite. A cat that eats less for 3 days needs a physical exam, not a new flavour: dental pain, nausea from CKD, hyperthyroidism, and intestinal disease are the common drivers. Weight loss of more than 5% in 6 months or a body condition score of 3/9 or lower justifies blood work including T4, renal panel and SDMA. Never use an appetite stimulant to mask a treatable cause.
Where senior and renal diets conflict
The conflict is real and it is measured in protein. A renal diet restricts protein and phosphorus to slow progression in IRIS stage 2–4 chronic kidney disease (CKD), while a senior maintenance diet is built to preserve muscle in a cat whose kidneys work. Putting a cat on a renal diet because it is 12 years old, without staging, removes protein it still needs.
Stage first: creatinine, SDMA, urine specific gravity, urine protein-to-creatinine ratio, blood pressure and phosphorus. In IRIS stage 1 with normal muscle and normal phosphorus, most internists keep a moderate-protein adult diet and recheck in 3 months. In stage 2 with muscle loss, a renal diet is reasonable; in stage 3–4 it is standard. If the cat refuses the renal diet for more than a week, a phosphorus binder on a palatable adult food usually beats forced acceptance of a food the cat will not eat.
Weighing, scoring and the 3-month review
Weigh weekly on the same kitchen scale, in grams, and record it. Body condition is scored 1–9, with 5 ideal; muscle condition is scored separately because a cat can be overweight and still lose muscle. A stable 4.5 kg cat eating 60 g of a 380 kcal/100 g food takes in about 228 kcal, near maintenance for an indoor neutered adult; recalculate after any food change rather than trusting the bag.
Terms like maintenance energy requirement, dry-matter basis and body condition score are defined in the site glossary if you need the exact formulas before a vet visit.
A four-step start
- Weigh the cat and 3 days of food, then calculate current grams per day and calories per day.
- Choose a food with 30% or more protein on a dry-matter basis and a taurine guarantee of at least 0.2%.
- Run the 7-day table with totals held constant, and add 30–40 mL warm water per meal.
- Weigh weekly for 8 weeks; if weight falls more than 2% in a week, stop the switch and book an exam.
Frequently asked
Should a healthy 12-year-old cat eat less protein?
No. Healthy older cats need adult-level protein, about 6–8 g per kilogram of body weight daily, because digestibility falls and lean muscle declines with age. Protein restriction is for staged chronic kidney disease with a documented reason, not for age alone. Check the dry-matter protein percentage rather than trusting the word senior on the bag.
How long can a cat refuse a new food before it is dangerous?
Three days of near-total refusal is the practical limit before you need a veterinary exam, because an overweight cat can start mobilising fat into the liver within 2–3 days of anorexia. Offer the old food again while you investigate rather than letting the cat fast through a transition, and never let a diet change turn into a hunger strike.
Can I mix a renal diet and a senior diet half and half?
It is a compromise, not a treatment. Half-and-half gives roughly half the phosphorus restriction and half the protein restriction, which is useful for a cat that will not accept a renal diet at all. Have the mix checked against the cat's IRIS stage and blood phosphorus at the next recheck, usually 4–8 weeks later.