fat in foods doesn't seem to be digesting. Any idea why?

If your gallbladder isn't functioning correctly, you may not have enough bile to help break down the fat.

doesnt sound like a very healthy diet, diets high in protein can cause kidney and heart problems as this polutes the blood with ketones, i suggest a diet more appropriate with the correct amount of protein for your dogs age and weight. A diet very low in carbohydrates will also mean your dogs are not producing as much energy as they should. Dogs are very hardy but what makes you think that they will not eventually go in to ketosis on such a diet? I suggest having the dogs urine tested for glucose and ketones and would be interested to see the how the hepatic system is going as well with a routine blood test. I wouldn't be surprised if your dogs are not as active as they would be if they were to receive a better quality food, I would also not be surprised if the dogs are very lean and the body has been breaking down muscle to produce energy on this diet. I am not familiar with the product at all but a diet consisting of only high protein and very low carbohydrates is not an adequate diet for a dog.

First consult your doctor. Personally, I take CLA which is amino acids that help break your food down and sometimes drinking lukewarm water may help the digestion of foods.

http://cat.inist.fr/?aModele=afficheN&cpsidt=18655...

This is pretty thick language, but there do seem to be such things as indigestible oils. What did you have for lunch, and what sort of fat do you believe was in it?