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Division of Energy Balance and Obesity, Aberdeen Centre for Energy Regulation and Obesity, Rowett Research Institute, Bucksburn, Aberdeen AB21 9BS, United Kingdom. E-mail: email@example.com
Objective: Mice divergently selected for high or low food intake (FI) at constant body mass differ in their resting metabolic rates (RMRs). Low-intake individuals (ML) have significantly lower RMR (by 30%) compared with those from the high-intake line (MH). We hypothesized that MLs might, therefore, be more likely to increase their body and fat mass when exposed to a high-fat diet (HFD).
Research Methods and Procedures: We exposed both lines to a diet with 44.9% calories from fat for 3 weeks while measuring FI, fecal production, and body mass and then returned the mice to standard chow.
Results: When exposed to the HFD, both lines significantly decreased their FI (MH, 40% to 45%; ML, 31% to 35%). This decrease occurred simultaneously with a significant increase in apparent energy absorption efficiency (AEAE). When returned to chow, FI and AEAE returned to the levels observed prior to HFD exposure. Because of the adjustments in FI, the absorbed energy was maintained in the MLs and, thus, body mass remained constant. The MH individuals overcompensated for the elevated energy content and AEAE on the HFD and, therefore, absorbed lower energy than when feeding on chow. These mice also did not significantly change their body mass when on the HFD and must have made adjustments in their energy expenditures. Both lines and both sexes increased in fat content on the HFD, but these effects were not different between lines or sexes.
Discussion: We found no support for the hypothesis that mice with low RMRs were more susceptible to weight gain when fed the HFD.
The largest component of the daily energy budget in individuals from western societies is resting metabolic rate (RMR),1 which accounts for between 60% and 80% of the total energy expenditure (measured by doubly labeled water) (1). Although much of the interindividual variation in resting metabolism can be accounted for by differences in lean tissue mass (2,3), there is still considerable residual variation once the effects of lean tissue mass are removed (4). Typical estimates of the residual coefficient of variation are ∼7%, meaning that those individuals with the highest 5% residual RMR expend ∼28% more energy each day on resting metabolism than individuals with the lowest 5% residual RMR. This difference amounts to ∼2 MJ/d (4). Such differences in RMR would be anticipated to have an enormous impact on energy balance if they are not compensated by differences in food intake (FI). In particular, it might be anticipated that individuals with low residual RMR would be susceptible to developing positive energy balance and would, thus, be susceptible to developing obesity. Direct evidence in this respect, however, has been mixed, with some longitudinal studies indicating that individuals with low RMR are susceptible to subsequent development of obesity (5,6,7,8), whereas other similar studies have failed to find such an effect (9,10).
A second disputed factor believed by some to be important in the etiology of obesity is the intake of dietary fat because the prevalence of obesity has risen dramatically with no recognized change in the amounts of fats eaten (11,12,13). Evidence with respect to the role of dietary fat is similarly mixed. This variability in response to high fat intake is the case not only in cross-sectional studies of humans but also in studies of animals, thereby eliminating the putative role of many potential social factors in these effects. Most domestic rodent strains reduce their intakes of energy when faced with high-fat diets (HFDs) (14), but in many cases, this reduction is insufficient to match their energy expenditures (15,16), and, thus, the majority show profound susceptibility to obesity (17); others, however, show remarkable resistance to HFDs (18), maintaining their body masses in the face of diets that contain up to 60% fat by calories. Similar resistance to HFD-induced obesity is evident in other species of nondomesticated small rodents such as the bank vole (Clethrionomys glareolus) (19), the marsupial mouse (Smithinthrops crassicaudata) (20), and Shaw's jird (Meriones shawi) (21).
One possible explanation of these confused data is that the two factors of resting metabolism and HFDs interact with each other. In other words, individuals that have low metabolic rates may be susceptible to developing obesity, but this difference is most likely to manifest itself when those individuals are exposed to an HFD. Moreover, HFDs may promote obesity, but this effect will be most evident in those that have low metabolic rates. This would potentially explain why some strains of mice and other rodents show differential susceptibility to weight gain on HFDs, and also why studies of links between residual RMR and development of obesity are similarly variable.
