The four "classic" symptoms of diabetes are:
Here is why these symptoms happen.
Every cell in the body needs energy in order to live. People get their energy by converting the food they eat into fats and sugar (glucose). This glucose travels in the bloodstream as a component of normal blood. Individual cells then remove some of that glucose from the blood to use for energy. The substance that allows the cells to take glucose from the blood is a protein called 'insulin'.
Insulin is created by beta cells that are located in the pancreas. The pancreas is an organ located next to the stomach.
A person with type 2 diabetes or gestational diabetes either cannot produce enough insulin, or they are "insulin insensitive", which means that their body can't use insulin properly. A person with type 1 diabetes produces little or no insulin.
Without enough insulin, the cells in the body do not have a way to use the glucose that is in the bloodstream, so the cells 'starve' while the glucose level in the blood rises.
In response to a lack of energy in the cells, the brain sends out signals that tell the person to eat more. Meanwhile, other cells in the body try to obtain energy by asking the body to break down fat and muscle protein. The liver can convert the muscle protein into glucose. A vicious cycle happens: more glucose is being created, but it cannot be turned into energy because there is not enough insulin to transfer the glucose into the cells of the body.
When too much glucose is in the blood, it 'leaks' into the urine. The urine of healthy people does not contain sugar. In diabetes, sugar in the urine draws water to it just like a dry sponge draws water. The person produces large amounts of urine because of all this water. All of that urination makes the person thirsty, so they drink excessively.
These responses to a lack of insulin lead people with diabetes to show the four classic symptoms of diabetes: they lose weight in spite of an increased appetite, drink excessively, and urinate excessively.