Decreasing linoleic acid with constant alpha-linolenic acid in dietary fats increases (n-3) eicosapentaenoic acid in plasma phospholipids in healthy men.

Research suggests that diets high in linoleic acid (LA) may negatively impact the metabolism of the omega 3 fatty acid alpha – linolenic acid (ALA) to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).  The positive effects of ALA in many chronic health conditions particularly in reducing the risk of CVD have been well documented.  The objective of the present study was to investigate the effect of replacing vegetable oils high in LA with oils low in LA, while keeping levels of ALA constant, on plasma omega 6 and 3 fatty acid concentrations.  

Twenty-two men aged 20-45 years of age participated in this 10-week randomized crossover study.  All subjects participated in a 2-week pre-study phase in which their usual diets were consumed, but fish and seafood were avoided.  Subjects were then randomly allocated to consume foods prepared with oils high in LA or low in LA, with constant ALA, for 4 weeks for each diet.  Dietary intake of ALA was held constant at approximately 1% to provide a total dietary LA:ALA ratio of 4:1 (low LA) or 10:1 (high LA) by modifying LA intake from vegetable oils and fats.  Intake of meat, poultry, eggs, dairy products, fruits and vegetables, cereals and grains, and all foods containing no vegetable fat were unrestricted though out the study.  Subjects were requested to consume no salad or cooking oils, mayonnaise, salad dressings, sauces, bakery foods, desert, or snack foods other than those supplied during the study.  Baked foods, mayonnaises, and salad dressings containing either high or low LA were provided to each participant at the beginning of each week, along with information sessions on how best to adhere to the test diet.  Foods prepared with flaxseed oil contributed approximately 70% of the total ALA intake during the study.  Dietary analysis was obtained via 24-hour food records taken 4 days apart during the 4-week study periods and analyzed for total fat and fatty acid intakes.  Fasting blood samples were also obtained for analysis of LA, EPA, and the arachidonic acid (ARA):EPA ratio.  Plasma phospholipid fatty acids were determined at week 0, 2, 4, 6, and 8.  Triglycerides, cholesterol, serum CRP, and IL-6, and platelet aggregation were determined at week 0, 4, and 8.   

During the course of the study, the mean fat intake from all sources was 32.5% of total energy, with 55% of total fat intake from test foods provided.  An LA: ALA ratio of 4:1 and 10:1 was achieved for the low and high LA diet periods, respectively, by altering only the vegetable fat potion of the diet.  Plasma phospholipid LA was higher and EPA lower during the high LA dietary phase compared to the low LA phase.  DHA levels were observed to decline over the 8-week period.  The ARA:EPA ratio in plasma phospholipids were 20.7 +/- 1.52 following the high LA diet, and 12.9 +/- 1.01 after consuming the low LA diet.  LA intake was also found to be inversely associated with EPA levels, and positively associated with the ratio of ARA:EPA.  A diet X time interaction was observed for the plasma phospholipid ALA values.  Subjects who consumed the low LA diet in the first 4-weeks had a significant increase in their PL ALA levels at week 2, followed by a decrease in ALA at week 4.  Men who consumed the low LA diet in the second 4-week period had a higher PL ALA levels following 4 weeks consuming the high LA diet.  LA intake did not exert any influence on ALA, ARA, DPA, DHA, or total, LDL or HDL cholesterol, CRP, IL-6, or platelet aggregation. 

A diet with 1% energy from ALA and a 4:1 ALA:ARA ratio resulted in higher PL EPA levels and a 40% lower ARA:EPA ratio when compared to a diet containing a LA:ALA ratio of 10:1. Elevated intakes of LA resulted in a decrease in PL EPA levels and an increase in the ARA:EPA ratio with no observed increase in ARA levels.