To test this interaction-of-factors hypothesis, we have examined in the present paper the responses to an HFD of two mouse lines that were divergently selected over 38 generations for high and low FIs at a fixed body mass (22,23). A known correlated response to this selection is that the high-intake line has RMRs that are ∼30% higher than mice in the low line (24). If low metabolism is a predisposing factor to obesity that is revealed only on exposure to an HFD, we predicted that when given ad libitum access to an HFD, the low-intake mice would be less able to compensate their caloric intake to match expenditure and would consequently gain more body fat than mice in the high RMR line.
Research Methods and Procedures
The origins of the mouse lines used in this study have been described previously (22). In brief, mice were divergently selected for over 38 generations for high and low FI. The origin of the selection lines was a three-way cross between an outbred (CFLP) and two inbred (CBA and JU) lines (25). The selection objective was specifically to increase (maintenance high, MH) or decrease (maintenance low, ML) FI between 8 and 10 weeks of age to target the maintenance needs in adult mice. The FI was corrected for body weight to counterbalance correlated selection effects on growth. For the first 23 generations of selection, three replicate lines in each direction were maintained. At generation 24, the replicates within direction of selection were crossed, and selection continued to generation 38 in a single replicate in each direction of selection. After 38 generations, the mouse lines underwent inbreeding by strict full-sibling mating to develop lines for the subsequent use in mapping studies. High-FI mice consumed, on average, 47% more food (standard rodent chow) than low-FI individuals (22).
A palatability study was performed to determine whether the mice found either of the diets aversive. All of the mice had previously experienced only the chow diet before the palatability study commenced. Eleven high-line and eight low-line male mice were used in this study. All animals were individually housed in Tecniplast 1290D cages (Tecniplast USA, Inc., Exton, PA) for the duration of the measurements. Because the mice had not previously experienced the higher fat diet (pelleted rodent diet D12451, Research Diets, Inc., New Brunswick, NJ), we were concerned that they might show a preference for the chow diet only because it was their usual food. To accustom the mice to the HFD, therefore, we alternated the food available to the mice between the two types daily for 7 days. The animals were then provided with a choice for 10 days, where both diets were available simultaneously in their hoppers. The position of the diet relative to the water bottle was altered daily to remove any positional biases. Daily FI for both diets was recorded at the same time each day. Gross compositions of the two diets are presented in Table 1. It should be noted that the HFD also had a higher sugar content than the chow diet.
Table 1. . Diet composition
Nutrient (% by mass)
Composition for lard and soybean oil were taken from the U.S. Department of Agriculture National Nutrient Database for Standard Reference, Release 15 (August 2002), http:www.nal.usda.govfnic.
Fat (% by energy)
Energy content (kcal/g)
Capric acid (C10:0)
Lauric acid (C12:0)
Myristic acid (C14:0)
Palmitic acid (C16:0)
Stearic acid (C18:0)
Myristoleic acid (C14:1)
Palmitoleic acid (C16:1)
Oleic acid (C18:1)
Gadoleic acid (C20:1)
Linoleic acid (C18:2)
Linolenic acid (C18:3)
Arachidonic acid (C20:4)
Clupanodonic acid (C22:5)
Fifteen 22- to 26-week-old mice of each line and sex were used to examine changes in body mass and FI over a 3-week period while on a normal diet or HFD. A further three to five mice of each sex and line were used to examine the digestive efficiency on each diet. The experimental animals were individually housed in M3 cages (NKP, Kent, United Kingdom) under a 12-hour light/12-hour-dark photoperiod at 20 ± 2 °C. All animals were provided with ad libitum food and water throughout the study.
After separation into individual cages, animals were provided with pelleted rat and mouse breeder and grower diet (Special Diets Services, BP Nutrition, Essex, United Kingdom), which will be referred to as chow diet for the rest of this paper. This diet had a gross energy content (GE) of 17.4 MJ/Kg (or 9.2% fat by energy) (26,27). Once accustomed to the new housing conditions (2-week acclimation), body mass and FI were measured daily (± 0.1 grams) over a 3-week period. Body temperature was also recorded (Model 2751-K and K type thermocouple; Digitron Instrumentation Ltd., Hertfordshire, United Kingdom) on a twice-weekly basis during a period when the mice were generally at rest and when the body temperature was at its lowest (between 2 and 4 pm).
The diet was then changed to pelleted rodent diet D12451 (Research Diets Inc.), which has a GE of 20.77 MJ/Kg (or 44.7% fat by energy), for a further 3 weeks; finally, the mice were returned to the chow diet for a final 3-week recovery phase. The same measurements of body mass, FI, and body temperature were recorded over these 3-week periods. Gross compositions of the two diets are presented in Table 1.
