School of Medicine investigators have found that eating a low-calorie yet nutritionally balanced diet lowers concentrations of a thyroid hormone and of a powerful inflammatory molecule, which ultimately leads to slowing the aging process.
Eating a calorie-restricted (CR) diet lowers concentrations of triiodothyronine (T3), a thyroid hormone that controls the body’s energy balance and cellular metabolism. It also decreases the circulating concentration of tumor necrosis factor alpha (TNFa), which occurs in many inflammatory diseases.
The investigators say the combination of lower T3 levels and reduced inflammation may slow the aging process by reducing the body’s metabolic rate as well as oxidative damage to cells and tissues.
Previous research on mice and rats has shown that both calorie restriction and endurance exercise protect them against many chronic diseases including obesity, diabetes, cardiovascular disease and some types of cancer. However, the research has shown that only CR increases the animals’ maximum lifespan by up to 50 percent.
These animal studies suggest that leanness is a key factor in the prevention of age-associated disease, but reducing caloric intake is needed to slow down aging.
For the new study, researchers examined 28 members of the Calorie Restriction Society who had been eating a CR diet for an average of six years. Although the CR group consumed fewer calories — averaging about 1,800 per day — they consumed at least 100 percent of the recommended daily amounts of protein and micronutrients. A second group of 28 study subjects was sedentary and ate a standard Western diet. A third group ate a standard Western diet, or about 2,700 calories per day, but also did endurance training.
The researchers found reduced T3 levels, similar to those seen in animals whose rate of aging is reduced by CR, only in the people on CR diets.
The findings are published in the online edition of the Journal of Clinical Endocrinology and Metabolism.
Interestingly, body fat levels did not affect serum T3 concentrations. The people in the CR group and the endurance-trained group had similar amounts and composition of body fat. But while the CR group had lower T3 levels, the endurance-trained group had T3 levels closer to those seen in the sedentary people on the standard Western diet.
“The difference in T3 levels between the CR group and the exercise group is exciting because it suggests that CR has some specific anti-aging effects that are due to lower energy intake, rather than to leanness,” said first author Luigi Fontana, M.D., Ph.D., assistant professor of medicine and an investigator at the Istituto Superiore di Sanita in Rome. “These findings suggest that although exercise helps prevent problems that can cut life short, such as obesity, diabetes and cardiovascular disease, only CR appears also to have an impact on primary aging.”
Primary aging determines maximal length of life. Secondary aging refers to diseases that can keep a person or an animal from reaching that expected lifespan.
This latest study targeted another marker of primary aging. The thyroid gland produces critical hormones that play an indispensable role in cell growth and development as well as in lipid and carbohydrate metabolism. Thyroxin, or T4, is the main product secreted by the cells of the thyroid gland, but most actions of thyroid hormone are initiated by T3. Fontana said T3 controls body temperature and cellular metabolism and appears to be involved with production of free radicals, unstable molecules that can damage cells. All are important aspects of aging and longevity.
Co-investigator John O. Holloszy, M.D., professor of medicine, and Fontana are preparing to launch a two-year study to look at the effects of calorie restriction. They are recruiting volunteers ages 25-45 who are willing to go on a CR diet for 24 months starting later this year.
Called the Comprehensive Assessment of the Long-term Effects of Reducing Intake of Energy (CALERIE) study, the goal is to get some clues about whether putting a normal-weight person on calorie restriction will lower their levels of inflammation and their serum concentrations of T3, improve their heart function and change other markers of aging, as Fontana and Holloszy have observed in members of the Calorie Restriction Society.
“We want to learn whether calorie restriction can reverse some of these markers of aging in healthy people,” Holloszy said. “It’s going to be many years before we know whether calorie restriction really lengthens life, but if we can demonstrate that it changes these markers of aging, such as oxidative damage and inflammation, we’ll have a pretty good idea that it’s influencing aging in the same way that CR slows aging in experimental animals.”
For more information or to volunteer for the study, call 747-3181 or 747-3180.