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Natural compounds with RCT evidence for metabolic rate effects: caffeine (100–150 kcal/day, Astrup 1990; tolerance-limited), EGCG from green tea (+80 kcal/day above caffeine, Dulloo 1999), capsaicin (+50 kcal/day, Ludy 2012; tolerance develops). L-carnitine has no evidence for metabolic rate increase in omnivores. B vitamins are essential cofactors but do not boost metabolism above baseline. Total achievable effect: ~200–350 kcal/day in non-tolerant individuals — less than one small meal.