What dog food or ingredients can help firm up my dog's loose stools?
The ingredients in the diet you are currently feeding are fine and quinoa would not be more beneficial than potatoes or carrots unless there was a known issue with them.
I see Luca's stool in the pictures and they are definitely loose. It also appears that he is straining to defecate. Without knowing more, I'm suspicious that there is a problem in the colon causing him to have diarrhea. This could be an infection, or an inflammatory bowel disease.
Infection is a common cause of diarrhea in younger dogs. Either way, it's best to have him evaluated by your vet, because I'm not sure that a diet change will help his current condition. A vet can test his stool ( make sure to take a fresh sample with you) and possibly do other testing to help identify the cause. Once the cause of this is identified, the correct treatment and diet can be advised.
You can help him get through this by feeding bland food (boneless chicken and rice is good). Feed small portions (1/2 cup or less) three times a day. You can also add probiotics to the food (can get this at the pet store), which has demonstrated some efficacy for improving diarrhea. Ensure fresh, clean water is always available.
I hope this helps and Luca gets better soon.
Updated on September 23rd, 2025