Dogged dieting: low-cal canines enjoy longer life. (This Week).

Holding back on the chow may be key to prolonging your pet's life. That's the message from a recently completed study of Labrador retrievers.

Since the 1930s, researchers have collected evidence that restricting the diets of rodents and invertebrates can extend their lives and delay the onset of age-related illnesses. In tests, the animals are typically fed a nutritionally complete diet that contains up to 40 percent less carbohydrate, fat, and protein than that given to control animals (SN: 10/5/91, p. 215). A handful of experiments in longer-lived animals, mainly rhesus monkeys, have yielded preliminary evidence that the diet-longevity link extends to larger animals, but most of these studies remain years away from completion (SN: 11/25/00, p. 341).

In the meantime, the Labrador study provides enlightening results.

In 1987, researchers led by Dennis F. Lawler at the Purina Pet Institute in St. Louis began a study of man's best friend. Lawler and his colleagues paired 48 retriever puppies by weight and gender. Between the ages of 8 weeks and 3.25 years, one of each pair was permitted to eat as much as it desired; its pair-mate was fed 75 percent that amount. From 3.25 years onward, food given to the all-you-can-eat group was limited to prevent obesity, while the low-calorie animals received 75 percent as much as the others.

Throughout the dogs' lives, the researchers monitored weight, health, and biochemical indicators of aging. The median lifespan of the diet-restricted dogs--the age by which half had died--was 13 years. That's about 22 months longer than that of their pair-mates, says Lawler.

The researchers also found that common age-related afflictions, such as cancer, osteoarthritis, and liver disease, hit the diet-restricted dogs at older ages and with less frequency than they did the pair-mates. The Purina team details its work in the May 1 Journal of the American Veterinary Medical Association.

Finding "a significant difference in median life expectancy is a dramatic and exciting result," says Joseph W. Kemnitz, director of the National Primate Research Center at the University of Wisconsin-Madison.

"This is really the first time that anyone has completed an entire [lifespan study] in an animal bigger than a rat," says George S. Roth of the National Institute on Aging in Baltimore. He notes that even though the calorie-restricted retrievers lived longer on average, no individual dog had a significantly longer lifespan than the longest-lived dogs in the fully fed group.

Just how calorie restriction leads to longer life isn't clear. Kemnitz speculates that it reduces the body's production of free radicals, which can expedite aging by damaging DNA and the cell's metabolic machinery.