Journal Name:
Nutrition Reviews
Article Title:
Update on alpha-linolenic acid
Date Written:
2008
Volume:
66
Number:
6
Page:
326
Author(s):
Stark, A.H.; Crawford, M.A.; Reifen, R.
Article:
For many years there was little interest in alpha linolenic acid (ALA) and issues were raised concerning the danger of consuming highly unsaturated fatty acids that were susceptible to peroxidation. ALA is abundant in certain plant foods including walnuts, canola, several legumes, flaxseed, and green leafy vegetables. ALA is the precursor of three important longer-chain n-3 fatty acids, eicosapentaenoic acid (EPA 20:5w3), docosapentaenoic acid (DPAw3 22:5w3), and docosahexaenoic acid (DHA 22:6w3), which have vital roles in brain development and function, cardiovascular health, and inflammatory response. *Alpha-linolenic acid is partially converted to EPA in humans (8–20%), while conversion rates of ALA to DHA are estimated at 0.5–9%. Studies in women of reproductive age showed a substantially greater (2.5-fold) rate of conversion of ALA to EPA than that measured in healthy men. Women have a lower partitioning of ALA to beta-oxidation, leaving more of it available for conversion to EPA. Other possible explanations include a direct effect of estrogen on conversion rates. In males it is estimated that only 0.5–4% of ALA is converted to DHA while in females the rates are thought to be as high as 9%. It is hypothesized that demands for DHA by the fetus during pregnancy may stimulate female physiology to more readily synthesize this fatty acid.
It has been shown that when ALA intake in the diet is increased, an increased proportion of both ALA and EPA is found consistently in both plasma and cell lipids. Metabolites of n-3 origin are anti-inflammatory and anti-arrhythmic. Several studies are available that strongly indicate that ALA may indeed be important in maintaining heart health. Because ALA is a precursor of the longer chained n-3 fatty acids, it contributes to the body pools of EPA and DHA that have been associated with improved vascular tone, heart rate, serum lipid levels, platelet function, inflammatory responses, arrhythmia, growth rates of atherosclerotic plaques, and blood pressure. Most of these beneficial effects have also been associated specifically with ALA consumption.
In most studies, particularly those in the American population, an inverse association between ALA consumption and incidence of CHD has been found. Data strongly support the direct role of ALA consumption in decreasing CHD risk and indicate that intake of plant sources of n-3 fatty acids may be of particular importance in sectors of the populations that do not eat fatty fish.
It is difficult to determine the exact impact of ALA on heart health because very few studies have focused exclusively on ALA. However, available data consistently support a beneficial effect. Large-scale randomized trials with ALA have not been carried out and observational studies alone cannot decisively establish causal relationships. However, the continual appearance of new studies and professional opinions of scientists from around the world support the ever growing body of research that ALA has cardioprotective effects in its own right.
A number of studies have shown that consuming ALA, systemic inflammation was significantly reduced, as measured by C-reactive protein, IL-6, E-selectin, soluble intercellular cell adhesion molecule 1, and soluble vascular cell adhesion molecule 1. Additional evidence that ALA consumption affects biomarkers of inflammation and endothelial activation was observed. Decreased levels of C-reactive protein and IL-1ra levels were associated with higher plasma ALA levels.
The present dietary reference intakes state that to achieve nutritional adequacy, ALA should provide 0.6–1.2% of energy with up to 10% provided by longer chained fatty acids. In contrast to ALA, which is essential, EPA and DHA have no minimum requirements, and in large doses, longer chain n-3 fatty acids present a health risk. The fact that several major scientific and medical associations have published nutritional guidelines including recommendations specifically for ALA emphasizes its perceived importance in health promotion and disease prevention.
The authors conclude that based on studies and research to date, it can be concluded that increasing ALA in the daily diet is a safe, viable option for meeting dietary requirements and maintaining the suggested n-6 : n-3 ratio. Foods naturally rich in ALA should be included in the diet and manufacturers can help the public meet dietary recommendations by increasing ALA content in processed foods. Results of many of the studies indicate that ALA is not only essential, it also has therapeutic properties. Nevertheless, additional well-designed research is needed in order to establish unequivocal support for health claims. It is important to remember that of the omega-3 fatty acids, ALA is the parent molecule, and greater attention should be paid to its independent physiological function.
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