Doughnuts are loaded with sugar, which makes them full of empty calories and almost void of nutritional value.  Also, when fat and sugar are combined, some areas of the brain are stimulated with feelings of pleasure and reward due to the release of the neurotransmitter dopamine. This addiction-like process causes someone to crave more of the same food to renew the experience. But over time, even more fat and sugar is required because the brain becomes desensitized to this stimulation.

Too much sugar raises blood pressure and blood glucose levels. It also triggers inflammation and elevated triglycerides. These factors will eventually induce leptin (a hormone that helps to regulate energy balance by inhibiting hunger) and insulin resistance, high cholesterol and impaired glucose tolerance, all of which contribute to heart disease. 

Too much sugar also contributes to obesity, increased risk of developing type 2 diabetes and cancer. 

Doughnuts also contain other harmful ingredients like artificial flavours and colours, additives, preservatives, dextrose and soybean oil. Dextrose is a sugar derived primarily from the very unhealthy and genetically modified (GM) form of high fructose corn syrup (HFCS). GM derived soybean oil is a partially-hydrogenated oil and as such, is an unhealthy trans fat that raises your bad cholesterol and lowers your good cholesterol. It also causes heart disease as well as kidney and liver problems, hypertension and strokes. 

Soybean oil is used because it is cheaper and creates a long shelf life. It is commonly used by fast food restaurants to deep-fry foods like doughnuts because it doesn’t have to be changed as often as other oils.