Over the course of the study the vibrated rats were healthy, tolerated the vibration well and exhibited no obvious signs of distress following adaptation to vibration. Twelve weeks of whole-body vibration resulted in significant body composition differences between the vibration and control groups. The whole-body vibration group weighed less, had less body fat and had a lower overall percentage of body fat than the age-matched non-vibration group. In addition, the vibrated rats had lower serum leptin levels than the control animals. However, differences in food intake were not observed between the two groups.
Serum leptin concentrations are strongly correlated with body fat and body mass index (BMI).29
Leptin acts on peripheral tissue and increases the inflammatory response by stimulating the production of tumor necrosis factor-α, interleukin-6, and interleukin-12. As a consequence, elevated leptin levels are considered to be an independent risk factor for increased cardiovascular disease, prostate cancer and breast cancer.30,31
Thus, the observed large reduction in serum leptin levels suggests that whole-body vibration may have beneficial effects on disease risk that are disproportionately large compared with the relatively small reduction in body weight.
Epidemiologic, cross-sectional and prospective correlation studies suggest an essential role for physical activity in weight-loss maintenance, and post-hoc
analysis of prospective trials shows a clear dose–response relationship between physical activity and weight maintenance.5
The mechanism responsible for the reduced fat accumulation in the vibration group is not known. Whole-body vibration has been proposed to increase energy expenditure via repetitive muscle contractions.5
Whole-body vibration has been reported to upregulate energy metabolism through an increased oxygen uptake comparable to moderate walking11,12
and corresponding increases in heart rate response, rating of perceived exertion and blood lactate levels expected when performing moderate exercise.11,12
Thus, increased energy expenditure without a concomitant increase in food consumption could explain the reduced weight gain in vibrated rats.
Previous whole-body vibration studies showing effects on bone mass and muscle strength have used frequencies ranging from 20 to 90 Hz.25,32–34
Recently, Rubin et al
demonstrated that growing mice receiving 15 weeks of high-frequency, low-magnitude mechanical vibration at 90 Hz, 0.2-g
peak acceleration developed far fewer fat cells than controls not receiving the treatment. Similar to our study, Rubin et al
. reported no difference in food intake between the two groups of animals. This suggests that vibration does not affect appetite in spite of decreased leptin levels and that an increase in energy expenditure may attenuate or decrease fat cell accumulation. Moreover, at the termination of the Rubin et al
the vibrated mice had nearly 28% less fat in the torso than control animals. In addition, levels of fatty compounds linked to type 2 diabetes, such as triglycerides and free fatty acids, were reduced by 43 and 39%, respectively, in the livers of the vibrated mice.13
The present study extends this work by showing that vibration also reduces fat accumulation in mature rats fed a normal diet.
Increased body weight is associated with higher BMD and lower incidence of hip fractures in older men and women,35
whereas the converse is true for low body weight. Weight loss is a risk factor for osteoporosis.14
The mechanisms for the osteo-protective effect of body weight are not completely understood. Data suggest that this may be a result of increased skeletal loading and/or increased plasma levels of bone-active hormones (for example, estradiol),35
as body weight and estradiol levels are considered to be major factors determining BMD in both women and men.35
Whole-body-vibration has been reported to increase bone mass in disabled children,34
young women with low bone mass32
and postmenopausal women.33
Thus, whole-body-vibration may potentially increase bone mass similar to increases associated with weight gain by mimicking the increased skeletal loading associated with increased weight. We did not detect an increase in total body bone mass in this study. However, the increase in vertebral BMC and BMD suggests that vibration may have resulted in local increases in bone mass and density. Importantly, our findings indicate that prevention of weight gain by vibration had no negative impact on muscle or bone mass.
Vibration reduced fat mass but not lean mass. Even though the whole-body vibration group weighed less, had less fat mass and a lower percentage of body fat, no differences were observed for lean mass and bone mass between the control and whole-body vibration group. Some studies have suggested that up to 30% of weight loss because of energy restriction may be due to a reduction of lean mass.36
Thus, it is possible that a weight reduction program supplemented with whole-body vibration may positively affect body composition by increasing the rate of fat loss while preserving lean mass. Further research is needed to test this possibility.
No differences in lean mass between the whole-body vibration and control groups are consistent with our finding that vibration had no effect on the mass or function of the oxidative SOL and glycolytic EDL muscles. There was a tendency for EDL mass and tetanic force to be reduced by 5–6% in the vibration group. Although our statistical power for this comparison was low (0.42–0.45), these findings deserve further investigation because whole-body vibration has been proposed to augment strength gains during human resistance training exercise programs.24,25
Previous whole-body vibration studies37,38
have reported acute increases in plasma concentrations of growth hormone and serum IGF-I. In our study, whole-body vibration had no effect on IGF-I levels. However, both the whole-body vibration and control groups had lower IGF-I levels than the baseline group. This is consistent with previous research showing serum IGF-I levels to decline with age in a predictable manner in healthy individuals. After reaching a peak during the pubertal growth spurt, mean circulating IGF-I levels decrease twofold by the third decade and continue to steadily decline with age.39
As our treatment groups were 12 weeks older than the baseline group, we attribute the lower IGF-I levels in the older rats to the natural effects of aging.
In conclusion, these studies show that whole-body vibration reduced age-related increases in fat mass in mature rats. This effect occurred without changes in lean body mass, bone mass or muscle mass and function and without alterations in food intake. These findings suggest that whole-body vibration exercise suppresses the accumulation of fat in young adult rats. Further studies are necessary to determine whether whole-body vibration can prevent fat accumulation in humans. If successful, this research could have implications in cases such as preventing childhood obesity and other metabolic disorders associated with weight gain.