Vitamin B12 enhances the phase-response of circadian melatonin rhythm to a single bright light exposure in humans.
Level 2 - randomized trial
Single-blind crossover trial in humans
PubMed 8981490 · doi:10.1016/s0304-3940(96)13247-x
What was done
Eight young males participated in a single-blind crossover study assessing whether vitamin B12 (methylcobalamin) modulates the phase-shifting effect of bright light on circadian melatonin rhythms. Participants received either methylcobalamin (0.5 mg/day intravenously at 12:30 for 11 days, followed by oral administration of 2 mg three times daily for 7 days) or a vehicle control. Plasma melatonin rhythms were measured via serial blood sampling under dim light (<200 lx) before and after a single bright light exposure (2,500 lx for 3 hours starting at 07:00).
What was found
Prior to light exposure, plasma melatonin rhythm amplitude was smaller during the vitamin B12 trial compared to placebo. Following morning bright light exposure, a significant phase advance in the melatonin rhythm occurred in the vitamin B12 condition, whereas no significant phase shift was observed with placebo. Specific numerical values for melatonin concentrations, amplitude, and phase shifts were not reported in the abstract.
Why it matters
The findings suggest that high-dose methylcobalamin may potentiate photic entrainment mechanisms in humans, altering melatonin amplitude and sensitizing the circadian clock to light-induced phase advances.
Limits
The study was conducted in an extremely small sample (n = 8) consisting exclusively of young males. It utilized a single-blind rather than double-blind design, and the abstract omits exact numerical values, effect sizes, variance estimates, and the duration of the washout period between crossover phases.
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