eadily switch between purchase R-547 glucose to lipid utilization for oxidative phosphorylation. To further test their ability to switch substrates from glucose to lipids, we performed fasting/refeeding experiments. When mice are fasted, RER drops to 0.7 as lipids are used as oxidation substrates and following refeeding, RER climbs to 1.0 as glucose becomes the predominant energy source. SirT1-null mice switch readily between lipid and glucose substrates as evidenced by the efficient changes in RER. Interestingly, the day after refeeding, RER in SirT1-null animals started to decline much faster than that in normal animals suggesting limited glucose utilization even when food is freely available. The observation that SirT1-null mice have higher rates of lipid oxidation than normal is consistent with previous observations that in older SirT1-null mice the lipid content of cells in white adipose tissue are smaller than those of normal tissues. We compared the total weight of some lipid storage tissues in young and old mice. In young animals, the inguinal fat pads from SirT1null mice were smaller than those of their littermates 18645012 and this tissue increased in mass with age much more slowly than normal in SirT1-null mice. The interscapular BAT depot was similar in size in young and in older mice perhaps reflecting its role in energy dissipation. The 24646995 brain weight of SirT1-null mice is about 20% smaller than normal mice and in both young and older animals. These results coupled with observations noted above are consistent with the notion that SirT1-null mice burn stored lipids at rates in excess of those in normal animals. Hormonal profile of SirT1-null mice Systemic 
energy metabolism is under the control of hormones. Thyroxine levels were slightly lower in the SirT1-null serum whereas peak corticosterone levels were normal. Fasted glucose levels of SirT1-null mice were higher than those of normal littermates but insulin concentrations were not different. Three hours after re-feeding, the insulin levels of normal animals rose by nearly 30 times whereas the rise in SirT1-null mice was increased only 10 fold. The postprandial blood glucose concentration was similar or lower in SirT1-null compared to normal mice, suggesting that SirT1-null mice might be insulin sensitive. A similar result was observed previously. However, insulin tolerance tests did not reveal a difference between SirT1null and normal animals. Blood lactate Liver mitochondria from SirT1-null mice are less efficient than normal SirT1-null mice are hypermetabolic but lethargic, suggesting that their energy generation system might be defective. We investigated the state 3 respiration rate as well as proton conductance in isolated mitochondria from liver and skeletal muscle. We detected no significant differences in the mitochondria from skeletal muscle. However, the SirT1 and Caloric Restriction rate of respiration under state 3 conditions was lower in SirT1-null liver mitochondria suggesting that these mitochondria would produce less ATP at full capacity than those from normal mice. Measurement of proton motive force under state 4 respiration conditions revealed that it was lower in SirT1-null liver mitochondria. This suggests that the inner membranes of the SirT1-null mitochondria are relatively permeable to protons and that less proton motive force would be generated to produce ATP. Uncoupling proteins such as UCP2 can diminish proton motive force and SirT1 is known to inhibit the expression of uc