Vitamin B3 deficiency, also known as niacin deficiency, occurs when the body does not get enough of this essential nutrient. Symptoms can include fatigue, skin rashes, and digestive issues. Severe deficiency can lead to a condition called pellagra, characterized by the three D's: dermatitis, diarrhea, and dementia. This deficiency is often linked to poor diet, certain medical conditions, or excessive alcohol consumption. Foods rich in niacin include meat, fish, nuts, and whole grains. Addressing the deficiency typically involves dietary changes or supplementation to restore adequate levels of this vital vitamin.