Dietary efficiency of the mice was measured on three to five animals of each line and sex by examining the GE of the diets and fecal deposits by adiabatic bomb calorimetry. To achieve this, each animal was housed individually in M3 cages where they underwent the same diet regime as previously described, although on Day 18 of each diet, they were provided with a clean cage and reduced bedding of wood shavings. Seventy-two hours later, these cages were removed, and new cages were provided. The feces collected over the 72-hour measurement period were separated from the wood shavings, weighed (±0.001; Ohaus analytical balance, Ohaus, Cambridge, United Kingdom), and dried in an oven along with a food sample (Gallenkamp, Loughborough, United Kingdom) at 60 °C for a period of 3 weeks. All fecal and food samples were again weighed to enable the percentage of water in the diets and feces to be calculated before analysis by Adiabatic Bomb Calorimetry (Gallenkamp).
Apparent dry mass absorption efficiency (ADMAE) and apparent energy absorption efficiency (AEAE) were calculated using the following equations:
An additional 6 to 12 mice of the same age, sexes, and strains used in the above protocol were culled after feeding on chow for 3 weeks or after feeding on chow for 3 weeks followed by 3 weeks of feeding on the HFD. The total sample of animals in this experiment was 80. Body compositions were determined by drying the carcasses to constant weight at 60 °C for 14 days and then extracting their body fat using a Soxtech (Tecator, Sweden) apparatus with petroleum ether as the solvent.
Minitab 13 (Minitab Inc., State College, PA) was used for all data analysis. All data are shown as least squared means ± SE and were analyzed using one-way ANOVA or general linear model (GLM) for pair-wise comparisons.
There were few significant differences in our data that could be attributed to the different sexes within the same line; therefore, unless otherwise stated, the data are described for the line effect alone, and it can be assumed that any sex differences were not statistically significant (p > 0.05).
Both the high- and low-line mice consumed some of each diet throughout the palatability study (Figure 1), showing that they did not find either of the diets aversive. In both lines the mice ate significantly more HFD than chow [ANOVA, high-line F (1, 17), 222.7, p < 0.001; low-line F (1, 17), 329.5 p < 0.001], indicating that they had a stronger preference for this diet.
There was no effect of time (Figure 2) on the level of FI in the high line during the initial period of chow feeding [GLM, F (12, 171), 0.58, p = 0.87]. In the low line, the effect of time was significant [GLM, F (12, 171) = 3.16, p < 0.001]; however, inspection of the trends with time revealed that the significant day-to-day variability in the low-line individuals was random and not directional. Therefore, we took the mean FI of each individual over the 3 weeks of the initial chow-feeding period and explored the factors influencing the interindividual variation in these levels. There was a significant effect of body mass on FI (Figure 3) during the initial period of exposure to the chow diet [GLM, F (1, 55), 4.72, p = 0.034]. In addition to the effects of body mass, there were also significant effects of the selection line [GLM, F (1, 55), 54.12, p = 0.001] and sex [GLM, F (1, 55), 6.45, p = 0.014]. The sex-by-line interaction was not significant [GLM, F (1, 55) = 2.69, p = 0.107]. Mean FI of the high line averaged 6.8 grams (SD = 1.8 grams, n = 30), for the low line averaged 4.4 grams (SD = 0.43 grams, n = 15), and for the low-line males averaged 4.3 grams (SD = 0.36 grams, n = 30).
When the mice were provided with the HFD, all individuals decreased their dry FI (Figure 4). The pattern of decline was similar in both lines. However, the response was faster in the high-line individuals. For both the female and male high-line mice, it took 3 days before the intake reached a new asymptote (defined by performing sequential Tukey comparisons of intake on different days and establishing the point at which intakes were no longer significantly different). In the low-line individuals, however, it took 5 days for the females and 6 days for the males to establish a new asymptotic level of intake. Once the new level had been established, the decreases in the high line were much greater than the changes in the low line. On average, the high line decreased their dry weight of intake by 43% on the HFD (Figure 4) (p < 0.001). In contrast, the low line decreased their intake by only 33%, which was significant in the males (p < 0.001) but marginally failed to reach significance in the females (p = 0.09).
