What role does B12 play in weight loss?

Vitamin B12 is frequently marketed as a weight-loss aid, but the scientific evidence is weak. B12 is essential for energy metabolism, DNA synthesis, and red blood cell formation. A deficiency can cause fatigue and weakness, which might indirectly reduce physical activity and energy expenditure. However, in individuals with normal B12 levels, supplementation does not lead to significant weight loss in well-controlled trials. Most claims come from mechanistic reasoning or anecdotal reports, not from randomized controlled trials (RCTs) with weight as a primary endpoint. Clinical guidelines recommend B12 testing only in at-risk groups (vegans, older adults, post-bariatric surgery). For weight loss, the evidence supports calorie restriction, exercise, and behavioral changes. Safety note: B12 is water-soluble and generally safe, but high-dose injections or supplements are unnecessary for most people. Evidence grade: mechanistic/practice, not human-strong.