I refuse to be made to feel guilty about yesterday’s bacon-enriched cornbread stuffing or the luscious leftover pumpkin pie, its spicy custard nestled in an all-butter crust and topped with fresh whipped cream, that I had for breakfast this morning. I don’t think you should feel guilty about your once-a-year feast either. Thanksgiving is my favorite holiday.

But then, in my life so far, I’ve been what University of Wisconsin researcher Scott Baum might call a pretty lucky monkey. Baum works at the university’s National Primate Research Center, in Madison, on a project monitoring the effects of a full-nutrition, extremely low-calorie diet on rhesus macaques. “Like the human population,” Baum says of the monkeys, “some are obese and some are not when eating everything they want.” Dieting monkeys in the study are given 30 percent fewer calories than normal (but with full nutrition); those in the control group get enough very healthy chow to eat as much as they want.

In September, I visited the primate center in Madison with a photographer for a photo essay about these dieting monkeys. Medicine professor Richard Weindruch is heading up the study of the diet, called calorie restriction. It has been known for decades that in mice, a diet of about 30 percent fewer calories than normal extends life span by about 30 percent, reduces the incidence of cancer, and prevents deterioration of learning and memory. Similar effects have been observed in organisms from yeast to fruit flies, but not yet in primates. Researchers of aging hope to discover the molecular mechanisms connecting the diet to healthy, long life and to mimic them with drugs–not to extend human life but to make old age healthier by eliminating suffering from cardiovascular disease, dementia, and other maladies. (See “Hungry Monkeys” for the photos, and “Do Dieting Monkeys Live Healthier and Longer Lives?” for more information on the study; see “Fountain of Health” for more on the effort to translate aging research into therapeutics.)

I met the Madison monkeys in the morning, before their first meal, when the dieters are most active. Baum put two of the monkeys–the stars of our photo essay–in clear cages so the photographer could get close-ups. The flabby control-diet monkey yawned and listlessly played with an orange dumbbell toy while his lean, calorie-restricted counterpart paced his cage in a darting, birdlike motion.

When the monkeys on calorie restriction heard Baum wheel in the food cart, they ran in circles in their cages and barked and squealed like a chorus of bull mastiffs and Chihuahuas; the others were alert but calm. When the calorie-restricted monkeys’ trays were filled with pellets, they seemed to eat them as fast as they could. I felt bad about delaying their meal, which was obviously the high point of the day.

There is strong evidence that the diet prevents diabetes, a major killer of captive rhesus monkeys, but it is too early to assess the diet’s effects on their life span. The Wisconsin researchers say it will take another ten years for survival differences to emerge.

Meanwhile, other research suggests that scientists hoping to tap into the molecular pathways that regulate aging are on the right track. A recent publication by David Sinclair at Harvard University shows that obese mice on a high-fat diet are almost as healthy as normal mice when they receive high doses of a compound found in red wine called resveratrol–and they seem to live longer (see “A Life-Extending Pill for Fat Mice”).

Wondering how long I’ll remain one of the lucky ones, I’ll continue to watch with great interest the work of Weindruch, Sinclair, and others in the field. And of course I’ll look forward to next Thanksgiving’s pie.