Effect of roasted purple laver (nori) on vitamin B12 nutritional status of vegetarians: a dose-response trial.

Purpose: To investigate the bioavailability of vitamin B₁₂ from nori and to evaluate the required dosage for improving vitamin B₁₂ nutritional status in vegetarians not using supplements.

Methods: The study design is an open-label, parallel, dose-response randomized controlled trial. Thirty vegetarians were assigned to control (no nori), low-dose (5 g nori, aiming to provide 2.4 µg vitamin B₁₂ per day), or high-dose (8 g nori, aiming to provide 4 µg vitamin B₁₂ per day) groups. The primary outcome was changes in vitamin B₁₂ status as measured by serum vitamin B₁₂, holotranscobalamin (holoTC), homocysteine (Hcy), and methylmalonic acid (MMA), and a combined score of these four markers (4cB12 score) during the four-week intervention. Dietary vitamin B₁₂ intakes were assessed at baseline and end of the trial with a 17-item food frequency questionnaire designed for vitamin B₁₂ assessment. General linear model was used to compare least square means of changes in each biomarker of vitamin B₁₂ status, among the three groups, while adjusting for respective baseline biomarker.

Results: After adjusting for baseline status, nori consumption led to significant improvement in serum vitamin B₁₂ (among-group P-value = 0.0029), holoTC (P = 0.0127), Hcy (P = 0.0225), and 4cB12 (P = 0.0094). Changes in MMA did not differ significantly across groups, but showed within-group pre-post improvement in the low-dose group (median [p25, p75] = -339 [-461, -198] nmol/L). Vitamin B₁₂ status appeared to plateau at low dose (5 g of nori), which compared with control group, improved serum vitamin B₁₂ (lease square mean [95% CI] = + 59 [25, 93] pmol/L, P = 0.0014); holoTC (+ 28.2 [10.1, 46.3] pmol/L, P = 0.0035); Hcy (-3.7 [-6.8, -0.6] µmol/L, p = 0.0226); and 4cB12 score (+ 0.67 [0.24, 1.09], p = 0.0036). High-dose resulted in similar improvements. There was no significant difference between low-dose and high-dose groups in all biomarkers of vitamin B₁₂.

Conclusions: Consuming 5 g of nori per day for 4 weeks significantly improved vitamin B₁₂ status in vegetarians. A higher dose (8 g) may not confer additional benefits.

Clinical trial registration: ClinicalTrials.gov Identifier: NCT05614960. Date of registration: November 14th 2022.