Efficiency of conversion of α-linolenic acid to long chain n-3 fatty acids in man.

Key Findings:

This review paper notes that plasma and tissues tend to be lower in ALA than n6 LA even when ALA is rich in the diet. ALA storage as triglycerides in adipose is relatively low. The authors provide data showing that humans of all ages, including preterm and very likely fetuses, convert 18:3n-3 to 22:6n-3. Total oxidation and partial oxidation with carbon recycling appear to the major fate of ALA. DHA is the least efficient formed n-3 long chain fatty acid from either ALA or EPA in humans. Conversion efficiency also appears to decrease as infants mature, and may be less efficient in adults. A high dietary n-6/n-3 ratio characteristic of modern diets will reduce conversion of ALA to DHA.

ABSTRACT:

Alpha-linolenic acid (18:3n-3) is the major n-3 (w3) fatty acid in the human diet. It is derived mainly from terrestrial plant consumption and it has long been thought that its major biochemical role is as the principal precursor for long chain polyunsaturated fatty acids, of which eicosapentaenoic (20:5n-3) and docosahexaenoic acid (22:6n-3) are the most prevalent. For infants,n-3 long chain polyunsaturated fatty acids are required for rapid growth of neural tissue in the perinatal period and a nutritional supply is particularly important for development of premature infants. For adults, n-3 long chain polyunsaturated fatty acid supplementation is implicated in improving a wide range of clinical pathologies involving cardiac, kidney, and neural tissues. Studies generally agree that whole body conversion of 18:3n-3 to 22:6n-3 is below 5% in humans, and depends on a concentration of n-6 fatty acids and, long chain polyunsaturated fatty acids in the diet. Complete oxidation of dietary 18:3n-3 to CO2 accounts for about 25% of 18:3n-3 in the first 24 h, reaching 60% by 7 days. Much of the remaining 18:3n-3 serves as a source of acetate for synthesis of saturates and monounsaturates, with very little stored as 18:3n-3. In term and preterm infants, studies show wide variability in the plasma kinetics of 13C n-3 long chain polyunsaturated fatty acids after 13C-18:3n-3 dosing, suggesting wide variability among human infants in the development of biosynthetic capability to convert 18:3n-3 to 22:6n-3. Tracer studies show that humans of all ages can perform the conversion of 18:3n-3 to 22:6n-3. Further studies are required to establish quantitatively the partitioning of dietary 18:3n-3 among metabolic pathways and the influence of other dietary components and of physiological states on these processes. (Author’s abstract)