Submitted on November 12, 2007
Accepted on February 8, 2008
Food Restriction and Simulated Microgravity: Effects on Bone and Serum Leptin
Kyunghwa Baek1, Alicia A. Barlow2, Matthew R. Allen3, and Susan A. Bloomfield2*
1 Health and Kinesiology, Texas A&M University, College Station, Texas, United States; Nutrition and Food science, Texas A&M University, College Station, Texas, United States
2 Health and Kinesiology, Texas A&M University, Texas, United States
3 Dept of Anatomy & Cell Biology, Indiana School of Medicine, Indianapolis, Indiana, United States
* To whom correspondence should be addressed. E-mail: sbloom{at}hlkn.tamu.edu.
Leptin is responsible for linking energy metabolism to bone mass. Because astronauts are commonly in negative energy balance during spaceflight, this study was designed to assess individual and combined effects of food restriction and simulated microgravity on bone mass and serum leptin. Six-mo-old male Sprague-Dawley rats were randomly assigned to 4 groups (n=12 each); two hindlimb-unloading (HU) groups fed 100% (HU100) and 70% (HU 70), and two cage-activity control groups fed 100% (CC100) and 70% (CC70), of their baseline food requirement. After 28 days, CC100 rats gained body weight (BW), whereas all other groups lost BW; this loss was greater in HU70 vs CC70 & HU100. Serum leptin decreased in CC70 and HU100 (-60% and -27%, respectively) and was non-detectable in HU70. Percent osteoid surface in CC70 and HU100 was lower than that of CC100 (7.80%, 8.60% vs 10.70%, respectively) and this decrease was more pronounced in HU70 (4.38%). Mineral apposition rate of CC70, HU100 and HU70 was lower than that of CC100 (1.5, 1.6, 1.5 vs 2.1 µm/day, respectively). Bone formation rate (BFR) of CC70, HU100 and HU70 was lower than that of CC100 (13.4, 13.1, 12.2 vs 40.8 mm3/mm2/day, respectively). The change in BFR was correlated with the change in serum leptin value over 28d (r2=.69, p=0.0007). We conclude that moderate caloric restriction may cause bone loss at susceptible bone sites to a similar degree as does the unloading effect of microgravity; serum leptin may be an important endocrine regulator contributing to this change in skeletal integrity.