When the mice were returned to the chow diet, they increased their FI again (Figure 5). This response was also not immediate. In high line, it took between 4 and 5 days to reestablish a new asymptote. The corresponding value for low line was 4 to 6 days. Hence, the response on return to chow appeared to be slower than the initial response to fat, and the difference between the lines was compressed. In the high line, where the reduction in dietary intake had been greatest, the corresponding increase on return to chow was also greater and significant (p < 0.001). The increase in low-line females by 1.7 grams from 3.0 to 4.7 g/d was only 40% of the increase observed in the high-line females (p = 0.001), but the increase in low-line males of 1.3 grams from 2.9 to 4.2 g/d was only 35% of the increase observed in the high-line males, and this marginally failed to reach significance (p = 0.09). FI levels at the end of the recovery phase did not differ significantly from the FI during the initial exposure period for any of the four groups (p > 0.05).
During the initial period when the mice were feeding on the chow diet, average fecal production was 1.17 (SE = 0.04) g/d. This level did not differ significantly between the different sexes [GLM, F (1, 16) = 1.11, p = 0.312] or lines [GLM, F (1, 16) = 1.74, p = 0.209]; however, there was a significant sex-by-line interaction [GLM, F (1, 16) = 5.74, p = 0.032]. Because the high-line mice were eating more food but producing the same amount of feces, the ADMAE was significantly different between the lines. On average, the ADMAE was 84% and 77% in the high-line females and males, respectively, but equivalent values for the low-line males and females were only 73% and 75%. This line difference was significant in the females [one-way ANOVA, F (1, 28) = 135.04, p < 0.001) but not in the males [one-way ANOVA, F (1, 28) = 0.001, p = 0.97].
When the mice were switched to the HFD, their production of feces was dramatically reduced to, on average, only 0.33 g/d. The low line reduced their fecal production by a higher degree than the high line [GLM, F (1, 16) = 14.86, p = 0.002]; however there were no differences between the sexes [GLM, F (1, 16) = 2.27, p = 0.156] in this amount. This reduction was, in part, because the mice ate less food on the HFD, but also because their apparent dry mass assimilation efficiency was increased. Across all four groups, the ADMAE averaged 91%. There were no significant differences between the lines [GLM, F (1, 59) = 0.48, p = 0.49] or sexes [GLM, F (1, 59) = 0.12, p = 0.73] in the ADMAE on the HFD.
The GE of the diets was 17.1 kJ/g for the chow diet and 20.3 kJ/g for the HFD. The energy contents of the feces were lower for mice on the chow diet (16.3 kJ/g) than for mice on the HFD (17.6 kJ/g). Combining these estimated energy contents with the dry matter absorption values calculated above, we were able to estimate the AEAE (Table 2). These trends mirrored almost exactly those for ADMAE, but, on average, because fecal energy contents were lower than the food, the AEAEs were ∼2% to 3% higher, averaging 79% on the chow diet and 94% on the HFD.
Table 2. . Least squared means for each parameter grouped by sex, line, and diet (±SE) over the 3-week measurement period
SE averaged over groups. Means sharing a common letter (a to i) are not significantly different (p > 0.05).
Dry food intake
Dry fecal deposits
Energy Intake and Absorption
There were no significant differences in the energy intake and energy absorption rates in the same line, between the initial chow-feeding phase of the study and during the recovery phase when the mice were also feeding on chow. Because there were no directional time effects on FI, there were similarly no directional effects on energy absorption; therefore, we calculated the mean daily energy intake and absorption for all individuals during the initial chow phase. Energy intake was a direct function of FI; thus, statistical effects of body mass, line, and sex were identical to those described above for FI. However, for energy absorption, the effects were different because different groups absorbed the energy in the diet with different efficiencies. There were significant body mass [GLM, F (1, 55) = 5.54, p = 0.022], line [GLM, F (1, 55) = 74.92, p = 0.001], and sex [GLM, F (1, 55) = 10.85, p = 0.002] effects, as well as a significant line-by-sex interaction [GLM, F (1, 55) = 8.23, p = 0.006] on energy absorption rates. The high-line females and males ingested, on average, 122 and 107 KJ/d, compared with only 75 and 73 KJ/d for the low-line females and males. Equivalent values for energy absorption rates were 104 and 83 KJ/d for the high-line females and males and 55 and 56 for the low-line females and males, respectively.
