Actually, I think a topical question as a separate issue is OK. You may also want to post your question (or cross post) on Health to get a broader response since the question isn't specific to Lantus. Also, there's a possibility that Dr. Lisa Pierson who's a vet with interests in feline nutrition will see your post. I think she commented in your condo previously about the food your vet had suggested (or didn't object to).
Cats are obligate carnivores. What that means is there is fundamentally no need for carbs in their diet. If they were living in the wild, the carbs they would get would be courtesy of what was in the digestive tracks of other animals and those carbs would be semi-digested. I feed a mostly raw diet -- it's about 1% carb -- with some Wellness that's in the 4 - 5% range.
Some cats are very carb sensitive. Jill's Alex is a cat that can eat a piece of chicken and have a food spike. Other cats can tolerate higher carbs with no effect whatsoever. As with many things, every cat is different (ECID) and you need to determine what is right for your cat. We encourage feeding under 10% carb. I think if you asked for a vote, most of us feed under or around 5% carb because that allows greater variety in choices of foods.
