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THE IMPACT OF ESSENTIAL FATTY ACIDS
ON THE AGING PROCESS (2)
Rashid Buttar, D. O. and Andrew Halpner, Ph. D.
Interestingly, LA and ALA themselves carry out few of the functions of essential fatty acids. For example, LA helps maintain the water impermeability of the skin, but without further metabolism, it is unable to carryout other functions. LA and ALA must be metabolized to other fatty acids before potent biological functions become apparent. Figure 1 shows the pathways by which LA and ALA from the diet are metabolized.
The first step in the metabolism of both LA and ALA is a desaturation by the enzyme D-6-desaturase. This enzyme inserts a double bond and converts LA and ALA into gamma-linolenic acid (GLA, 18: 3n-6) and stearidonic acid (18: 4n-3), respectively.
After further desaturation and elongation, LA is ultimately converted to arachidonic acid (AA, 20: 4n-6) and docosapentaenoic acid (DPA, 22: 5n-6). ALA is converted to eicosapentaenoic acid (EPA, 20: 5n-3) and docosahexaenoic acid (DHA, 22: 6n-3).
Because of our knowledge of these metabolic pathways, EPA, DHA, and GLA are now commonly categorized as EFAs, as they depend on the presence of LA and ALA for their synthesis. However, unlike other animals, such as the rat, we have a limited ability to convert ALA to EPA and DHA. Consequently, we depend greatly on dietary sources of EPA and DHA.
Omega-3 fatty acids
EPA and DHA are commonly referred to as "omega-three EFAs" or "omega-3 fatty acids (FAs)," since the first double bond is 3 carbons from the methyl end, as previously mentioned. These omega-3 FAs comprise the smaller family of EFAs, and are typically found in higher concentrations in fish oils and linseed (flaxseed) oil. Omega-3s are also found in many of the green leafy vegetables, where they are associated with the chloroplasts, and in the meat of animals that feed on grass (herbivores). Interestingly, it is only within the chloroplasts of plants that enzymatic reactions can desaturate linoleic acid (n-6) to yield alpha-linolenic acid (n-3).
Our ancestors consumed high concentrations of omega-3 FAs, as their diets included only what they could hunt (meat) or gather (green leafy vegetables).
The human brain is high in omega-3 FAs. Scientists have attributed the neurological evolution and development of modern humans to the high omega-3 FA diets that our ancestors consumed. However, as humans have evolved, a systematic erosion of omega-3 FAs has occurred our diet. This is most evident in the last 100 years, and especially in Western society. Some medical authors claim that the largest known nutritional deficiency in modern-day society is that of omega-3 FAs. Some scientists and clinicians have postulated and proven that many of the chronic, insidious disease processes that are typically attributed to aging, actually reflect a chronic state of omega-3 deficiencies. These deficiencies sometimes occur throughout most of the patients' life spans.
Omega-3 (as well as omega-6) FAs serve as precursors for a vast number of signal molecules (hormone-like substances that act as messenger molecules). These signal molecules include prostaglandins, leukotrienes, thromboxanes, and other eicosanoids that are involved in numerous biological functions. The omega-3 FAs are incorporated within all the phospholipid bilayers of cell membranes, and interact with nuclear receptor proteins. However, they do not have the same susceptibilities as other acid substrates.
Omega-6 fatty acids
The omega-6 FAs comprise the larger of the two EFA families. Omega-6 FAs are predominantly found in most seed and vegetable oils, including primrose oil, borage seed oil, corn oil, safflower oil, and sunflower oils. Like omega-3 FA deficiencies, chronic omega-6 FA deficiencies have also been proven to impair the human system. (This will be discussed later in this chapter.) Some studies indicate a correlation between excess consumption of omega-6 FAs and the risk for developing certain diseases. This is very plausible, considering our modern society's increased dependency on vegetable oils, especially over the last 100 years.
Benefits of EFAs
Classic signs of EFA deficiency are dermatitis, growth retardation, and reproductive failure. However, EFAs are now known to exert many other wide-ranging, health-promoting effects. This chapter will focus on the importance of EFAs in various physiologic functions.
How EFAs work
Science is learning more about the biochemistry and physiology of EFAs. We are beginning to understand how they function beyond their ability to prevent classic signs of deficiency.
A number of mechanisms have been explored in an attempt to explain the essentiality of these compounds, including their effect on membrane structure. This is a seemingly simple role, but it should not be overlooked. A change in the fatty acid composition of the diet can easily modify membrane fluidity and structure. Even the insertion of one additional double bond into a membrane can significantly change the properties and physiologic functions of a membrane. These shifts can take the form of altered protein-protein interactions, altered protein-lipid interactions, changes in cellular receptors and their substrate-binding abilities, loss or gain in the ability to transport certain molecules across the membrane, and other functions that can have a profound impact of how a cell or tissue functions.
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