When exposed to the HFD, both lines significantly reduced their gross intake of energy. The patterns were identical to those for FI. The reductions relative to the initial chow feeding phase once the new asymptotes had been reached were 41.3 and 31.2 KJ/d in the high-line females and males, respectively, which was equivalent to 32.8% and 28.4% relative to the initial intakes. In the low-intake line, the reductions were more modest at 13.3 and 15.8 KJ/d for females and males, respectively, which were 17.1% and 21.9% of the initial energy intakes.
Because the AEAE on the HFD was much greater than for the chow diet, the reduction in gross energy intake on high fat was not translated uniformly into a reduction in the apparent net energy absorption. Assuming that the changes in energy absorption measured at the end of each phase happened immediately after the diets were switched, there was a transient increase in energy intake during the period immediately after the switch to high fat and a transient decrease in energy absorption after the return to chow. These transient changes corresponded to the times during which the animals adjusted their FI levels to reestablish new asymptotic intakes (Figure 6; Table 2).
For the low-intake line, once the new asymptotic level had been reached, the apparent net energy absorption did not differ significantly between the periods when the mice were feeding on the chow and high-fat foods. In low females, the apparent net energy absorption averaged only 2.9 KJ/d higher on the HFD (p = 0.97), and in low males, it was 1.7 KJ/d lower (p = 1.00). The low-line individuals, therefore, adjusted their intake almost exactly to maintain their apparent net energy absorption rates constant. However, even taking into account the increase in the AEAE on the HFD for the mice from the high line, there was still a reduction in apparent net energy absorption at the new asymptotic level when the mice were feeding on this diet. In females, the reduction amounted to 27.9 KJ/d (22.0% relative to initial phase), and in the males, the reduction was more modest at 11.6 KJ/d (10.5% relative to the initial phase). The difference in females was highly significant (p < 0.001) but in males marginally failed to reach statistical significance (p = 0.073). The high line seemed, therefore, to respond differently to the HFD, overcompensating their reduction in energy intake with a resultant lower energy absorption rate that in females was significant.
Body Mass and Composition
During the initial chow feeding phase, there was no significant difference in body mass between the high and low lines (Figure 7) [GLM, F (1, 55) = 0.71, p = 0.404] (Table 2). However, there was a significant sex effect, with males, on average, being heavier than females [GLM, F (1, 55) = 30.1, p = 0.001]. The line-by-sex interaction was not significant. When exposed to the HFD, the body masses of all four groups increased slightly (by 1.7 grams for the high-intake line and by 1.1 grams for the low-intake line). In both lines, the increase in body mass was not significant (p > 0.05). On return to the chow diet, these slight increases in body mass continued, so that after 3 weeks of recovery, the high line had increased by a further 0.9 grams, and for the low-line individuals, the increase averaged 1.0 grams. Again, neither of these changes achieved statistical significance.
There were significant sex and line effects on body fatness, with female mice having a higher body fatness than males [GLM, F (1, 76) = 53.83, p < 0.001] and low-intake mice having a higher body fatness than high-intake mice [GLM, F (1, 76) = 20.69, p < 0.001] (Table 3). There was also a significant effect of diet, with each group having a higher fat content after 3 weeks of high-fat feeding compared with feeding on chow only [GLM, F (1, 76) = 27.34, p < 0.001]. The interactions between sex and diet [GLM, F (1, 76) = 1.49, p = 0.23] and between line and diet [GLM, F (1, 76) = 0.06, p = 0.80] were not significant. This indicates that body fat increase followed the same pattern in all four groups and that neither line was more prone to fat deposition on the HFD. The increases in deposited fat, averaging between 1.0 and 4.7 grams in the different groups, were offset by reductions in the body water contents in all four groups of between 0.3 and 2.7 grams and changes in the lean tissue mass that increased in one group by 0.3 grams but decreased in the other three by between 0.3 and 1.5 grams. Together, these opposite trends in the changes in fat mass and other body components meant that the differences in total body mass were much lower than the changes in the fat mass when the mice were exposed to the HFD and were consistent between the two experiments.
