First, though, let's start with some human data and take a tour of some parts of the brain. Let's introduce ourselves to the prefrontal cortex. Okay, so when Tom Naughton does his head bangs on his desk, I'm guessing he's whacking his prefrontal cortex (which we are going to call the PFC). It's the area that hangs out over your eyes. Fortunately this habit has not seemed to harm his brain function, if his recent terrific articles are any measure.
Now the prefrontal cortex is more developed and extensive in humans than any other primate, and it is responsible for what is called "executive function." That is, the PFC helps us predict outcomes, prioritize, modulate our emotions to socially acceptable norms, and helps us sort out the best options given conflicting data (reasoning, basically). It is a bit like a policeman for your brain - sure, it would be super fun to get drunk as a skunk and throw beer bottles off your roof at the neighbors - but your policeman says, er, no, that might get you in trouble. Drinking alcohol, in fact, disinhibits the PFC which enables you to ask out the girl you wouldn't have approached sober. Of course, if you are making a selection with impaired reasoning, the girl you ask out might not look quite so good to you sober…
But back to obesity. Our brains do play a major role in whether we gain fat or not. And part of what happens in obese humans is that the prefrontal cortex seems to be less active than in lean humans. This finding is especially interesting in obesity, as the PFC sends nerve fibers to the core appetite regulation part of the brain (the "central orexigenic network") - presumably, when fully active, the policeman is shaking his night stick at you when you want to go to the fridge for that second helping of ice cream. Nuh uh. You have had enough. If the policeman is offline, it may be easier to consume extra helpings. (I'm not sure I like the policeman analogy so much - too close to lack of willpower or gluttony and sloth, but it does fit into the model of weird modern food poisoning our brains, so that the reasoning piece of our appetite regulation machinery is shot.)
Now the question is, obviously - do obese humans start out with underactive PFCs, or is it acquired along with obesity? Well, women who were obese with underactive PFCs regained their frontal lobe function with successful weight loss.* This evidence would suggest that underactive PFCs aren't hard-wired, but depend upon the environment, including nutrition. However, a study going the opposite direction - starting with lean humans and making some obese with controlled overfeeding for an extended period is a tough sell to the institutional review board these days. So it is easier to use mini-pigs, who also seem to have particularly well-developed PFCs.
Let's look at the experiment. 17 pigs, 9 kept lean and 8 made obese. One of the SPECT scans in one of the obese pigs was "unusable" so the data is for 9 lean and 7 obese.
The standard diet was composed of 33% barley, 25% wheat bran, 12% soy shell, 10% wheat, 10% sunflower meal, 6% soy meal, and other minor components. Fat provided 2.17% of the total nutritional value.Well, if in much of pig evolutionary history they were set loose in a warehouse of a cardiologist's favorite Power Bar ingredients, perhaps this is what the minipigs would eat these days and stay nice and lean and metabolic syndrome free. In this experiment the pigs were fed 102 calories per kilogram each morning, and the pigs dutifully ate their swill in one meal and did their piggy things and stayed lean. Since calories are calories…though I will get back to the standard diet later...
Now the obese diet:
…eight animals were fed with a Western Diet (WD) enriched with carbohydrates and lipids offered ad libitum during 5 months (one ration offered at 0900 hours and calculated to exceed daily calorie consumption of the animals. The WD was composed of 32.65% wheat, 15% soy meal, 12% wheat bran, 10% barley, 10% sunflower oil, 10% cornstarch, 5% saccharose, and other minor components. Fat provided 22.74% of the total nutritional value.Gak! Enough said. I wonder if this was that high-oleic sunflower oil. Anyway….
The results of the experiment - well, one interesting thing is that the dietary pattern of the fattening pigs changed. The control pigs ate all their food at once, each morning, but as the "Western Diet" pigs became more obese they would eat 4-5 meals a day, and then spontaneously fast for a day or several days. By the end of the 5 months, the lean pigs weighed 38 kg on average (which is about where they started). The obese pigs weighed 67.1 kg. That's pretty impressive for five months. The day before the final brain imaging, the lean pigs ate 1561 calories each, and the obese pigs ate 2183 calories each.
And, as expected, the brains of the obese pigs did indeed have decreased activity in the PFC, both in the dorsolateral prefrontal areas and the anterior prefrontal cortex. In addition, there was a lessening of activity in some brain areas associated with the "reward system" (specifically the nucleus accumbens, the ventral tegmentum, and the nucleus pontis), which is consistent with the addiction literature - people who are addicted to something have less activation of the reward areas of the brain in response to the addictive stimulus than people who are not addicted. Thus addicted people need more and more of the stimulus to feel reward.
A key finding is that the decreased activation of the PFC correlated significantly with the final weight of the pigs - so the more obese they became, the more depressed their PFC function tended to be.
So, we have learned that in mini-pigs, sunflower oil, wheat bran, cornstarch, and sacchralose is a quick recipe for obesity, and that pigs on a similar diet minus so much cornstarch and oil will stay the same weight as long as you feed them controlled calories… (which is something of a weakness of the study if they were trying to prove that sunflower oil and starch make you fat, which they weren't, but it would have been interesting to see what happened if both groups were fed ad libitum. )
So, in all likelihood, given the corresponding human data in the reverse trial and observationally, the PFC does indeed play an important role in feeding signals and hunger/satiety states. And I'll quote the researchers here: "Whether the alteration of the brain dopamine system and prefrontal cortex metabolism is a cause or a consequence of obesity is still unknown. The answer brought by our study is that less activation of the prefrontal cortex is definitely an acquired anomaly related to obesity, and not a "hard-wired" feature."
*I framed that sentence the way the mini-pig paper did, using "regained frontal lobe function" - however, in the original paper the womens' frontal lobe function was not measured prior to the weight loss, so it is also possible that the women who successfully lost weight were a subset who had better frontal lobe function - but in that study the obese women had decreased PFC metabolism, the lean and formerly-obese women had PFC metabolism indistinguishable from each other.
Also - for more dopamine/frontal lobe/obesity discussion - a post from last year, ADHD and Obesity.