Dossier: Polyunsaturated fatty acids in biology and diseasesThe importance of the ratio of omega-6/omega-3 essential fatty acids
Introduction
The interaction of genetics and environment, nature, and nurture is the foundation for all health and disease. In the last two decades, using the techniques of molecular biology, it has been shown that genetic factors determine susceptibility to disease and environmental factors determine which genetically susceptible individuals will be affected 〚81〛, 〚82〛, 〚84〛, 〚87〛. Nutrition is an environmental factor of major importance. Whereas major changes have taken place in our diet over the past 10,000 years since the beginning of the Agricultural Revolution, our genes have not changed. The spontaneous mutation rate for nuclear DNA is estimated at 0.5% per million years. Therefore, over the past 10,000 years, there has been time for very little change in our genes, perhaps 0.005%. In fact, our genes today are very similar to the genes of our ancestors during the Paleolithic period 40,000 years ago, at which time our genetic profile was established 〚21〛. Genetically speaking, humans today live in a nutritional environment that differs from that for which our genetic constitution was selected. Studies on the evolutionary aspects of diet indicate that major changes have taken place in our diet, particularly in the type and amount of essential fatty acids (EFA) and in the antioxidant content of foods 〚21〛, 〚64〛, 〚75〛, 〚76〛, 〚77〛 (Table 1, Fig. 1). Using the tools of molecular biology and genetics, research is defining the mechanisms by which genes influence nutrient absorption, metabolism and excretion, taste perception, and degree of satiation, and the mechanisms by which nutrients influence gene expression.
Whereas evolutionary maladaptation leads to reproductive restriction (or differential fertility), the rapid changes in our diet, particularly the last 100 years, are potent promoters of chronic diseases such as atherosclerosis, essential hypertension, obesity, diabetes, and many cancers. In addition to diet, sedentary lifestyles and exposure to noxious substances interact with genetically controlled biochemical processes leading to chronic disease. This paper reviews the scientific evidence for a balanced intake of omega-6 and omega-3 EFA, focusing on the evolutionary aspects of diet, the biological and metabolic functions, and the health implications. Appendix A is a portion of the summary of the workshop on the essentiality of and recommended dietary intakes (RDIs) for Omega-6 and Omega-3 Fatty Acids, held at the National Institutes of Health (NIH) in Bethesda, Maryland, USA, April 7–9, 1999, which provides recommendations for adequate intakes (AI) of EFA for adults and infants 〚83〛.
Section snippets
Evolutionary aspects of diet with emphasis on omega-6 and omega-3 EFA
The foods that were commonly available to pre-agricultural humans (lean meat, fish, green leafy vegetables, fruits, nuts, berries and honey) were the foods that shaped modern humans’ genetic nutritional requirements. Cereal grains as a staple food are a relatively recent addition to the human diet and represent a dramatic departure from those foods to which we are genetically programmed and adapted 〚13〛, 〚67〛, 〚78〛. Cereals did not become a part of our food supply but only very recently,
Large-scale production of vegetable oils
The increased consumption of omega-6 fatty acids in the last 100 years is due to the development of technology at the turn of the century that marked the beginning of the modern vegetable oil industry, and to modern agriculture with the emphasis on grain feeds for domestic livestock (grains are rich in omega-6 fatty acids) 〚36〛. The invention of the continuous screw press, named Expeller® by V.D. Anderson, and the steam-vacuum deodorization process by D. Wesson made possible the industrial
Biological effects and metabolic functions of omega-6 and omega-3 fatty acids
Mammalian cells cannot convert omega-6 to omega-3 fatty acids because they lack the converting enzyme, omega-3 desaturase. LA and ALA and their long-chain derivatives are important components of animal and plant cell membranes. These two classes of EFA are not interconvertible, are metabolically and functionally distinct, and often have important opposing physiological functions. The balance of EFA is important for good health and normal development. When humans ingest fish or fish oil, the EPA
Clinical intervention studies and the omega-6/omega-3 EFA balance
The Lyon Heart Study was a dietary intervention study in which a modified diet of Crete (the experimental diet) was compared with the prudent diet or Step I American Heart Association Diet (the control diet) 〚19〛, 〚20〛, 〚56〛. The experimental diet provided a ratio of LA to ALA of 4/1. This ratio was achieved by substituting olive oil and canola (oil) margarine for corn oil. Since olive oil is low in LA whereas corn oil is high, 8% and 61%, respectively, the ALA incorporation into cell membranes
Conclusions
In conclusion:
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Human beings evolved on a diet in which the ratio of omega-6/omega-3 EFA was about 1, whereas in the Western diets, the ratio is 15/1 to 16.7/1. Such evidence comes from studies on the evolutionary aspects of diet, modern day hunter-gatherers, and traditional diets. Agribusiness and modern agriculture have led to decreases in omega-3 fatty acids and increases in omega-6 fatty acids. Such practices have led to excessive amounts of omega-6 fatty acids, upsetting the balance that was
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