New research suggests that while exercise helps ward off chronic ailments such as obesity, diabetes, and heart disease that can shorten a normal lifespan, only cutting calories appears to slow down the primary aging process.

Investigators at Washington University in St. Louis have found that eating a low-calorie, nutritionally balanced diet lowers concentrations of a thyroid hormone known as T3, which controls body temperature, cell metabolism, and it appears the production of free radicals - all of which are important aspects of aging and longevity. Calorie-restriction also decreases levels of the inflammatory protein tumor necrosis factor (TNF)-alpha.

The combination of lower T3 and TNF levels may slow the aging process, Dr. Luigi Fontana and colleagues report in the Journal of Clinical Endocrinology and Metabolism.

In earlier work, Fontana’s group found that both cutting back on calories and exercising protects lab animals from heart disease, diabetes and obesity. However, only cutting back on calories increases the animals’ lifespan. This hints that leanness is the key factor in preventing age-related disease, but that reducing the amount of calories consumed is needed to slow down aging.

The team’s latest research seems to supports this idea.

In the study, Fontana and colleagues compared thyroid hormone levels of 28 adults consuming a self-imposed calorie-restricted but nutritionally packed diet for 3 to 15 years, with that of 28 sedentary adults consuming a typical Western diet and 28 endurance athletes who were also consuming a Western diet.

The calorie-restricted group consumed about 1,800 calories per day, whereas those on the Western diet took in about 2,700 calories per day.

Fontana’s group observed reduced T3 concentrations only in the calorie-restricted group. Levels of two other thyroid hormones - T4 and TSH - were normal, indicating that the calorie-restricted group was not suffering from thyroid disease called hypothyroidism.

Interestingly, according to the team, body fat levels did not affect T3 levels. This is a key finding, they say, because it suggests that decreased T3 levels are likely related to calorie restriction not changes in body fat.

T3 concentrations were 30 percent lower in the calorie-restricted eaters compared with the exercisers, even though percent body fat was similar between the two groups.

The difference in T3 levels between the calorie-restricted group and the exercise group is “exciting,” Fontana said in a statement, “because it suggests that calorie restriction has some specific anti-aging effects that are due to lower energy intake rather than to leanness.”

SOURCE: Journal of Endocrinology and Metabolism, online May 23, 2006.