Brenna JT, Salem N Jr, Sinclair AJ, Cunnane SC; International Society for the Study of Fatty Acids and Lipids, ISSFAL. (2009) Prostaglandins Leukot Essent Fatty Acids. Feb-Mar;80(2-3) 85-91.
File Download: Download the ISSFAL Statement on Omega-3 (documenting the inefficiency of ALA to DHA conversion) (192.53 KB)
Blood levels of polyunsaturated fatty acids (PUFA) are considered biomarkers of status. Alpha-linolenic acid, ALA, the plant omega-3, is the dietary precursor for the long-chain omega-3 PUFA eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA).
Studies in normal healthy adults consuming western diets, which are rich in linoleic acid (LA), show that supplemental ALA raises EPA and DPA status in the blood and in breast milk. However, ALA or EPA dietary supplements have little effect on blood or breast milk DHA levels, whereas consumption of preformed DHA is effective in raising blood DHA levels.
Addition of ALA to the diets of formula-fed infants does raise DHA, but no level of ALA tested raises DHA to levels achievable with preformed DHA at intakes similar to typical human milk DHA supply.
The DHA status of infants and adults consuming preformed DHA in their diets is, on average, greater than that of people who do not consume DHA. With no other changes in diet, improvement of blood DHA status can be achieved with dietary supplements of preformed DHA, but not with supplementation of ALA, EPA, or other precursors.
This comprehensive review of the evidence shows that humans cannot effectively convert ALA (the shortest-chain omega-3, found in some plant, nut and seed oils) into DHA (a longer-chain omega-3 found in fish and seafood) in sufficient quantities to achieve optimal tissue DHA levels.
This means that pre-formed DHA is a dietary essential.
As the authors note, the poor conversion of short- to long-chain omega-3 observed in human studies may partly reflect the very high intakes of short-chain omega-6 - mainly linoleic acid (LA) from industrially processed seed oils - in humans eating modern. western-type diets, as LA competes with ALA for conversion within the body.
However, normal genetic individal differences, and/or a lack of vitamin and mineral co-factors essential to the enzymes involved may also help to explain the very limited and inefficient conversion of ALA to DHA seen in humans.
Sex hormones also affect conversion, giving females a well-documented advanage over males in converting short-chain omega-3 (or omega-6) to the long-chain versions that are most biologically important. Detailed UK studies show that although young women could make at least some DHA from dietary ALA, the young men studied appeared to make none at all:
This paper is a 'consensus statement', representing the official position of ISSFAL (the International Society for the Study of Fatty Acids and Lipids) - an independent organisation that includes most of the world's leading scientists in this area.
This is an extremely important finding, because the health benefits associated with consuming 'omega-3' fatty acids relate primarily to EPA and DHA - not to ALA.
This particularly applies to brain health - because DHA is the most abundant omega-3 in the brain and nervous system, and adequate supplies are essential for normal brain structure and function.
The scientific consensus that pre-formed DHA is a dietary essential, because dietary omega-3 ALA alone cannot meet human needs for DHA, should encourage public health authorities, food and supplement producers, and consumers, to do more to ensure adequate intakes.
Implications for vegans and vegetarians.
Pre-formed DHA suitable for vegans can be obtained from special forms of algae - and this is already used in infant formula, and some supplements and fortified foods.
Vegans and vegetarians could also achieve higher tissue DHA by reducing their intake of short-chain omega-6 as far as possible. This would reduce competition from omega-6 LA, allowing more omega-3 ALA to be converted to the long-chain omega-3 EPA and DHA, which are the most biologically important forms. All traditional pre-industrial diets provided a far lower ratio of short-chain omega-6 to omega-3 than do modern western-type diets (the vast majority of traditional diets also provide at least some DHA ready-made, mainly from fish and seafood, but also from organ meats, and/or eggs).
Ensuring a good intake of all vitamins and essential minerals needed as co-factors for the conversion enzymes can also help to maximise synthesis of DHA from ALA.