A diet dog food is a dish that has been specially tailored to meet the nutritional needs of pets suffering from certain health conditions. For example, there are diet foods for dogs with diabetes or with kidney problems, and these should only be fed to your dog after consultation with your vet. Therefore, a diet dog food is not always a suitable choice for weight loss.
However, a light dog food is one that has a reduced calorie content. This should be your first port of call in ensuring your dog does not eat too much and can therefore lose some weight.