How do I know if I have a vitamin deficiency?

The most reliable way to know if you have a vitamin deficiency is through a blood test ordered by your doctor. They can measure levels of specific vitamins like D, B12, iron, and others directly in your bloodstream. That said, some common symptoms can hint at deficiencies. Fatigue and weakness may suggest low iron or B12. Bone pain or muscle weakness can indicate low vitamin D. Bleeding gums or slow wound healing may point to vitamin C deficiency. Hair loss, brittle nails, or skin issues can relate to biotin or zinc. However, these symptoms overlap with many conditions, so self-diagnosing is unreliable. See a doctor for proper testing before starting any supplements.