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Fatty acids represent a substantial part of lipids in human body and are important sources of energy. They are either saturated or unsaturated carbosylic acids containing carbon chains between 2 and 36 carbon atoms in length. Although more than 60 fatty acids have been found in blood plasma and tissues, only a fraction of them is relevant from the biological perspective.
The large body of epidemiological evidence about total fat content, fatty acids and human health show that major groups of fatty acids are associated with diverse health effects. When the diet lacks adequate amounts of specific fatty acids, deficiency symptoms will arise as specific clinical entities. Conversely, increased content of saturated fatty acids may result in dyslipidemia.
Two main fatty acids essential in the diet are linoleic (or omega-6) fatty acid and alpha-linolenic (or omega-3) acid. Both of them are polyunsaturated fatty acid, which means that they possess two or more double bonds and lack several hydrogen atoms that are found in saturated fatty acids.
Linoleic acid keeps the skin impermeable to water, but to exert other effects the compound must undergo specific metabolism. First step is conversion to gama-linolenic acid by delta-6-desaturation. Gama-linolenic acid is subsequently converted to dihomo-gamma-linolenic acid, which is in turn converted to arachidonic acid.
Arachidonic acid can form prostaglandins and thromboxanes – hormone-like lipids that promote blood clotting, induce inflammation and cause smooth muscle contraction. In alternative pathway it can also form leukotrienes, which are one of the most potent inflammatory agents in the human organism.
The necessity of metabolism is reflected by the increasing potency of each substance in the form of an essential fatty acid, as it moves down the chain from linoleic to arachidonic acid; hence to achieve full range of activities, linoleic acid must be metabolized to other substances. Therefore it can be considered as analogous to provitamines.
In infants, delta-6-desaturase is too immature to provide the desired metabolism of linoleic acid, which is a reason why human milk contains gamma-linoleic acid, dihomo-gamma-linoleic acid and arachidonic acid. In contrast, conventional infant formula milks have only linoleic and alpha-linolenic acid, which can lead to a deficiency state in formula-fed infants.
Conjugated linoleic acids (CLA) refers to a heterogeneous group of constitutional and geometric isomers of linoleic acid, which are predominantly found in milk, milk products, meat and meat products of ruminants. The effect of CLA on human health became the subject of interest after one study demonstrated inhibitory effects on mouse epidermal neoplasia.
CLA can provoke a wide spectrum of beneficial effects in various cell culture and animal models of disease. However, some studies have reported ambiguous or harmful effects of CLA supplementation and there is less evidence from direct studies on humans, thus careful consideration and further investigation is warranted.
Research has shown that CLA has a significant inhibitory effect on the establishment and progression of atherosclerosis in animal models. Both LDL cholesterol to HDL cholesterol and total cholesterol to HDL cholesterol ratios are reduced when CLA is fed to the test animals.
There is also proof of CLA blocking the growth and spread of malignant tumors, primarily by influencing cell replication and mechanisms of carcinogenesis. The increase in mineralized bone formation was demonstrated in several experiments as well.
On the other hand, it has been shown that CLA can induce insulin resistance and fatty liver. Also, a deeper and more detailed investigation into the complex network of anti-obesity regulatory pathways is necessary to improve our understanding how exactly this compound can affect body weight control.
Recommended CLA daily intake is currently between 0.35 and 1 g per day. More controlled studies of CLA usage in defined populations are needed, as possible health consequences of prolonged treatment periods are unknown at the moment. Mutual comparisons of different and well-defined mixtures of isomers are also essential to secure long-term effects and safety.