HUFA stands for "Highly Unsaturated Fatty Acids" and is a term which is very important as it pertains to aquaculture and its role in freshwater and marine diets. This article will go into some detail to explain the following:
What are Fatty Acids? Fatty acids (FA) are building blocks of fats. They consist of a hydrocarbon chain with a polar, carboxyl functional group (COOH) on one end. The carboxyl group is what makes the molecule an acid and at biological pH values is found in an ionized form where the hydrogen of the hydroxyl (OH) group is missing leaving a negatively charged oxygen atom. This polar end of the fatty acid is hydrophilic, or attracted to water. The carbon-hydrogen bonds of the hydrocarbon chain are nonpolar and so are hydrophobic and thus, excluded by water. The hydrophobic interactions of the hydrocarbon chains of fats explain the separation that can be seen in such everyday items as salad dressing where the oil separates from the water. The hydrocarbon tails pack together on the inside of the oil droplets while the polar carboxyl groups associate with the water.
The character of the hydrocarbon chain will determine whether a fatty acid is considered "saturated" or "unsaturated". Saturated FA have the maximal number of hydrogen atoms bonded to each carbon in the chain. Such chains contain carbon to carbon single bonds.
A class of enzymes called desaturases can convert saturated FA to unsaturated FA, which have one or more carbon to carbon double bonds. Carbon atoms participating in a double bond have fewer than the maximal number of hydrogen atoms bonded to them. The double bonds are formed as the desaturase enzyme removes one hydrogen atom from each carbon. The positions of the double bonds are designated by their location relative to the last carbon in the chain using the symbol ω (the last letter in the Greek alphabet, omega). Of particular importance to nutrition are the ω3 FA, where the double bond begins 3 carbon atoms from the end of the chain. The introduction of a double bond to a hydrocarbon chain puts a "kink" in its structure. The kinks are commonly shaped in what is known as the "cis" configuration. When the hydrocarbon tails of FA associate together in avoidance of water, kinks will prevent them from packing too closely and help to lower the freezing point (keeping the FA fluid at lower temperatures).
Fats, also known as lipids, play a very important role in animal physiology. They are the major component of biological membranes which envelop every cell of an animal's body. The presence of certain types of lipids in specific proportions is essential to life. While this article will not discuss the importance and functions of biological membranes, it is important to point out that membranes have very complex functions, specific to each cell type. What is most important to understand is that membranes are NOT solid, static structures. They are fluid and formed from the hydrophobic interactions of the lipids with each other rather than with water (as discussed earlier-think of the droplets of oil formed in salad dressing). Cell membranes are actually lipid bi-layers. One can picture this as two concentric circles of lipid molecules where the polar portions associate with the water on the inside and outside of the cell while the hydrophobic tails of each layer point towards each other away from the water on either side of the membrane. The lipids, along with proteins and other components of the membrane, are all considered a part of a "Fluid Mosaic". The "fluid" nature allows movement of the membrane components within the plane of the membrane. The extent of a specific membrane's fluidity (and therefore its ability to perform specific functions) is determined in part by its lipid composition-the ratio of saturated to unsaturated lipids, concentrations of specific FA, etc. The FA of a membrane will have marked effects on the temperature at which the membrane freezes (becomes solid) and no longer functions. The kinks of unsaturated fatty acids prevent freezing and lower the freezing/melting point of the membrane-keeping it fluid at lower temperatures. Pond goldfish have the ability to change the composition of their membranes to adapt to the winter season each year. As the temperature drops, they replace some of the saturated FA with unsaturated FA in the cell membranes of certain organs such as the brain-allowing the processes of the organs to continue despite the drop in temperature. HUFA act as anti-freeze for cold-blooded animals. Keep in mind that these FA also serve as precursors to many biologically important molecules which are involved in a variety of other important processes in the animal body.
It is now clear that unsaturated FA play an important role in fish health. HUFA are a subset of unsaturated FA which contain 20 or more carbon atoms and multiple double bonds. Common HUFA encountered in fish nutrition are EPA and DHA-both ω3 FA. EPA and DHA are essential for fish health. These HUFA must either be obtained in the fish's diet, or made through the conversion of other FA such as Linolenic Acid-although not all fish are capable of this or efficient at the process, making incorporation in the diet important. Studies suggest that fish benefit most from directly consuming the EPA and DHA as evidenced by increased rates of growth and survival.
The increased adaptability of a fish due to the anti-freeze qualities of HUFA helps to explain the observed increase in survival rates of fish. However, this doesn't explain the increase in growth rates which have been noted. Here, it helps to understand that fats are very efficient means of energy storage-thanks to their hydrocarbon chains. The gasoline that runs our cars contains hydrocarbons which, through combustion reactions, release explosive energy. The cells of an animal body do the very same thing with fats. One gram of fat contains two times more energy than a gram of carbohydrate-meaning fish can be fed a smaller amount of a higher-fat content food and still consume as much energy as if they were fed a larger portion of a lower-fat food. Aside from the benefit of stretching your dollar, feeding less also reduces pollution of tank water which is a valuable benefit all its own.
Highest Quality Foods Contain HUFA Clearly, the best foods to feed to your fish will be those which are rich in HUFA-due either to their ability to produce it themselves or through the consumption of other HUFA-rich organisms. EPA and DHA are all derived from plants either directly or through the indirect conversion of linolenic acid from plants to EPA and DHA. Phytoplankton are similar to plants. They are autotrophic and many are able to make HUFA. The zooplankton which consume them benefit the most by consuming those which are rich in HUFA. This transfer of HUFA up the food chain to organisms such as mysis and brine shrimp leads to the incorporation of the HUFA in fish's cells. The greater the proportion of HUFA in the food fed to fish, the greater amount of energy for growth and survival is obtained from it. Fats are very high in energy and there is evidence to suggest that foods with higher levels of HUFA are more nutritious as they are a more efficient means of transferring energy up the food chain. Foods which are more efficient at providing energy are higher-quality foods and a better value for your money. This is why dried foods such as mysis, plankton, cyclops, and others are an important staple of a well-rounded aquatic diet. Evidence suggests that frequent feedings of at least two or more different dried foods each week will greatly enhance the growth and reproduction of most fish.
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