7.3 Fats & Lipids

Considering the role of fats & lipids in the diets of teleosts, the following are very general. Fats are the chief form of stored energy. This energy retention allows fish to exist in “feast or famine environments.” Natural foods are not guaranteed in most cases. Environmental conditions often alter the “normal” cycle of multiple feed sources. Whether the species is herbivore, omnivore, or carnivore, they all are obligate eaters. Studying the feed stream in feral animals is difficult but rewarding to see how nature is interrelated.

Fats provide a stable energy source for work and heat. Fats insulate the body from temperature variations. The fat forms “pads’ that protect organs from physical trauma. Phospholipids are a major component of cell membranes. This category of nutrients is the precursor to bioactive compounds such as eicosanoids.

Some fats are essential to the diet for the energy source emphasized and are integral to body tissues. Cholesterol is an extremely important component of the plasma membrane around the cells. It is in the fats/lipids part of this discussion because of its lipid component. A very brief review of cholesterol biochemistry finds it is a major cell membrane component, a precursor for bile acid generation, and a precursor of all the steroid hormones such as estrogens, androgens, glucocorticoids, and mineralocorticoids.^[1] Some cholesterol will be absorbed from the diet, and the liver creates the remainder.

Fatty acids

Fatty acids (FA) are the building blocks of fats. FA’s are described in several ways. FA’s can be saturated or unsaturated. The saturated FA’s contain no double bonds and can be found in animal fats and plant oils. The saturated FA’s are organic acids with straight chains and even numbers of carbon atoms (4-16).^[2], ^[3]

Unsaturated fatty acids contrast with saturated fatty acids as carbon chains with double bonds between carbon atoms. When there is one double carbon-carbon bond, it is called monounsaturated fatty acid (MUFA). When more double bonds occur, the fatty acid is termed polyunsaturated fatty acid (PUFA)

FA chemistry permits almost innumerable creations. Animals have many uses but can also be held back when some essential ones are missing. The names we put on them are simple and follow one of three nomenclature systems. The Delta system, common names, and the Omega system are named here for future reference. In this part of our discussion, we will use the Omega system. This system creates identifying numbers by noting the position of the first double bond from the terminal methyl carbon. (File that definition for trivial but beneficial if you enter the world of diet formulation and try to understand fatty acid construction.) For introductory purposes, most readers have come across the important Omega-3 and Omega-6 fatty acids (denoted as Ω-3 and -6 or n-3 and-6). Fish need both of the groups in the diet.

Omega-3 is an FA-like linolenic acid (18:3 n-3) commonly found in fish oil, flax seed, primrose, and algae. These may give rise to longer Omega-3’s such as eicosapentaenoic acid (EPA) (20:5 n-3) and docosahexaenoic acid (DHA) (22:6 n-3). Omega-6 fatty acids like linoleic acid (18:2 n-6), dihomo-gamma-linolenic acid (DGLA)(20:3 n-6), and arachidonic acid (20:4 n-6), which may be gotten from soy oil and canola oil. Some fish can produce other Omega 3 & 6 fatty acids as long as some are in their diet, i.e., supplement linolenic acid and EPA can be generated.

Fish need to ingest highly unsaturated fatty acids to allow cold water tolerance. The higher amount of unsaturated fats in cell membranes permits a lower temperature before freezing overtakes the cell membranes and kills the fish.

We know fish to be a valuable source of Omega 3 fatty acids for the human diet. However, fish do not synthesize but need a source in their diet. Depending on the species, the source could be algae, plankton, or fish prey. The challenge in aquatic livestock production is to get these fatty acids into farm-raised fish. Livestock need at least 0.5-2% of the diet as unsaturated fatty acids to contribute much to the human diet. Wild fish are likely to provide higher levels of Omega-3 to the consumer.

Fish are usually fed diets with higher fat diets than most terrestrial animals. As mentioned before, fat is an excellent energy source. Typically diets are below 20% to avoid a condition called fatty liver in the harvested fish. Trouble comes with high-fat diets and especially unsaturated fatty acids because of rancidity. It is common to find hatcheries holding fish feed in coolers or even freezers as an assurance to preserve the fats. Fish are particularly sensitive to rancid fats and turn away from diets not properly stored.