Docosahexaenoic Acid is an Essential Nutrient
Very Little DHA is Produced from ALA in Humans
Aug 25, 2009 Stephen Allen Christensen
Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have garnered a good deal of attention in recent scientific literature and in the popular press. These omega-3 fatty acids are touted as anti-inflammatories, anti-carcinogens, preventives for Alzheimer’s disease, and lipid-lowering agents.
Since EPA and DHA affect the production of prostaglandins and other prostanoid molecules, their metabolic influences are far-reaching.
Rodent-based studies performed in the mid-1970s—and subsequently reproduced by various researchers—showed that rats could produce sufficient levels of EPA and DHA from dietary alpha-linolenic acid, an organic molecule found in many vegetable oils. (Lamptey M, Walker B. A possible essential role for dietary linolenic acid in the development of the young rat. J. Nutr. 1976;106[1]:86-93)
Can Humans Make Their Own Docosahexaenoic Acid (DHA) and Eicosapentaenoic Acid (EPA)?
The assumption that humans efficiently utilized the same metabolic pathway as rats to manufacture EPA and DHA led both Health and Welfare Canada and the United States Food and Nutrition Board to recognize alpha-linolenic acid (ALA) as an essential fatty acid. Age-related recommendations for minimal dietary intakes of ALA were developed to prevent DHA deficiencies in the general population.
Unfortunately, numerous controlled trials have now confirmed that there is limited conversion of dietary ALA to both EPA and DHA in human subjects, and vanishingly little production of DHA occurs via this metabolic pathway in some individuals.
In one revealing study conducted among lactating women, large doses of ALA from flaxseed oil (seven times the recommended daily dosage for pregnant women) failed to produce any net increase in the already low levels of DHA in the test subjects’ breast milk. (Francois C, et al. Supplementing lactating women with flaxseed oil does not increase docosahexaenoic acid in their milk. AJCN. 2003;77: 226-233)
Lest anyone point out that breast milk is not an ideal medium for evaluating the efficiency of ALA-to-DHA conversion, there are far more compelling data showing that, for most people, this conversion is inefficient at best.
Sophisticated techniques that measure metabolism of tagged forms of ALA in both men and women demonstrate that the average efficiency of conversion of ALA to total EPA and DHA is less than 0.5%, and that of ALA to DHA alone is less than 0.1%. (Hussein N, et al. Long-chain conversion of [13C]-linoleic acid and a-linolenic acid in response to marked changes in their dietary intake in men. J Lipid Res. 2005;46: 269-280, and Pawlosky R, et al . Physiological compartmental analysis of alpha-linolenic acid metabolism in adult humans. J Lipid Res. 2001;42[8]:1257-1265)
Although the ability to convert ALA to EPA and DHA varies widely among individuals—some fortunate people may convert up to 4% of an ingested dose of ALA to DHA—even the most metabolically efficient individual cannot produce enough DHA from ALA obtained in the typical American diet. Furthermore, it is difficult to distinguish efficient converters from inefficient ones.
Therefore, the assertion that plant-based oils alone (e.g., flax, borage, evening primrose, etc.) can supply sufficient levels of EPA or DHA should be reexamined. Marketing claims that plant oils are adequate sources of these nutrients should be questioned.
What Are Good Sources of Eicosapentaenoic Acid (EPA) and Docosahexaenoic Acid (DHA)?
The oil found in the flesh of cold-water fish (salmon, tuna, herring, mackerel, sardines, etc.) has long been recognized as a good source of omega-3 fatty acids. Sadly, continued pollution of our planet’s waters has necessitated the restriction of fish consumption.
Since fish obtain EPA and DHA by feeding on plankton and other invertebrates, new industries have evolved around the extraction of omega-3s from these sources; contaminants that are problematic in animals higher in the food chain aren't prevalent in lower organisms.
Some of these enterprises quickly encountered difficulties with mass production: companies that recovered oils from, say, arctic krill had to curtail their harvests due to dwindling resources.
For persons who wish to regularly consume fish, it is important to verify that those fish are free of mercury, PCBs, and other toxins. Due to feeding and coloring practices, farmed fish are suspect, but some farm sources can be certified as safe.
Contaminant-free supplements are still readily available; for many, this may be the safest, cheapest, and most convenient source of EPA and DHA.
For vegans who eschew animal-based products, there are algae-based sources of omega-3 fatty acids that will ensure adequate intake.
Because the conversion of ALA to EPA and DHA is limited in healthy humans, and because the capacity to accomplish this conversion is widely variable and unpredictable, dietary DHA should be considered an essential or “conditionally essential” fatty acid.
Essential Fatty Acid (DHA/EPA) Capsules
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