A nutritional disorder is any disease or dysfunction caused by inadequate, excessive, or imbalanced nutrient intake. The category spans deficiency diseases such as iron-deficiency anaemia, iodine deficiency disorders, vitamin A deficiency, and scurvy; protein-energy malnutrition in its wasting and stunting forms; and disorders of excess including obesity, type 2 diabetes, and diet-related cardiovascular disease. Some conditions arise from malabsorption or increased needs rather than poor diet alone, as in the anaemia of chronic disease or the bone disease of kidney failure. Diagnosis combines dietary assessment, clinical signs, and biochemical markers, while treatment addresses both the nutrient gap and its underlying cause. Nutritional disorders remain among the world’s largest disease burdens, with undernutrition and overnutrition now coexisting in many countries as the double burden of malnutrition.