The three symptoms of diabetes mellitus are polyuria, polyphagia, polyphagia and weight loss. When blood glucose is elevated above the renal glucose threshold, glucose is not completely reabsorbed by the renal tubules through glomerular filtration, and subsequently osmotic diuresis develops, so an increase in urine volume develops. The increase in osmotic pressure due to hyperglycemia leads to polyuria, followed by excessive water loss and intracellular dehydration, a condition that gradually raises plasma osmolality and stimulates the thirst center leading to thirst and polyuria, which is further aggravated by polyuria. Diabetic patients experience hunger and subsequently hyperphagia and polyphagia because blood glucose cannot be metabolized normally, while high blood glucose stimulates insulin secretion. Although the appetite and food quantity of diabetic patients maintain normal or even increase, their weight will gradually decrease, mainly because of the relative or absolute lack of insulin or insulin resistance, the body cannot make full use of glucose to produce energy, then the amount of fat and protein decomposition increases, and weight loss will occur when too much fat and protein is consumed. Therefore, the three poly and one less in diabetes are polyuria, polyphagia, polyphagia and weight loss, and these symptoms affect each other and promote each other.