Conversion of α-linolenic acid in humans is influenced by the absolute amounts of α-linolenic acid and linoleic acid in the diet and not by their ratio

BACKGROUND: Human in vivo data on dietary determinants of α-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