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Conversion of -linolenic acid in humans is influenced by the absolute amounts of -linolenic acid and linoleic acid in the diet and not by their ratio^1,2,3

¹ From the Department of Human Biology, Maastricht University, Maastricht, Netherlands (PLLG and RPM); the Technical University of Munich, Nuclear Medicine Department, Klinikum rechts der Isar, Germany (MES); the Division of Human Nutrition, Wageningen University, Wageningen, Netherlands (PLZ and MBK); and Wageningen Centre for Food Sciences, Wageningen, Netherlands (PLZ, MBK, and RPM)

Background: Human in vivo data on dietary determinants of -linolenic acid (ALA; 18:3n–3) metabolism are scarce.

Objective: We examined whether intakes of ALA or linoleic acid (LA; 18:2n–6) or their ratio influences ALA metabolism.

Design: During 4 wk, 29 subjects received a control diet (7% of energy from LA, 0.4% of energy from ALA, ALA-to-LA ratio = 1:19). For the next 6 wk, a control diet, a low-LA diet (3% of energy from LA, 0.4% of energy from ALA, ratio = 1:7), or a high-ALA diet (7% of energy from LA, 1.1% of energy from ALA, ratio = 1:7) was consumed. Ten days before the end of each dietary period, [U-¹³C]ALA was administered orally for 9 d. ALA oxidation was determined from breath. Conversion was estimated by using compartmental modeling of [¹³C]- and [¹²C]n–3 fatty acid concentrations in fasting plasma phospholipids.

Results: Compared with the control group, ALA incorporation into phospholipids increased by 3.6% in the low-LA group (P = 0.012) and decreased by 8.0% in the high-ALA group (P < 0.001). In absolute amounts, it increased by 34.3 mg (P = 0.020) in the low-LA group but hardly changed in the high-ALA group. Nearly all ALA from the plasma phospholipid pool was converted into eicosapentaenoic acid. Conversion of eicosapentaenoic acid into docosapentaenoic acid and docosahexaenoic acid hardly changed in the 3 groups and was <0.1% of dietary ALA. In absolute amounts, it was unchanged in the low-LA group, but increased from 0.7 to 1.9 mg (P = 0.001) in the high-ALA group. ALA oxidation was unchanged by the dietary interventions.

Conclusion: The amounts of ALA and LA in the diet, but not their ratio, determine ALA conversion.

Key Words: Ratio of -linolenic to linoleic acid • conversion • oxidation • stable isotopes • humans

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