It's true that no one has ever been able to prove beyond a doubt that humans can become addicted to any food. And, as a dietitian, I was a skeptic myself until I did the research for my book Breaking the Bonds of Food Addiction. One of the obstacles to belief is a lack of agreement as to what constitutes an addiction. Yet by standards many experts, and certainly most food addicts, would find both acceptable and applicable, sugar has been shown to be addictive. Certainly, addictive behavior has been demonstrated again and again, both clinically by lab rats fed pure sugar and anecdotally by humans with especially bad eating habits. Just as there is a difference between a social drinker and an alcoholic, there is a difference between someone who eats a sweet dessert after dinner and a compulsive overeater who scarfs down enough chocolate pudding for twelve people.
Bart Hoebel, a neuroscientist at Princeton University, has been tempting rats with sugar syrup for many years. Early in his research, Hoebel discovered that when rats are allowed to binge on sugar, and then are deprived of the substance, the animals develop more receptors for pleasure-inducing chemicals in an area of the brain that motivates eating behavior. That means it makes them feel better to eat more sugar, or to put it another way, they need to eat more sugar in order to feel good. When those receptors were blocked, the rats showed all the typical signs of withdrawal in a human, such as chattering teeth and changes in brain chemistry common to anyone who is deprived of a chemical on which they've become dependent. At the same time, rats that maintained a steady diet that included sugar syrup showed none of these signs.
More recently, Hoebel and his rat pack provided another link between substance abuse and sugar addiction. Changes in the brain function of rats who went through a period of withdrawal after binging on sugar for a month showed that the rats were willing to work harder to satisfy their sugar cravings, ate more sugar and, while in withdrawal, were more sensitive to other substances of abuse, such as alcohol and amphetamines. It is well established that addiction often has a long-lasting effect on the brain that causes, say, an alcoholic or drug addict to have an increased sensitivity to other addictive substances.
Ask a food addict what happens when s(he) does and doesn't eat sugar.