It is noted in the Figure 2 that elevations of DHA plus EPA in the human body (tissues and cells) can be readily provided for by the direct dietary consumption of DHA and EPA. Unlike LA which is present at considerable levels in most cellular lipids (particularly membrane phospholipids) throughout various tissues and cells, ALA does not usually accumulate to particularly high concentrations in cellular/tissue lipids/phospholipids even when ingested at relatively high dietary levels. This is partly due to the fact that much of the ALA which is consumed in the diet undergoes beta oxidation in the mitochondria and only a limited amount is available for the very limited conversion of ALA to EPA plus DHA. Figure 2 also indicates that dietary DHA has the potential to undergo some reverse metabolism (retroconversion) back to EPA as indicated by the dotted line in the figure.
Some commentary on the so-called omega-6:omega-3 ratio which is readily referred to in the popular press and in the marketing of various nutritional supplements needs to be briefly addressed herein. The omega-6:omega-3 concept originates primarily in the early rodent experiments where high levels of LA (omega-6) in the diet were found to partially suppress the conversion efficiency of dietary ALA to EPA plus DHA in the body. As indicated in the previous figure, LA and ALA are metabolized to their corresponding products via common enzyme systems (including initial delta-6 desaturation). Early animal studies that used excessively high levels of dietary LA (n-6) relative to ALA (n-3), giving rise to very high omega-6:omega-3 ratios, resulted in a smaller rise in DHA/EPA levels in tissues due to the competitive inhibitory effect of LA and ALA at the level of the initial desaturation reaction. Thus, lower ratios of omega-6:omega-3 were found to provide for a somewhat better conversion efficiency of ALA to DHA/EPA as compared to higher omega-6:omega-3 ratios even when the amount of ALA was fixed at the same amount. These animal experiments have influenced subsequent dietary recommendations, such as those from Health and Welfare Canada in 1990 where they recommended that attempts should be made to reduce the omega-6:omega-3 ratio in the Canadian diet to approximately 10:1 down to 4:1. Subsequent human studies have indicated that lowering the LA(n-6):ALA(n-3) from higher levels (e.g., 27:1 down to 3:1) does allow for a somewhat moderately enhanced conversion of dietary ALA to EPA as revealed by moderately higher levels of EPA in blood samples taken from subjects given varying n-6:n-3 ratios and amounts of ALA. Thus, higher intakes of ALA and much lower ratios of LA: ALA is one strategy for moderately enhancing the conversion of ALA to EPA via the desaturation/elongation reactions presented in Figure 2. However, it is most interesting to note that numerous human studies which have lowered the n-6:n-3 ratio (as LA: ALA ) have not shown a significant rise in DHA with the lower ratios or even with higher intakes of ALA despite the moderate rise in EPA as mentioned previously. Furthermore, it is becoming apparent that the direct consumption of pre-formed DHA and EPA provides for a highly efficient elevation of these important long-chain omega-3 fatty acids in cells and tissues such that dependency on lower LA:ALA ratios becomes questionable since enrichment of the body in DHA plus EPA is then not dependent on the desaturation/elongation reactions for such enrichments. In conclusion, the omega-6:omega-3 concept, based on animal studies focusing upon LA: ALA ratios alone, needs to be reconsidered in the context of dietary/health situations where DHA plus EPA are consumed directly in their preformed state.
The omega-6 product formed from the desaturation plus elongation of LA is arachidonic acid (AA, 20:4n-6) which accumulates to very high concentrations in a wide variety of human tissues and cells. While small levels of AA in the body do have some important functions, such as in reproduction and other processes, excessively high levels of AA are considered to be potentially problematic in the development and/or progression of some chronic health conditions.