Because fat is energy rich and provides 9kca/g of energy, humans are able to obtain adequate energy with a reasonable daily consumption of fat- containing foods. Dietary fat is stored in adipose (fat) cells located in depots on the human frame. The ability to store and use large amounts of fat enables humans or survive without food for weeks and sometimes months.
Some fat deposits are not used effectively during a fast and are classified as structural fat. Structural fat pads hold the body organs and nerves in position and protect them against traumatic injury and shock. Humans also have a subcutaneous layer of fat that insulates the body, preserving body heat and maintaining body temperature.
Dietary fat is also essential for the digestion, absorption, and transport of the fat-soluble vitamins and phytochemicals such as carotenoids and lycopenes. Fat also conveys important textural properties to foods such as ice creams (smoothness) and baked goods (tenderness-due to"shortening" of strands of gluten).
Fatty Acids
In general, fats with shorter fatty acid chains or more double bonds are liquid at room temperature. Saturated fats, especially those with long chains (e.g., beef tallow), are solid at room temperature; but a fat such as coconut oil, which is also highly saturated, is semiliquid at room temperature because the predominant fatty acids are short (8 to 14 carbons). Because fatty acids with double bonds are vulnerable to oxidative damage, humans and other warm-blooded organisms store fat predominantly as saturated palmitic fatty acid (Cl6:0) and stearic fatty acid (C18:0).
To achieve this requirement, membrane phospholipids contain one SFA and one highly PUFA, the most abundant of which is arachidonic acid (C20:4). Omega -3 fatty acids (n-3) are polyunsaturated fatty acids that have their first double bond three carbon atoms from the methyl carbon atoms.
Common Fatty Acids
In omega notation a lower case omega(ω) or n is used to refer to the placement of the first double bond counting from the methyl end (referred to as the fatty acid's omega number). Thus arachidonic acid (20:4 ω -6 or 20:4 n-6), the major highly polyunsarurated fat in membranes of land animals, is an omega-6 fatty acid. It has twenty carbons and four double bonds, the first of which is six carbons from the terminal methyl group.
Animals including humans can only place double bonds as low as the omega-9 carbon and therefore cannot synthesize omega-6 and omega-3. Because of this ability, the term essential fatty acid, especially as it refers to linoleic and linolenic acids, be replaced by referring to omega-3 and omega-6 fatty acids more generally. It is the longer-chain fatty acids that are required (essential), but if sufficient amounts of shorter- chain precursors occur in the diet they need not be supplied directly.
The longer chain fatty acids are important components of the cell membranes and as precursors of eicosanoids such as prostaglandins, thromboxanes, and leukotrienes. Eicosanoids can alter the size and permeability of the blood vessels, alter the activity of platelets and contribute to blood clotting, and modify the processes of inflammation. Omega -3 fatty acids (n-3) have been shown to lower serum triglyceride levels, reduce blood pressure , and decrease factors involved in blood clotting and stroke.
It is not known whether the reduced risk of sudden cardiac death among people who eat one fish meal per week results from consumption of n- 3 fatty acids, or of some other component in fish , or both. Although both omega-6 and omega-3 fatty, acids are essential in the diet, excess omega-6 fatty acids in the diet prevent conversion of ALA into longer EPA and DHA forms. Haag(2003) concluded that the omega- 6/omega-3 ratio in the diet influences neurotransmission and thus brain function.
The optimal omega-6/omega-3 ratio has been estimated to be 2:l to 3:1; therefore it is recommended that humans consume more omega- 3 fatty acids from vegetable and marine sources. Alph-linolenic acid , which is found in green leafy vegetables , flax oil, canola oil, soybean products, and hazelnuts, is the n-3 fatty acid in plants. To a limited extent humans can convert alpha-linolenic acid to EPA and DHA in the body.
Sources of the longer EPA and DHA omega-3 fatty acids are primarily marine: cod liver oil, mackerel, salmon, and sardines, as well as crab, and shrimp,.
Trans-Fatty Acids
Membrane function depends on the three- dimensional configuration of membrane fatty acids found in phospholipids. The cis double bonds in the membrane bend, allowing the fatty acids to pack loosely, thus making the membrane fluid. Because proteins embedded in a membrane float or sink, depending on the membrane's fluidity, membrane viscosity is important for membrane protein function.
Trans fatty acids do not bend; they pack into the membrane as tightly as if they were fully saturated. Clinical and epidemiologic studies suggest that higher intakes of trans-fatty acids are associated with increased risk for coronary heart disease, cancer, and other chronic diseases (including type 2 diabetes and allergies, possibly because of their potential to influence membrane fluidity. Until more is known about the extent of their risk, it is recommended that dietary consumption of hydrogenated and saturated fatty acids be reduced.
Department of Agriculture Dietary Guidelines for Americans recommends limiting intake of trans-fatty acids and saturated fatty acids to as little as possible.
Essential Fatty Acid Deficiency
Deficiencies of omega-6 essential fatty acids also have clinical implications, including growth retardation, skin lesions, reproductive failure, and fatty liver,. Fat-free diets may lead to essential fatty acid deficiencies and eventually death if the missing nutrient is not provided. Linolec acid (n-6) and alpha- linolenic acid (n-3) are termed essential fatty acids because they cannot be synthesized in the body.
Essential fatty acids are important for maintaining healthy skin and promoting normal growth in children.
Phospholipids
