Obesity is a biochemical alteration in the brain promoting leptin resistance with resultant weight gain and secondary changes in behavior to maintain energy balance. The apparent character defects of gluttony and sloth are not the cause of the problem; they are the result of the problem. The biochemistry drives the behavior, not vice versa. The linchpin in this biochemical alteration is the hormone insulin. The majority of humans, regardless of weight, release double the insulin today that we did thirty years ago for the same amount of glucose. Now we’re left with the $147 billion (the annual financial cost of obesity) question: If insulin is the bad guy and we’re all hyperinsulinemic as never before in the history of humankind, where did the excess insulin come from? And how do we reverse it?
The plot thickens.
Food Addiction—Fact or Fallacy
Salvador is a fifteen-year-old Latino boy with obesity, a fatty liver, and high blood pressure. He drinks four sodas a day. His mother does not buy them for him or keep them in the house. Rather, he buys them at the convenience store on the way to and from school. Salvador enrolls in our research study whereby each day, for ten days, he will consume the same number of calories from our hospital’s Metabolic Kitchen, which will provide all his food, prepared by a chef and sugar free. Nonetheless, each day, he buys a can of soda and brings it home, putting it on his dresser, next to those from the day before. He tells his mother, “When the study is over, I’m drinking them all.” Indeed, the evening of the end of the study, he drinks every last one, to his mother’s chagrin. He may not have been addicted physically, but the mental obsession and craving indicated dependence, and could not be suppressed.
Life’s too short to eat bad food, even if it’s cheap. Eating is supposed to be an enjoyable experience, especially when the food is special. There’s nothing quite like going to a nice restaurant with the sights, sounds, and smells of a well-prepared meal. It’s one of the true enjoyments of life. Yet familiarity breeds greater cravings. Ask Philadelphians about their cheesesteaks, New Orleans denizens about their Po-Boys and beignets, or Memphians about their barbecue. Surprise! Those are among the three most obese cities in the country. Coincidence?
As prodigious as some American cuisine is, is there really anything special about a soda, a French fry, or any item in a fast food restaurant? Yet we devour fast food as if it were going out of style. Americans consume Big Macs as if each one might be our last. (Given the mortality rates in the obese, each one just might be.) Fast food comprises a growing portion of food eaten outside the home. In the United States of the 1950s, fast food accounted for 4 percent of total sales of food outside the home. In 1997 it accounted for 34 percent. Each day, 30 percent of U.S. adults eat at a fast food outlet, and McDonald’s feeds forty-six million Americans.
What about the rest of the world? They didn’t experience fast food growing up, yet it’s now the biggest seller in developing countries. There is no familiarity here; they weren’t raised on the stuff; they’re consuming it de novo. Why do they eat fast food when it’s not their traditional fare? Because it’s cheap? It certainly isn’t abroad. Why do the locals frequent Taco Bell in Mexico when the original tacos are cheaper and ostensibly healthier? Something more is going on here. Is the world addicted to fast food? The biology of addiction is at the center of this question.
Might as Well Face It, We’re Addicted to…
Our brains are wired for reward—it is the primary force behind human survival. Reward is the reason to get up in the morning. If you take away reward, you take away the reason to live. We know this from recent experience with the anti-obesity drug rimonabant, which was deep-sixed after it failed to gain approval from the FDA in 2007. Rimonabant is an endocannabinoid antagonist, or the “anti-marijuana” medicine—which means it’s also “anti-munchies.” It inhibits the sense of reward. While it worked to promote weight loss, 20 percent of the subjects who used it experienced serious psychiatric side effects, especially depression, and there were several suicides. Kill the reward system, and you just might want to kill yourself.
Although the brain’s reward system is complex and has many inputs, it can be reduced to the “hedonic pathway.” This pathway is where primal emotions, reproductive drive, and the survival instinct are all housed and expressed. These reward mechanisms are thought to have evolved to reinforce behaviors that are essential for perpetuation of the species and survival: such as sex for reproduction and the enjoyment of food so that you eat. This is also the pathway that reinforces the positive and negative aspects of drugs of abuse such as nicotine, cocaine, morphine, and alcohol. In order to maintain eating as one of the most powerful urges in animal and human behavior, evolution has also made it a rich source of pleasure and reward.
The hedonic pathway comprises a neural conduit between two brain areas: the ventral tegmental area (VTA) and the nucleus accumbens (NA, also known as the reward center), both of which are deep-brain structures. Pleasure occurs when the VTA signals the NA to release dopamine, a neurotransmitter. It’s a signal from one brain center to another. When the released dopamine binds to its specific dopamine D2 receptor in the NA, the sense of pleasure is experienced.1
So what are neurotransmitters and receptors? Think of keys and locks. Each neuron is a cell body, and at its end is an axon (special fiber of the neuron that sends information). This axon has a synapse, or pathway, that connects to the dendrites (specialized fibers of the nerve cell that receive information) of the next neuron. When a neural impulse is generated in the first cell, it pulses down to the end of the axon, which contains little packets of neurotransmitters that are then released. These are the keys. They travel across the synapse to the receptors (locks), located in the dendrites of the next cell. There are many keys that take the path along the synapse, and not all of them make it to their destination. Along their way via the synapse, some are metabolized and some are “re-uptaken.” Dopamine is one of these types of keys traveling to fit into the locks of the D2 receptors in the next cell, thus determining the triggering and firing of the next cells down the chain.
Food intake is just one readout of the hedonic pathway.2 It appears to mediate feeding on the basis of palatability rather than energy need: I’m stuffed, but that chocolate cake looks so good. When functional, the hedonic pathway helps to curtail food intake in situations where energy stores are replete: I don’t need to finish that macaroni and cheese. However, when dysfunctional, this pathway can increase food intake, leading to obesity.
If you feed a rodent a palatable food (e.g., a high-fat, high-sugar food such as cookie dough), the animal experiences reward because dopamine is released from the VTA and binds to the D2 receptor in the NA. As long as that continues, the animal will continue to eat and experience reward. There are three processes that modulate this system in one direction or another:
1. Anything that increases the dopamine transmission to the NA increases the feeling of reward.
2. Anything that clears dopamine from the NA will extinguish the feeling of reward.
3. Anything that reduces the number of D2 receptors in the NA, or binding of dopamine to those receptors (such as chronic overuse of a substance), will shortchange reward. You then need more dopamine, and hence more of the substance, to get the same feeling of pleasure.
These