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Re: Making you own fish food!?
At 11:47 AM 11/22/00 -0800, Wright wrote, amongst other things:
>Fatty degeneration of the internal organs is the prime cause for early death
>in otherwise healthy aquarium fish, according to a study some years ago at
>the Calif. Academy of Science. High fat does not belong in any maintenance
>foods.
I would have to disagree, Wright. First, "fatty degeneration of the
internal organs" probably equates to hepatic lipidosis (abnormal
accumulation of fats, i.e. triglycerides in liver cells). I am not aware of
specific studies done on fish, but the pathogenesis of this condition is
probably similar in fish to other animals. It is generally not a
manifestation of excess dietary fat intake. Rather it can reflect a
(usually sudden) decrease or cessation of food intake, such that body fat
stores are mobilized as free fatty acids, which then are transported to the
liver, where they are processed into triglyceride. The re-export of these
is a rate limiting step so under such conditions, fat can accumulate in the
liver. If there is hepatic dysfunction, export can also be reduced, again
resulting in fat accumulation. Finally, other metabolic alterations can
cause hepatic lipidosis, such as changes in insulin resistance. I would
like to see that study, if you have it available. I wonder who looked at
the histopathology (microscopic lesions). For an inexperienced person it
could be very difficult to differentiate the vacuolation of tissue cells
that one sees in Mycobacterial infections (a subject dear to your heart)
and that due to lipid accumulation.
In these days of "fat is bad" mentality, which is based on the inability of
humans to regulate their fat intake, we have developed a false mantra that
diets should contain as little fat as possible. That is quite wrong. Good
diets contain something like 20% available energy as fat. Human diets often
contain over 30%. Many animals depend heavily on fats as their energy
source, including some that you might not suspect. Ruminants, such as
cattle, utilize free fatty acids produced in the rumen as a result of
bacterial metabolism as their major energy source - a true symbiotic
relationship. Horses and other hind gut fermenters similarly utilize FFAs
produced in the cecum. In fact, there is a muscle disease in horses called
Polysaccharide Storage Myopathy that seems to result from feeding horses
too much readily digestible carbohydrates as their energy source. Horses
are probably mal-adapted to such diets. The treatment is to remove grains
and add cooking oil (poly-unsaturated fats) to their diet, bringing fat to
about 20% of available energy. My wife, incidentally, is well known for her
work in this area (she is also a veterinary pathologist). Lastly, free
fatty acids are the preferred energy source for muscle cells.
The bottom line is that one should be careful in trying to extrapolate
between species. Unfortunately we don't know enough about natural diets and
dietary needs for many fish species, but as you all know, different groups
have evolved different dietary niches, and so have different needs. The
other bottom line is that, in general, and in appropriate amounts, FAT IS GOOD.
Barry
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