Caffeine is one of the few supplements — alongside Creatine Supplementation and protein powder — with strong evidence behind it, but its value is primarily performance-based rather than a direct muscle-building agent.
Caffeine's biggest practical impact may be behavioral rather than physiological: by boosting energy and motivation, it can be the difference between someone training hard (or training at all) and skipping/under-performing a session. Since training itself is the primary driver of muscle growth (nutrition and supplements are permissive, not causative — see Caloric Surplus Size & Bulking Strategy), caffeine's real value may lie in supporting consistent, higher-quality training rather than any direct anabolic effect.
Fat burners claim to work via appetite suppression (fewer calories in) or thermogenesis (more calories out). A recent meta-analysis found no added benefit of fat-burner/thermogenic supplements over exercise, or diet plus exercise, alone.
Caffeine and green tea extract do have a real, measurable thermogenic effect, but it's small relative to diet and exercise. Caffeine remains worth taking pre-training for its direct performance benefits, not specifically for fat loss — be skeptical when caffeine is repackaged alongside other unproven herbal ingredients as a branded "fat burner."
Source: Jeff Nippard, "The Worst Supplements Everyone Takes For Muscle Growth" (2026).