| Digestibility & Waste
I often come across comments that a food is "more digestible" as proved by the fact that there is "less waste". I do not understand the supposed connection.
A fish is largely composed of water. If you have ever had a fish jump out and dry out completely, you know that the dry weight of the fish was a very small percentage of the natural weight. Very little of any dry food is going to be absorbed by a fish through the digestive process. Virtually everything that is consumed will be expelled. So, how can there be a noticeable difference in waste volume?
Seems to me that there are two possibilities: (1) the food is so nutritious that less dry weight of it is fed [note: not less volume, but less dry weight...some foods are full of air], but I've not noticed people commenting that they feed less of particular foods on a weight basis; or, (2) the waste is just as voluminous, but not as easily observed.
Wheat germ is often identified as a highly digestible food. In mammals, wheat germ has a mild laxative effect. I expect the same is true of fish. During cool weather this can be very desirable, because we do not want food rotting in clogged-up intestines. However, the reason wheatgerm has a laxative effect is because it contains indigestible fiber, high oil content, and low starch levels (as well as high protein content). It results in less visible waste because the starches which bind waste into visible stools are very low. This does not necessarily mean the fish are getting more nutrient out of the food.
So, I see the photo on the Saki-Hikari label showing filter mats with less visible waste trapped and I wonder: If the waste is no longer getting captured in the matting, where is it? [If there is 50% less visible waste, are the koi gaining dry weight every meal equal to 50% of the weight of the food fed? ... Of course not. ] Wherever it is, is it better for the fish that the waste is not visibly trapped in the matting? Do I want waste that does not settle out in the vortex chamber? Or, do I want solid stools that I can see and that do settle out in the vortex without breaking apart?
I really do not know the answers. I have purposefully phrased my thoughts to create doubt about the usual "less visible waste means better digestibility" concept. That concept may be wrong, but the foods involved could still be the best for a different rationale.
Anyone aware of published scientific studies on digestibility in fish and absorption of nutrient from different food sources? |