Fto and Postnatal Growth Figure 6. Metabolic parameters in Fto complete knockout mice. (A) Average hourly oxygen consumption of 16,17-week-old male Fto+/D and FtoD/D mice during the light and dark period. P = 0.1044 (light), and P = 0.0098(dark). (B) Average hourly oxygen consumption divided by lean mass. P,0.0001. (C) Average hourly carbon dioxide production of 16,17-week-old male Fto+/D and FtoD/D mice during the light and dark period. P = 0.0463 (light), and P = 0.011(dark). (D) Average hourly carbon dioxide production divided by lean mass. P,0.0001. (E) Accumulative food intake over a period of 72 hours of 16,17-week-old male Fto+/D and FtoD/D mice. P = 0.3015. (F) Food intake divided by lean mass. P = 0.0001. The number of mice used was 7 for each genotype. Statistical analyses were performed by unpaired t-test. **P,0.01. All values are mean 6 s.e.m. doi:10.1371/journal.pone.0014005.g006 power and the oversimplification of normalization method [39,40]. We also measured the metabolic rate (in terms of O2 consumption and CO2 production) using indirect caloriometry. While the unnormalized O2 consumption and CO2 production rates were decreased in Fto knockout mice, the correction for lean mass using either direct division or ANCOVA reversed the result, pointing to an increase of metabolic rates in the mutant mice. On the other hand, both ours and the previous study all indicated an increase in food intake caused by Fto deficiency [31]. A more recent report demosntrated that manipulating the FTO levels in the arcuate nucleus of hypothalamus in rats could affect the food intake [33]. Thus, it appears that the status of Fto has an impact on both the energy expenditure and food intake. In the absence of Fto, both of the two parameters are increased. Given the trend of more fat accumulation in our Fto knockout mice, the increase in the energy expenditure seemed overwhelmed by the increase in food intake. A number of human studies suggested that the risk SNPs in FTO associate with increases in food intake [16–21], and rejected an effect on energy expenditure [18–20,22]. It should be noted that the effect of those SNPs on FTO itself is still unknown. Thus, the phenotypes of Fto knockout mice do not necessarily contradict with the results from the human studies. The SNPs in FTO that are associated with obesity in humans reside in the intronic region and their effect on the function of FTO remains elusive. The limited expression studies in humans PLoS ONE | www.plosone.org 8 November 2010 | Volume 5 | Issue 11 | e14005