A practical example of the use of different approaches to the analysis of energy metabolism in the mouse. (a) Twenty control mice were fed ad libitum (ad lib) and 58 mice were fed a calorie-restricted diet (60% of ad libitum) for three months⁴⁴. Raw data with the average metabolic rate were calculated without any correction for body mass. Note that the mice fed a calorie-restricted diet had a significantly (one-way ANOVA: P = 0.01) lower metabolic rate than the ones fed ad lib. (b) Metabolic rate expressed per gram of BW. Note that the opposite result was found: the mice on calorie-restricted diet had significantly (one-way ANOVA: P < 0.001) higher metabolic rates. (c) Resting metabolic rate (RMR) data as a function of body mass. Note that there is some overlap between the groups and a general positive trend of greater RMR at higher body masses (BM). (d) RMR versus fat-free mass (FFM) (measured by dual energy X-ray absorptiometry) shows a much greater overlap between the groups. (e) Dividing RMR by FFM revealed no significant effect of the treatment group on RMR (one-way ANOVA: P = 0.275). Values represent means ± s.e.m.

Problems of analysis and interpretation of energy expenditure demonstrated on hypothetical data. (a) Experimental genotype (W, white) is compared to the wild-type genotype (B, black). Study A and study B are different experimental manipulations of genotype. Energy expenditure per whole animal (kJ day⁻¹) is plotted against lean body mass (top). In all panels, lines show fitted regressions. Average of raw data across the individuals for each genotype plotted in a histogram (bottom). (b) Energy expenditure divided by lean body mass versus lean body mass is plotted for each data point (top) and shown as histogram (bottom). Values represent means ± s.e.m.

Flowchart for mouse energy metabolism phenotype analysis for mouse models. (a) Analysis with higher BW in comparison with wild-type littermate controls. (b) Analysis for mouse models with lower BW in comparison with wild-type littermate controls. EI, energy intake; EE, energy expenditure; FI, food intake; KO, knockout. *For discussion of advantages and pitfalls of pair-feeding, see Supplementary note 3.