Estimated conversion of α‐linolenic acid to long chain n‐3 polyunsaturated fatty acids is greater than expected in non fish‐eating vegetarians and non fish‐eating meat‐eaters than in fish‐eaters
Authors: Welch A.A. , Bingham S.A. , Khaw K.T. 
Published: July 2008

Background:  It is believed that n‐3 polyunsaturated fatty acid (PUFA) status is important for cardiovascular health. However, the major sources of the essential dietary PUFA, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are fish and fish oil supplements. Limited conversion of α‐linoleic acid (ALA), found in plant foods (flaxseeds, walnuts, soya and rape‐seed oils), to EPA, DPA (docosapentaenoic acid) and DHA occurs in men (0.2–6% for EPA and DPA, ≤0.05% for DHA) but is greater in younger women (Burdge, 2006). Therefore, n‐3 PUFA status may be compromised in individuals not eating fish or fish‐oils, compared with fish‐eaters. An investigation into n‐3 PUFA status and estimated potential conversion of ALA in middle‐aged women with different dietary habits in the EPIC‐Norfolk population was conducted.

Methods:  Data were collected via a nurse examination between 1993 and 1997 (Welch, 2006). Circulating plasma phospholipid n‐3 PUFA were measured using gas liquid chromatography (HP 5980 gas chromatograph) (Welch, 2006). Diet was estimated using a 7‐day food diary and data were entered using the DINER data entry program (Welch, 2001). Only participants with dietary data and plasma fatty acids, who did not consume fish oils, were included in this study (n = 2256). Ethical permission was granted by the Norfolk and Norwich Ethics Committee. Participants were divided according to dietary habit into non‐fish‐eating vegetarians, non‐fish‐eating meat‐eaters and fish‐eaters (97% meat‐eaters). The ratio of circulating long chain n‐3 PUFA (LCn‐3PUFA are the total of EPA, DPA and DHA) to intake of ALA was calculated (PLCn‐3PUFA:DALA). A larger ratio implies greater conversion of ALA. Statistics were performed with STATA 10.0 and as ALA conversion may be affected by dietary EPA, DHA and linoleic acid, age, body mass index (BMI) and smoking habit, the ratio was adjusted for these covariates. The P for the difference between groups was calculated using anova.

Results:  Although intakes of total n‐3 PUFA were ≈30% lower in vegetarians and meat‐eaters than fish‐eaters, circulating plasma LCn‐3 PUFA were only 12% and 9% lower, respectively. The PLCn‐3PUFA:DALA was greater in vegetarians and meat‐eaters than in fish‐eaters.

 Population characteristics, diet, n‐3 PUFA status and the ratio of circulating long chain n‐3 PUFA to intake of dietary ALA (PLCn‐3PUFA:DALA) in 2256 women aged 39–78 years

Vegetarian non‐fish‐eaters
Meat eating non‐fish‐eaters
Fish‐eaters
P*

N
56
309
1891

Mean
SD
Mean
SD
Mean
SD

Age
59.0
9.5

61.8
9.3

62.4
8.7

0.01

BMI (kg m−2)
25.7
4.1

27.0
4.7

26.7
4.4

0.09

Diet n‐3 PUFA g d−1
0.86
0.38

0.89
0.34

1.24
0.46

<0.001

Plasma LCn‐3 PUFA μM
360
199

373
166

408
169

<0.001

Mean
95% CI
Mean
95% CI
Mean
95% CI

PLCn‐3PUFA:DALA†
0.149
0.123
0.171
0.142
0.133
0.152
0.129
0.125
0.133
0.002

PLCn‐3PUFA:DALA‡
0.156
0.135
0.177
0.149
0.139
0.158
0.128
0.124
0.131
0.011

*Difference between groups.

†Unadjusted data.

‡Adjusted for age, BMI, smoking habit, dietary EPA and DHA and circulating linoleic acid.

Discussion:  Although these are epidemiological findings and not the result of careful metabolic studies, the fact that the PLCn‐3PUFA:DALA ratio is ≈22% is higher in vegetarians and meat‐eaters than in fish‐eaters indicates that there is greater conversion of ALA than in fish‐eaters and may explain the smaller than expected differences in n‐3 PUFA status between fish and non‐fish consumers.

Conclusions:  If this finding were replicated in a controlled feeding study and it were confirmed that maintenance of n‐3 PUFA status was feasible from plant sources there could be implications for the requirements for fish intake.

References  Burdge, G.C. (2006) Metabolism of alpha‐linolenic acid in humans. Prostaglandins Leukot. Essent. Fatty Acids75, 161–168.

Welch, A.A., Bingham, S.A., Ive, J., et al. (2006) Dietary fish intake and plasma phospholipid n‐3 polyunsaturated fatty acid concentrations in men and women in the European Prospective Investigation into Cancer‐Norfolk United Kingdom cohort. Am. J. Clin. Nutr. 84, 1330–1339.

Welch, A.A., McTaggart, A., Mulligan, A.A., et al. (2001) DINER (Data Into Nutrients for Epidemiological Research) – a new data‐entry program for nutritional analysis in the EPIC‐Norfolk cohort and the 7‐day diary method. Public Health Nutr. 4, 1253–1265.