April 4, 2006 -- Trimming calories may affect certain markers of longevity, but it's not clear if that means living longer.
The news comes from a study in The Journal of the American Medical Association. The study included 48 overweight people in Baton Rouge, La.
Participants were split into four groups, three of which had their calories curbed to varying extents. Six months later, the dieters showed several changes in markers of longevity.
The study was too short to check life span effects. Restricting calories has been tied to longevity in rodents and other short-lived species. But it's not clear if that's also true for people, write the researchers.
They included Leonie Heilbronn, PhD, and Eric Ravussin, PhD. Heilbronn worked on the study at Louisiana State University's Pennington Biomedical Research Center, where Ravussin is a professor. Heilbronn now works at the Garvan Institute for Medical Research in Darlinghurst, Australia.
"I think it's exciting data," Ravussin tells WebMD, cautioning that he and his colleagues didn't prove that cutting calories lengthens life.
The study tracked "biomarkers of aging," Ravussin says, including blood sugar (glucose) levels, insulin (a hormone that controls blood sugar) levels, and core body temperature. The results showed fasting insulin levels, body temperature, DNA damage, and metabolism were lowered with calorie restriction.
"Those are preliminary data suggesting indeed calorie restriction in nonobese people can potentially extend life span in people," Ravussin says.
He adds the study was "way too short" and had "way too few subjects." The next step is to do a similar study lasting at least two years to get "a little bit more definitive answers," Ravussin says.
- Comparison group: No changes in daily calories
- Calorie restriction group: Calories cut by 25%
- Calorie restriction plus exercise group: Calories cut by 12.5%, calories burned in exercise raised by 12.5%
- Very low calorie group: Calories cut to 890 daily calories until 15% of body weight lost, followed by weight-maintenance diet
Participants were in their mid- to late 30s, on average. Their average BMI (body mass index) at the study's start was about 27, which is overweight but not obese.
Sticking With It, Losing Weight
The researchers tailored the diets to each participant and provided meals and snacks. Weekly group meetings and conference calls were also held to help participants stay on track.
Those assigned to exercise were told to walk, run, or bike five days per week. When their workouts weren't supervised, they wore heart rate monitors to prove they had exercised.
All but two participants stuck with the study. One dropout was in the comparison group. The other was in the very low calorie group.
All four groups lost weight. Here is each group's average percentage of weight lost during the six month study:
- Comparison group: 1%
- Calorie restriction group: 10%
- Calorie restriction plus exercise group: 10%
- Very low calorie group: 14%
All dieters also lost body fat. But slimming down wasn't the point.
"Our intention was really not weight loss," Ravussin says. Instead, he and his colleagues were more interested in the longevity markers.
By the study's end, fasting insulin and DNA damage had dropped for all participants whose calories had been limited, but not for the comparison group. The dieters also burned fewer calories than before the experiment, the study shows.
Core body temperature dropped for the calorie restriction and calorie restriction plus exercise groups. Core body temperature didn't change for the comparison group or those on the very low calorie diet.
Those changes went beyond what would be expected with weight loss, Ravussin says.
Longer, larger studies are needed to see if the effects last and affect human aging, the researchers note. A journal editorial agrees.
Editorialist Luigi Fontana, MD, PhD, works in St. Louis at Washington University's medical school and at Rome's Istituto Superiore di Sanita. Fontana and colleagues recently reported in the Journal of the American College of Cardiology that a low-calorie,may help hearts stay younger longer.
In his editorial in The Journal of the American Medical Association, Fontana says Heilbronn, Ravussin, and colleagues have shown for the first time that DNA damage declined in people with calorie restriction. The study adds "considerable information to current understanding of the effects of calorie restriction in humans," Fontana writes.
It's "not likely" that many people would adopt a calorie-restricted diet, he writes. But such studies suggest new clues about human aging.