Conversion of alpha-linolenic acid in humans is influenced by the absolute amounts of alpha-linolenic acid and linoleic acid in the diet and not by their ratio.
Conversion of alpha-linolenic acid in humans is influenced by the absolute amounts of alpha-linolenic acid and linoleic acid in the diet and not by their ratio.
Year: 2006
Authors: Goyens, PLL, Spilker, ME, Zock, PL, et al.
Publication Name: Amer. J. Clin. Nutr.
Publication Details: Volume 84, Page 44.
Abstract:
The conversion of ALA to the longer chain fatty acids EPA and DHA appears to be limited. High amounts of linoleic acid (LA) in the diet negatively impacts this conversion as LA and ALA compete for the desaturase and elongase enzymes responsible for the conversion to longer chain PUFAs Increasing ALA intake and/or decreasing LA intake may favorably affect ALA conversation. It has been suggested that the dietary ALA:LA ratio is a better determinant of ALA conversion than the absolute intakes of both fatty acids. Results from human studies investigating the effect of ALA:LA ratio on ALA metabolism have been mixed. An investigation on whether intakes of ALA or LA, or their ratio has an influence on ALA metabolism was conducted. In this research, twenty-nine men (n=14) and women (n=15) participated in a 10-week double-blind, nutritional intervention trial. During a 4-week run in period, all participants consumed a run in diet for the first four weeks providing 7% of energy as LA and 0.4% as ALA and an ALA:LA ratio of 1:19. For the remaining 6-weeks, participants were randomly allocated into one of three experimental diet groups based on sex, age, and total cholesterol concentrations as determined at the initial screening visit: 1) Control group (n=9): subjects continued to consume the same diet as for the run-in period (7% of energy as LA; 0.4% as ALA; ratio of ALA:LA of 1.19); 2) Low LA group (n=10): 3% of energy was consumed as LA and 0.4% of energy was consumed as ALA; 3) High ALA group (n=10): 7% energy was consumed as LA and 1.1% of energy consumed as ALA. Ratio of ALA:LA was 1.19 for the control diet and 1:7 for the other two diets. All three diets were comparable with respect to protein (15% energy), fat (35% energy as fat), and carbohydrate (50% energy as carbohydrates). The composition of the 3 diets was achieved through guidance of dietary intake of experimental and non-experimental food items that had to be followed precisely. The experimental dietary fats supplying the ALA and LA were provided in the form of a margarine, which in turn was also used to prepare bakery items for the participants to consume. The source of ALA was primarily canola oil, while monounsaturated fat and LA was provided primarily via olive and sunflower oils, respectively. [U13-C] ALA was administered orally for a 9-day period 10-days prior to the end of each study period. The oxidation of ALA was then determined from breath samples, and conversion estimated via compartmental modeling of [13-C] and [12-C] n3 fatty acid concentrations in fasting plasma phospholipids. The incorporation of ALA into phospholipids increased by 3.6% in the low-LA group and decreased by 8.9% in the high-ALA group. In terms of absolute amounts, an increase of 34.3 mg of ALA was shown in the low-LA group, with only a slight change observed in the high-ALA group. Almost all of the ALA phospholipid pool was converted into EPA; however, conversion of EPA to DHA in all dietary groups was <1% of dietary ALA. In absolute amounts, no changes in EPA+DHA were reported in the low-LA group, while an increase from 0.7 to 1.9 mg was found in the high-ALA group. No changes in ALA oxidation were observed following the dietary treatments. Based on these results, the authors conclude that the dietary ALA:LA ratio appears not to be a significant determinant of ALA conversion to EPA. Lowering dietary LA resulted in an increase in EPA levels. Increasing dietary ALA increased DHA synthesis. Substituting dietary LA for ALA appears to provide for adequate synthesis of EPA and should be considered when recommendations for omega 3 intakes are made.
