What and how to feed a dog so as not to harm

There are more than 522 million dogs in the world. And each one must be fed!

And feed wisely, because nutrition affects literally everything in a dog — from its well-being and coat quality to activity and mood. A dog's body has a considerable reserve of resilience, which allows it to «mask» the negative effects of improper feeding, and we will only notice changes when they have gone too far...

• How to feed a dog so that it not only looks healthy but is truly healthy?
• How to understand that the food provides the body with all the necessary vitamins?
• How to determine whether it is obesity or just a «wide bone»?
• How to protect a pet from allergies and toxic products?

And in general, what is this strange science — dietetics?