Don't use Hills, Iams or anything else like that. If corn is in there do not feed it as a lot of dogs have allergies to corn and it basically means the food is crap and full of fillers.
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While premium foods cost more per bag, you usually feed less as they are more nutrient rich and have better quality ingredients.
Swimming is great exercise for arthritic dogs. A dog boarding place about an hour from us has an indoor pool you can take your dogs to do laps in so maybe see if anything near you has a similar set-up.
Our older dog had arthritis and her spine fused, and gentle walks were the best exercise for her. I would not run an arthritic dog as the concussion on the joints (particularly if running on asphalt or the sidewalk) may make things worse.
Cutting down on food, using a premium brand dog food, and slowly increasing exercise is going to help take the weight off. We kept our dog on the lean side as if she was any heavier she would not have been able to get around.