Comfort Food, for Monkeys
By JOHN TIERNEY
Published: May 20, 2008
The ladies who lunch do not obsess about their weight in the rhesus monkey compound at the Yerkes National Primate Research Center in Atlanta. Food is freely available, and the high-status females do not pride themselves on passing it up. They don't seem to stigmatize obesity — there is no equivalent of a Kirstie Alley joke — and they certainly don't turn themselves into Social X-Rays.
In fact, the dominant females ordinarily eat a little more than the subordinates. The lower status monkeys can get as much food as they want but seem to have less of a desire to eat, perhaps because of the higher level of stress hormones in their brain. The anxiety of constantly toadying to their social superiors seems to curb their appetite, researchers suspect, at least when their regular high-fiber, low-fat chow is on the menu.
But suppose you tempted them with the equivalent of chocolate and potato chips and ice cream? Mark Wilson, a neuroscientist at Emory University, and a team tried that experiment at Yerkes by installing feeders with a constant supply of banana-flavored pellets — not exactly Dove bars, but they had enough sugar and fat to appeal even to human palates. (In the interest of science, I sampled a few pellets.)
Once these foods were available, the low-status monkeys promptly developed an appetite. They began eating significantly more calories than their social superiors. While the dominant monkeys dabbled in the sweet, fatty pellets just during the daytime, the subordinate monkeys kept scarfing them down after dark.
These results may not surprise any stressed-out wage slave who has polished off a quart of Häagen-Dazs at midnight while contemplating the day's humiliations. But the experiment intrigues scientists studying human junk-food binges, which are hard to understand because there are so many confounding factors.
Monkeys' cravings aren't so complicated. The female monkeys weren't dieters who tasted one forbidden food and then couldn't stop themselves from binging. They were not rebelling against the thin mandate from tyrannical fashion magazines. They weren't choosing junk food because they couldn't find healthier fare. They weren't seduced by commercials telling them they deserved a break today.
For the monkeys the situation seems simple. They get some sort of comfort that is particularly appealing to the subordinate monkeys. One possibility is that the fatty foods help block the monkeys' stress responses. Studies with rodents have shown that high-calorie foods cause a metabolic change that tamps the release of stress hormones like cortisol.
Another possible explanation, the one favored by the Yerkes researchers, is that the snacks activated the reward pathways in the brain. They may have provided the same sort of dopamine reward as cocaine, which was studied in a previous experiment with monkeys by researchers at Wake Forest University.
In that experiment, the dominant monkeys didn't show much interest in pressing a lever that administered an intravenous dose of cocaine. But the subordinate monkeys, who started off with compromised dopamine receptors, kept pushing the lever to get more cocaine, just as the subordinates in the new study kept munching on the fatty pellets. Dr. Wilson suggests that the snackers are reinforcing the dopamine systems that had been diminished by stress.
"Essentially, eating high-calorie foods becomes a coping strategy to deal with daily life events for an individual in a difficult social situation," Dr. Wilson said. "The subordinates don't get beat up, but they get harassed by high-ranking monkeys. If they're sitting somewhere and a dominant monkey comes over, they give up their seat and move away. They're always looking over their shoulders."
These results seem to jibe with the famous Whitehall study of British civil servants, which found that lower-ranking workers were more obese than higher-status workers. Even though the subordinate workers were neither poor nor lacked health care, their lower status correlated with more health problems.
The new monkey data also jibe with an American study that looked at women's snacking tendencies. After they worked on puzzles and recorded a speech, the women were tempted with an array of chocolate granola bars, potato chips, rice cakes and pretzels provided by the research team, led by Elissa Epel, a psychologist at the University of California, San Francisco.
The women who seemed most stressed by the tasks, as measured by their levels of cortisol, ate more of the sweet, high-fat snacks, the same pattern observed in the subordinate monkeys with high cortisol levels. But as Dr. Wilson and others caution, there are plenty of other factors besides status and stress that affect humans' diets and waistlines.
Debra A. Zellner, a psychologist at Montclair State University, tested both men and women by putting bowls of potato chips, M&Ms, peanuts and red grapes on a table as the participants in the study worked on solving anagrams. Some of the people were given unsolvable anagrams, and they understandably reported being more stressed than the ones given easy anagrams.
The stress seemed to affect snacking in different ways for each sex. The women given solvable puzzles ate more grapes than M&Ms, while the women under stress preferred M&Ms. The men ate more of the high-fat snacks when they were not under stress, apparently because the ones who got the easy anagrams had more time to relax and have a treat.
Dr. Zellner says these gender patterns are probably because of a simple difference between the sexes: more of the women were on diets. Previous studies have shown that such "restrained eaters" are more likely than nondieters to keep scarfing snacks once they yield to temptation. This might be because they're hungrier, but it might also be because all restraint disappears once a diet is broken — the "what the hell" theory of binging.
Humans are not as lucky as monkeys in one way. "Female humans report that they eat high-calorie foods to make themselves feel better when stressed," Dr. Zellner says, "but they actually don't feel better after eating them. Instead, because they are restrained eaters, they feel guilt and actually feel worse. Female monkeys don't have that cognitive baggage."
Only the monkeys, it seems, find comfort in comfort foods.
