Three numbers decide whether an old cat with soft stool, vomiting or a flat appetite improves on a GI prescription diet: digestibility above 85%, fiber between 3% and 8% of dry matter, and 4 small meals a day instead of 2 large ones. A cat that also carries a kidney diagnosis needs a second set of numbers, and the two prescriptions pull in opposite directions.
What a GI formula actually changes
Three levers, in order of effect. First, digestibility: prescription GI diets run 85–92% apparent dry-matter digestibility, against 75–80% for a typical maintenance kibble, so less unabsorbed protein and fat reaches the colon to feed the wrong bacteria. Second, a fiber blend fixed between 3% and 8%. Third, energy density of 3.9–4.4 kcal/g, so a cat that manages only 35 g of food still takes in 140–155 kcal.
Low fat is not part of the package by default. Restricting fat to 10–15% of dry matter belongs to lymphangiectasia, recovery from pancreatitis, or a confirmed fat intolerance. Applied to a thin 13-year-old it strips out the cheapest calories available and the weight keeps sliding.
Fiber: the 3 to 8 percent window
At 3–5% dry matter, fiber mostly changes transit and stool form: psyllium or beet pulp at that level firms loose stool without cutting calories much. At 6–8% the target shifts to the colon, where fermentable fiber feeds short-chain fatty acid production. Above 10%, energy density drops below 3.5 kcal/g and a cat with a small stomach capacity cannot eat enough volume to hold weight.
Read the guaranteed analysis, then convert. Fiber listed at 4% as-fed on a 10% moisture food is about 4.4% dry matter, and that is the number that matters. A pâté at 78% moisture listing 1.5% fiber lands near 6.8% dry matter, much higher than the label suggests.
Four meals a day, sized in grams
A 4.5 kg senior cat at rest needs roughly 220–240 kcal a day, the same energy target used in the general senior cat diet plan; what changes here is how the total is split. Split 4 ways that is 55–60 kcal per meal: 14–16 g of a 3.9 kcal/g kibble, or about 45–55 g of pâté at 1.1 kcal/g. Two or three meals of 75–110 kcal each is the pattern behind the 3 a.m. bile vomit in cats with slow gastric emptying; 4 meals spaced 5–6 hours apart usually ends it within 7–10 days.
Weigh the portion. A scoop drifts 10–20% between bags, which is 25–50 kcal a day and 0.1–0.2 kg a month on a 4 kg cat. Feed the last meal within 2 hours of bedtime if the vomit is yellow and frothy.
When CKD and GI conflict
The conflict is protein and phosphorus. A GI formula typically carries 30–35% protein and 0.8–1.0% phosphorus on a dry-matter basis; a renal formula sits at 26–28% protein and 0.3–0.6% phosphorus. You cannot honor both at once, so one axis gets priority for a defined period, and the bloodwork decides which.
Let the kidney win when creatinine is above 2.5 mg/dL, SDMA above 25 µg/dL, or phosphorus above 1.6 mmol/L. Those cats do better on a renal diet with a soluble fiber supplement added at 1–2 g per meal. Let the gut win for 4–8 weeks when values are IRIS stage 1–2 with phosphorus under 1.4 mmol/L, then recheck. A middle route: renal formula plus 0.5–1 g psyllium and 3 meals a day, which handles mild constipation without touching protein.
| Parameter | GI formula | Renal formula | Senior maintenance |
|---|---|---|---|
| Protein, dry matter | 30–35% | 26–28% | 32–38% |
| Fat, dry matter | 18–22% | 18–22% | 15–20% |
| Fiber, dry matter | 3–8% | 2–5% | 3–5% |
| Phosphorus, dry matter | 0.8–1.0% | 0.3–0.6% | 0.7–1.1% |
| Energy density | 3.9–4.4 kcal/g | 3.9–4.3 kcal/g | 3.6–4.1 kcal/g |
| Cost per 100 kcal | USD 1.20–1.80 | USD 1.30–2.00 | USD 0.45–0.80 |
Switching over 14 to 21 days
A damaged gut tolerates change slowly, and the 14-day minimum is not marketing. Hold each ratio for 3–4 days and extend by 2 days if stool score drops below 4 on the 1–7 scale.
| Days | Old food | New food | Hold or extend when |
|---|---|---|---|
| 1–3 | 75% | 25% | stool 4–5, no vomiting |
| 4–7 | 50% | 50% | stool 4, appetite steady |
| 8–11 | 25% | 75% | stool 3–4, no gas |
| 12–14 | 0% | 100% | weight within 2% of start |
| 15–21 | 0% | 100% | score muscle weekly |
Budget for the cost before the first bag. A prescription GI diet runs USD 1.20–1.80 per 100 kcal, so a 240 kcal cat costs USD 86–130 a month, or USD 1,050–1,580 a year against USD 400–700 on maintenance food. The annual prescription cost breakdown is worth reading before a 3-month trial starts.
Red flags that outrank any diet change
Vomiting more than 3 times in 24 hours, blood in stool, weight loss above 5% in 30 days, jaundice, or a cat that has refused food for 24–36 hours. Two to three days without intake puts an overweight cat at real risk of hepatic lipidosis, and that cat needs to be seen the same day rather than switched to another bag.
Track three things for the first 21 days: stool score, body weight to 0.05 kg, and how long a meal takes. A cat that improves on stool but keeps losing 1–2% of body weight a week has a second problem, usually dental or kidney, and the diet is not the fix.
Five steps that hold
- Confirm the problem first: fecal score, weight trend, and bloodwork including SDMA and total T4 within 14 days of starting.
- Pick the axis by number, not by habit — creatinine above 2.5 mg/dL means renal formula, under that means GI formula for 4–8 weeks.
- Weigh every portion in grams and split it into 4 meals 5–6 hours apart.
- Run the 14–21 day transition and extend any step where stool score falls below 4.
- Recheck weight and bloodwork at 4 weeks, then every 3 months on a prescription diet, using the cost calculator to plan refills.
Terms mentioned in this article
Frequently asked
How do I know the 85% digestibility figure applies to my cat's food?
Most prescription GI diets publish digestibility as a range rather than a guarantee, and 85% is the floor you should insist on. Ask the manufacturer for the dry-matter apparent digestibility figure for the specific formula, not the whole brand. If the number is unavailable, treat the diet as unproven and judge it on stool score and weight over 21 days instead of on the label.
Can I just add psyllium to my cat's normal senior food?
Sometimes, and it is cheaper than a prescription bag. Start at 0.5 g per meal mixed into wet food with 10 ml of water, twice daily, and increase to 1 g per meal after 5 days if stool is still loose. This handles mild fiber-responsive diarrhea but does nothing for fat malabsorption, protein loss, or a cat that is vomiting, so bloodwork comes first.
The renal diet made the vomiting worse. What now?
Check the fat level and the portion size before blaming the protein. Renal formulas at 18–22% fat are still rich for a cat with slow gastric emptying, so split the daily ration into 4 meals of 50–60 kcal and warm it to 38 degrees C. If vomiting continues past 7 days, or weight falls more than 5% in a month, ask for a GI formula plus a phosphorus binder instead.