Long ago our ancestors, being omnivores, placed great emphasis on identifying foods that are nutritious, tasty and above all not toxic or deadly; a process that must have involved lots of trial and error. Once a good food source was found, it was wise to remember it and seek it out because a lot of things out there, especially plants, are downright dangerous to eat. There are brain mechanisms that enforce strong connections with what feels good and brings pleasure, and avoidance of what makes us feel bad or sick. This is one of the roles of the ventral tegmental nucleus (VTN) and especially the nucleus accumbens (often called the brain’s pleasure center) which controls the secretion of dopamine. It is a part of the brain we cherish, and frankly cannot live without, because it is responsible for joy, euphoria and romantic love. Activation of nucleus accumbens guarantees that once you find something delicious, you want more. Dopamine is a dominant transmitter there and its receptors are the usual targets of antidepressants as well as drugs of abuse from nicotine to heroin (http://bit.ly/1ulI5pF).
Take opium for example; the poppy produces a toxic substance to deter omnivores like us from eating it. We did not evolve to crave opium. Opium (and refined forms like heroin) induces euphoria, albeit with intestinal distress, in people by usurping the natural pleasure pathways to increase dopamine release in nucleus accumbens as well as activating endorphin receptors in the same region to produce reward. Bad luck from a societal perspective; clever design on the part of the poppy because it wildly stimulated its cultivation. Think about that.
What does this have to do with sugar?
Foods that we find particularly tasty stimulate dopamine release in nucleus accumbens in the same way as drugs of addiction with the same outcome, feelings of well being that bring us back to those foods. Sugar is particularly powerful in causing large release of dopamine and that is the hook that attracts us repeatedly to sugary, carbohydrate laden foods. With prolonged sugar uptake, the positive feelings attenuate so you have to eat more to get the same effect, a process called tolerance. If you cut back, you experience withdrawal. Tolerance and withdrawal are hallmarks of addiction.
With the increasing epidemics in AD, obesity and type-2 diabetes, it is time for us to take a good look at the effect of sugar on the brain and think twice about the role sugar plays in our diet. To make the point very clear, consider this; binging on sugar lowers the activity of neurons in the hypothalamus that produce oxytocin, a peptide hormone that promotes satiety and feeding termination. This is serious; the decrease in oxytocin promotes overeating and engages an insidious and deadly feedback loop (http://1.usa.gov/18Y5pAm).