Lipids are any one of a group of fats or fatlike substances, characterized by their insolubility in water and solubility in fat solvents such as alcohol, ether, and chloroform. . The term is descriptive rather than a chemical name such as a protein or carbohydrate. It includes true fats (esters of fatty acids and glycerol); lipoids (phospholipids,, cerebrosides, waxes); and sterols (cholesterol, ergosterol). All lipids like carbohydrates contain carbon, hydrogen and oxygen but the proportion of oxygen in lipids is much lower. In addition, phosphorus is found in some of the more complex lipids.
Triglycerides which are also called neutral fats are commonly known as fats when solid or oils when liquid. Triglycerides are made up of two types of building blocks, fatty acids and glycerol, in a ratio of 3:1 of fatty acids to glycerol. Triglycerides are large molecules and ingested fats and oils must be broken down into their building blocks before they can be absorbed. Oils or fats do not mix with water because polar and non polar molecules do not interact. Triglycerides provide the body’s most efficient and compact form of stored energy, and when they are oxidized, they yield large amounts of energy. Triglycerides are found mainly beneath the skin, where they insulate the deeper body tissues from heat loss and protect them from trauma.
Fatty acid chains with only a single covalent bond between carbon atoms are referred to as saturated. Their fatty acid chains are straight and, at room temperature, the molecules of a saturated fat are packed closely together, forming a solid. Fatty acids that contain one or more double bonds between carbon atoms are said to be unsaturated (monounsaturated and polyunsaturated). The double bonds cause the fatty acid chains to kink so that they cannot be packed closely enough to solidify. Hence, triglycerides with short fatty acid chains or unsaturated fatty acids are oils (liquid at room temperature) and are typical of plant lipids. Examples include olive and peanut oils (rich in monounsaturated fats) and corn, soybean, and safflower oils, which contain a high percentage of polyunsaturated fatty acids. Longer fatty acid chains and more saturated fatty acids are common in animal fats such as butter fat and the fat of meats, which are solid at room temperature.
Trans fats which are common in many margarines and baked products, are oils that have been solidified by the addition of hydrogen atoms at the sites of double carbon bonds. Trans fats are very harmful to body cells and should be avoided altogether in the animals diet. Trans fats have been linked to atherosclerosis and are considered by many scientists to be carcinogenic.
Phospholipids are modified triglycerides with the addition of a phosphorous-containing group. Phospholipids are the chief material for building cellular membranes.
Steroids are basically flat molecules made of four interlocking hydrocarbon rings. Steroids are very important molecules which the animal gets from ingestion of animal products such as eggs, meat and cheese. The steroids cholesterol, bile salts, Vitamin D, sex hormones, and adrenocortical hormones are extremely important to the animals health. Cholesterol is the structural basis for manufacture of all body steroids and is a component of the cells membrane. Bile salts are manufactured by the liver from the breakdown of cholesterol and are stored in the gall bladder for release into the small intestine where they aid fat digestion and absorption by helping to emulsify fats into smaller absorbable molecules.Vitamin D is a fat-soluble vitamin produced in the skin on exposure to UV radiation and is necessary for normal bone growth and function. The steroid sex hormones estrogen, progesterone and testosterone are necessary for reproductive function. Adrenocortical hormones like cortisol is a metabolic hormone necessary for maintaining normal blood glucose levels. Aldosterone another adrenocortical hormone helps to regulate salt and water balance in the body by targeting the kidneys.
Other lipid substances include the fat soluble Vitamins A, D, E, and K. Vitamin A is ingested in orange-pigmented vegetables and fruits and are converted in the retina to retinol and are a part of the photoreceptor pigments involved in vision. Vitamin E is ingested in plant products such as wheat germ and green leafy vegetables and protects the cell membrane against oxidative damage. Vitamin K is made by bacterial synthesis in the intestine and is also found in green leafy vegetables and it regulates the formation of clotting factors in the blood.
Eicosanoids (prostaglandins, leukotrienes and thromboxanes) are a group of molecules that are derived from fatty acids found in all cell membranes. Prostaglandins stimulate uterine contractions, regulate blood pressure, control gastrointestinal tract motility and secretory activity. Both prostaglandins and leukotrienes are involved in inflammation. Thromboxanes are powerful vasoconstrictors.
Lipoproteins are lipoid and protein based substances that transport fatty acids and cholesterol in the blood stream. The major lipoproteins are high-density lipoproteins (HDL) and low-density lipoproteins (LDL). HDL is healthy for the body and high levels of LDL is lousy for the body.
Essential Fatty Acids (EFA) are fatty acids that the animal can not synthesize themselves and so it is essential for the animal to get these in its diet. Essential fatty acids in the dog and cat include linolenic acid, linoleic acid and arachidonic acid. Adult dogs are able to synthesize linolenic and arachidonic acid from linoleic acid. Adult cats, however, can synthesize linolenic acid but not arachidonic acid. Arachidonic acid is only found in fat sources from animals and for this reason cats are obligate carnivores.
Deficiencies in lipids can cause a dry coat and scaly skin. A reduction of lipids on the skin can cause pyoderma. Wound healing can also be impaired due to the requirement of phosopholipids within the cells membranes. If there is a chronic deficiency of lipids, especially of the EFA, symptoms of alopecia, edema and moist dermatitis can occur. Severe deficiencies can result in emaciation of the animal. Deficiencies of Vitamin A can cause night blindness, infertility, crusting lesions of the nares, seborrhoeic coat conditions, and increased susceptibility to microbial infections. Deficiencies of Vitamin D can cause rickets in the young and osteomalacia in adults. Deficiencies of Vitamin E can cause pancreatitis, skeletal muscle dystrophy, reproductive failure and impairment of the immune response in dogs. Deficiencies of Vitamin K is rare due to its synthesis in the intestine.
In the next post on animal nutrition we will wrap up the subject of nutrients by covering water soluble vitamins and minerals.