Dietary α-linolenic acid and health-related outcomes:: a metabolic perspective

alpha-Linolenic acid (alpha LNA; 18 : 3n-3) is essential in the human diet, probably because it is the substrate for the synthesis of longer-chain, more unsaturated n-3 fatty acids, principally EPA (20 : 5n-3) and DHA (22 : 6n-3), which confer important biophysical properties on cell membranes and so are required for tissue function. The extent to which this molecular transformation occurs in man is controversial. The present paper reviews the recent literature on the metabolism of alpha LNA in man, including the use of dietary alpha LNA in beta-oxidation, recycling of carbon by fatty acid synthesis de novo and conversion to longer-chain PUFA. Sex differences in alpha LNA metabolism and the possible biological consequences are discussed. Increased consumption of EPA and DHA in fish oil has a number of well-characterised beneficial effects on health. The present paper also reviews the efficacy of increased alpha LNA consumption in increasing the concentrations of EPA and DHA in blood and cell lipid pools, and the extent to which such dietary interventions might be protective against CVD and inflammation. Although the effects on CVD risk factors and inflammatory markers are variable, where beneficial effects have been reported these are weaker than have been achieved from increasing consumption of EPA+DHA or linoleic acid. Overall, the limited capacity for conversion to longer-chain n-3 fatty acids, and the lack of efficacy in ameliorating CVD risk factors and inflammatory markers in man suggests that increased consumption of alpha LNA may be of little benefit in altering EPA+DHA status or in improving health outcomes compared with other dietary interventions.