A small amount of the starch in potatoes resists digestion (this is called ‘resistant starch’): this particularly occurs when potatoes are eaten cold after being cooked, such as in a potato salad. Resistant starch acts in the body in a similar way to fibre.I think the effect is that the person has a feeling of fullness after eating, and isn't so hungry, yet the body can't digest the cold potato, so doesn't gain weight.
However, I'm not going to feed potato to Penny until I know more about this, as I've never fed her potato before.
And I came across a site that looks useful, because it discusses the value of home-prepared meals without the angst that sometimes accompanies this topic. The author, who appears to be a vet, with a PhD, also says that if you're going to buy prepared foods, you should buy the reputable ones from a vet, because at least you know a properly qualified staff of nutritionists works for the company. I suppose that is logical. Interestingly, he says something that I've always thought should be true, that since dogs have lived with us for about thirty thousand years, they've adapted to the same sort of diet as we eat. (With notable exceptions such as chocolate, grapes, raisins, garlic, onions, macadamia nuts, tomatoes and avocado.)
I thought the discussion of eggs was interesting - he says eggs should be eaten cooked, not raw, which is news to me. Also, he discusses the fact that we need to assess the danger of high level of mercury in fish against the nutritional value of the omega-3 fatty acids.
The discussion of the best proportions of fat, protein and carbohydrates is one of the clearest I've read.