? What does negative carb % mean?

[FarmKitty]

Heya, I am looking into Weruva's new Wx Phos Focus food line and some of their foods have a negative carbohydrate percentage. Anyone know what this means? I am looking for a new low-carb, low-phosphorus food because they are discontinuing my current one.

 

[Larry and Kitties]

It appears that they measure the protein and the fat and then subtract that from 100 to calculate the % carbs. Because of errors in measuring the protein and fat you can get a negative carbs.