Food Funct., 2018, doi: 10.1039/c8fo00735g.

Effects of a low and a high dietary LA/ALA ratio on long-chain PUFA concentrations in red blood cells.

Greupner, T. Kutzner, L. Pagenkopf, S. et al.



There is a debate about the optimal dietary ratio of the parent n6 fatty acid linoleic acid (LA) and n3 fatty acid alpha-linolenic acid (ALA) to promote an efficient conversion of ALA to EPA and DHA, which have implications for human health. The aim of the present study was to compare the effects of a low-LA/high-ALA (loLA/hiALA) diet with a high-LA/low-ALA (hiLA/loALA) diet on fatty acid concentrations in red blood cells (RBCs). Fifteen omnivore healthy men (mean age 26.1 ± 4.5 years) with a low initial EPA/DHA status (sum (∑) EPA + DHA% of total fatty acids in RBC at baseline: 4.03 ± 0.17) received both diets for two weeks with a nine-week wash-out phase in between. Fatty acid intake of the subjects was tightly controlled. Concentrations [μg mL-1] and relative amounts [% of total fatty acids] of fatty acids in RBCs were analyzed at baseline (day 0), day 7 and 14 by means of GC-FID. The dietary LA/ALA ratios were 0.56 ± 0.27 : 1 and 25.6 ± 2.41 : 1 and led to significantly different changes of ALA, LA, EPA and ∑EPA + DHA concentrations in RBCs. In the course of the loLA/hiALA diet ALA and EPA concentrations and relative amounts of ∑EPA + DHA increased, whereas LA concentrations decreased. The DHA concentration was unaffected. The hiLA/loALA diet led to slightly decreased EPA concentrations, while all other fatty acid concentrations remained constant. Compared to our previous study, where we simply increased the ALA intake, our results show that ALA supplementation combined with a reduced LA intake (loLA/hiALA diet) more efficiently enhanced EPA blood concentrations. The absence of changes in the PUFA pattern in consequence of a LA/ALA ratio of 25.6 ± 2.41 : 1 suggests that the high LA/ALA ratio of the Western diet already leads to a saturation and a further increase of the ratio does not affect the PUFA pattern.

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Key Findings

The aim of this study was to compare the effects of two different dietary LA/ALA ratios on fatty acid concentrations in RBC with special emphasis on LC n3 PUFAs. A homogenous study collective of healthy, non-smoking men within a narrow range regarding age (mean age 26.1 ± 4.53 years) and BMI (24.0 ± 1.65 kg m−2) was chosen to prevent/minimize the influence of gender, smoking, age, and BMI on PUFA metabolism. With the two experimental diets a low-LA (2.78 en%) and a high-ALA (4.98 en%) diet and a high-LA (6.95 en%) and low- ALA (0.27 en%) diet were achieved, which correspond to a LA/ALA ratio of 0.56 ± 0.27 : 1 and 25.6 ± 2.41 : 1, respectively. These ratios can be classified as extreme examples of a desirable presumably health-promotive LA/ALA ratio (loLA/hiALA diet, <5 : 1) and an unfavorable LA/ALA ratio (hiLA/loALA diet,≫5 : 1) as it is typical for Western diets. Total PUFA intake was significantly higher in the loLA/hiALA diet compared to the hiLA/loALA diet due to the higher PUFA
content of linseed oil (69.8% of total fatty acids) compared to sunflower oil (62.5% of total fatty acids).   The loLA/hiALA diet was effective in increasing ALA and EPA concentrations in RBC membranes and resulted in a strong increase of ALA concentrations of 332 ± 40% (day 7) and 354 ±47% (day 14) in RBCs.   EPA concentrations in RBCs significantly increased by 35.0 ± 13% after 7 days and by 57.6 ± 18% after 14 days following the loLA/hiALA diet is a likely result of an increasing conversion of ALA to EPA, since no EPA was ingested via the background diet. ALA conversion to EPA can be further enhanced by reducing the LA content. The relative amount of ΣEPA + DHA in RBCs significantly increased by 11.3 ± 3.7% (4.03 ± 0.17% to 4.76 ± 0.20% of total fatty acids) in only 14 days after the loLA/hiALA diet. The high ALA intake may lead to a competitive saturation of the delta-6 desaturase, and thus the conversion of EPA to DHA may be inhibited by the conversion of ALA to EPA. In this study, an increase in RBC EPA concentrations when a high ALA intake was combined with a reduced LA intake (loLA/hiALA diet) was found. The data support that a high LA intake might impede the ALA conversion to EPA. Further studies are needed to investigate the influence of high LA doses on the n3 PUFA status, especially in view of the high LA and low ALA intake and low ΣEPA + DHA status in many Western countries. Minor changes in the fatty acid profile in consequence of the hiLA/loALA diet suggest that the LA/ALA ratio of 25.6 : 1 is similar to that of the Western diet.

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