Effect of roasted purple laver (nori) on vitamin B(12) nutritional status of vegetarians: a dose-response trial.

PURPOSE: To investigate the bioavailability of vitamin B(12) from nori and to evaluate the required dosage for improving vitamin B(12) 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 B(12) per day), or high-dose (8 g nori, aiming to provide 4 µg vitamin B(12) per day) groups. The primary outcome was changes in vitamin B(12) status as measured by serum vitamin B(12), 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 B(12) intakes were assessed at baseline and end of the trial with a 17-item food frequency questionnaire designed for vitamin B(12) assessment. General linear model was used to compare least square means of changes in each biomarker of vitamin B(12) 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 B(12) (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 B(12) status appeared to plateau at low dose (5 g of nori), which compared with control group, improved serum vitamin B(12) (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 B(12). CONCLUSIONS: Consuming 5 g of nori per day for 4 weeks significantly improved vitamin B(12) 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.