No vitamin causes weight loss alone. But several micronutrient deficiencies make fat loss significantly harder by impairing metabolism, energy, hormones, and exercise capacity.
Vitamin D — The Most Important
Deficiency affects ~1 billion people and is strongly correlated with obesity. A 2011 study found higher vitamin D at baseline predicted significantly greater fat loss over 12 weeks. It improves insulin sensitivity, regulates leptin, and enhances calcium absorption (which supports fat metabolism). Test first; supplement with 2,000-4,000 IU D3 daily if deficient. Always pair with 100-200mcg K2 (MK-7).
B Vitamins (B12, B6, B9)
Essential coenzymes in energy metabolism. B12 deficiency causes fatigue that makes exercise much harder. Common in women, vegans, and those over 50. Doesn't directly burn fat, but restoring energy enables more effective exercise.
Magnesium
Required for 300+ enzymatic reactions including blood sugar control, muscle function, and sleep quality. Poor sleep raises cortisol and ghrelin — actively counteracting fat loss. 300-400mg magnesium glycinate at night improves sleep for many people.
Iron
Deficiency anaemia causes fatigue and reduced exercise capacity. Women of reproductive age are most vulnerable. Test ferritin (stored iron), not just haemoglobin, for a complete picture.
What Doesn't Work
Most "weight loss vitamin" supplements are B vitamins + green tea extract with minimal evidence. Green tea EGCG does modestly boost metabolism (~80 extra calories/day) but this is far less than advertised. Correct real deficiencies first — they're far more impactful.