How fast can I switch formulas?

Seven days, moving in 25% steps every two days. Here is the reasoning behind that answer, and the exceptions worth knowing.

The short answer

Seven days, moving in 25% steps every two days.

Gut flora needs about a week to adapt. Faster switches cause loose stools that owners often blame on the new food, when the real culprit was the speed of change.

Why it works that way

Feeding decisions compound: a 10% overshoot becomes 2–3 kg a year, and joint disease follows weight rather than age. That is why portion control and body condition scoring outrank brand loyalty in every credible feeding guideline.

• Weigh food rather than scooping by eye for at least two weeks.
• Score body condition monthly using ribs, waist and belly tuck.
• Change one variable at a time so you can tell what worked.

When to ask your vet instead

See a vet rather than adjusting diet if you notice weight loss without diet change, persistent loose stools, vomiting, or a sudden refusal to eat. Those are clinical signs, not feeding problems.

Adjust slowly, measure consistently, and let body condition be the referee.