One thing I learnt from Jiggers is that sometimes a food just doesn't work for a dog. The only sign I had that the PC food didn't work for Jiggers was that I just couldn't keep weight on him. If the vet says nothing is medically wrong but she is underweight it might be the wrong food for her.
I went through the exact same issue with both of mine. I really wanted to feed Earthborn. I love the food, love the company, and my dogs enjoyed it as well, but it just could not keep weight on them. I would increase the amount in an attempt to get them to fill out a little and end up with runny poos from digestive overload.
I ended up having to switch to Diamond Naturals Extreme Athlete (my dogs are neither extreme nor atheltic) but it is a nutrient dense formula and they finally got to a good management weight. Not to mention because it was so dense, I was able to feed them less and maintain a good body condition.
+ Caution: I absolutely would not recommend Extreme Athlete for a pup under 1 year old. The calcium/phos balance is high and the fat is quite high as well. It is a good maintenance formula for adult dogs that have trouble keeping weight on.
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