Substances of abuse used to be the subject of much hand-wringing. It started with opium dens, moved to speakeasies, then to crack houses, then to "smoking permitted" anterooms. Since Nancy Reagan's "Just Say No," the war on drugs has taken a back seat, but not because it has been won. Rather, because a different war has cluttered the headlines -- the war on obesity. And a substance even more insidious, I would argue, has supplanted cocaine and heroin. The object of our current affliction is sugar. Who could have imagined that something so innocent, so delicious, so irresistible -- just one glucose molecule (not so sweet) plus one fructose molecule (very sweet) -- could propel America toward economic deterioration and medical collapse?

The brain's pleasure center, called the nucleus accumbens, is essential for our survival as a species. We learned how important it was with the introduction in 2003 and subsequent removal in 2007 of the anti-obesity drug rimonabant. This medication blocked the effect of endocannabinoids, molecules that affected the pleasure center like marijuana -- sort of like the anti-munchies. Patients lost interest in food and lost weight, but 20 percent became clinically depressed, and many committed suicide. Turn off pleasure, and you turn off the will to live.

But long-term stimulation of the pleasure center drives the process of addiction. Rich people are addicted to money, power, gambling; middle-class people are addicted to cocaine, amphetamine, caffeine, nicotine, alcohol, heroin. The poor, well, all they've got is sugar.

Pleasure is exciting. Happiness is transcendent. More importantly, pleasure is dopamine. And happiness is serotonin.

When you consume any substance of abuse, including sugar, the nucleus accumbens receives a dopamine signal, from which you experience pleasure. And so you consume more. The problem is that with prolonged exposure, the signal attenuates, gets weaker. So you have to consume more to get the same effect -- tolerance. And if you pull back on the substance, you go into withdrawal. Tolerance and withdrawal constitute addiction. And make no mistake, sugar is addictive.

By all estimates, obese people are not happy. The question is whether their unhappiness is a cause or a result of their obesity. At this point we can't say for sure, but it is entirely possible that both answers are correct. Here's how. The serotonin hypothesis argues that deficiency of brain serotonin causes severe clinical depression, which is why serotonin reuptake inhibitors (SSRIs) such as Wellbutrin and Prozac, which increase brain serotonin, are effective.

Not by chance, these medications are also used for obesity. Serotonin is made from an amino acid called tryptophan, which is the rarest amino acid in our diet. And the amino acid transporter that gets tryptophan into the brain is easily perturbed. If you're serotonin-deficient and depressed, you're going to want to boost your serotonin any way you can. Eating more carbohydrate, especially sugar, initially does double duty -- it facilitates tryptophan transport, and it generates a dopamine response for pleasure in the short-term. But as the dopamine signal down-regulates, more sugar is needed for the same effect, driving a vicious cycle of consumption to generate a pleasure that withers in the face of persistent unhappiness.