Table 3. . Body composition on each diet
High fat (g)
18.29 ± 0.46
25.28 ± 0.36
4.39 ± 0.49
17.53 ± 0.30
24.54 ± 0.70
9.09 ± 0.57
21.87 ± 0.33
30.72 ± 0.49
5.41 ± 0.51
20.35 ± 0.25
29.66 ± 0.75
9.02 ± 1.03
18.06 ± 0.40
25.17 ± 0.56
7.89 ± 0.66
15.26 ± 0.32
23.67 ± 0.78
12.67 ± 1.35
19.63 ± 0.34
27.15 ± 0.50
5.63 ± 0.46
19.37 ± 0.86
27.33 ± 1.32
6.58 ± 0.56
Body temperature (Table 2) was slightly higher in females than in males of the same line [GLM, F (1, 210) = 8.31, p = 0.004], and body temperature decreased when on the HFD in comparison with the chow diet [GLM, F (1, 210) = 6.23, p = 0.002].
The two selection lines that we used in this experiment had been previously selected for >38 generations for divergence in FI at fixed body mass. The 30% to 40% difference in FI between the lines when initially exposed to the chow diet was, therefore, entirely consistent with this selective background. In fact, Hastings et al. (22) estimated the divergence in FI between the lines to be 30%, and Bunger et al. (23) found a similar divergence of 34%, both of which are in the same range as the divergence we found. The difference in apparent dry matter absorption efficiency between the lines, with the high line having greater absorption efficiency in addition to greater intake, has not been previously described. In particular, both ADMAE and AEAE were higher in the high- compared with the low-line females. This difference in absorption efficiency disappeared, however, when they were fed the HFD. The reasons for these differences and the changes on an HFD are unclear. Previous measurements indicate that the livers of high-line mice are larger, but there are no gross morphological differences in the sizes of the alimentary tracts between the lines (24). The difference in FI between these lines while at fixed body mass implies that energy expenditure of the high-intake line exceeds that of the low-intake line. Our previous measures have shown that part of this elevation is an increase in the RMR of the high-intake line (24).
Given this difference in RMR between the lines and our original hypothesis that the effects of RMR and fat intake might interact, we predicted that the low-intake lines would be susceptible to weight gain when exposed to an HFD. We used the Research Diets D12451 diet in our studies, which had been previously used in several studies to precipitate diet-induced obesity (28,29). We predicted, on the basis of previous studies exposing mice to HFDs (17), that the intake of energy would be greater than expenditure on the HFD and that this effect would have a more profound effect on weight gain in the low-RMR compared with the high-RMR individuals. However, although the HFD was highly palatable and preferred in a direct comparison with chow, we found that the mice from the low line, over a short period of ∼4 days, reduced their intake of energy to almost exactly match the altered energy content and absorption efficiency for this diet. Hence, there was no significant increase in body mass in the male or female mice from this group. The response of the high line was similar in that a profound reduction in intake occurred; however, this overcompensated for the increased energy and absorption efficiency of the diet so that the animals actually absorbed less energy on the HFD than on the relatively low-fat chow. The high-line mice also responded to the diet faster than the low-line mice. Because there was no significant change in the body mass of the high-line individuals over the 3-week exposure to the HFD, this suggests that the high-line individuals made additional modulations of their expenditure to balance their energy budgets. The duration of our study, including 3 weeks of exposure to high fat, was relatively short, but in other studies where rodents have been shown to be susceptible to weight gain on high fat, such an effect is already apparent after 3 weeks (29,30,31,32).
Our measurements of body composition in a separate group of animals exposed to the same protocol showed that, in fact, over the 3-week period, both lines and both sexes did deposit statistically significant amounts of body fat. This increase in fat was, however, obscured by reductions in both water and lean tissue contents (Table 3), resulting in the low overall change in total body mass. The differences in body fat were, therefore, masked when only body mass was measured in the original group of animals. Nevertheless, there were no differences between the lines in the levels of fat deposition (diet-by-line interaction not significant), further supporting our interpretation that differences in RMR did not make the low-RMR line susceptible to gaining body fat. The significant accumulation of fat, however, despite being partially offset in three of the groups by reductions in lean tissue mass, indicated that both lines failed to exactly match expenditure and intake. Understanding the modulations that occur in expenditure parameters of these lines under HFD is a future goal. Nevertheless, these data clearly indicate that the mice had an incipient capability to regulate their energy intake closely, despite the large change in the macronutrient content of the incoming diet.
The speed of response of the mice to the diet switch appeared to differ between the lines. If we assumed that the change in absorption efficiency was instantaneous, the failure of the mice to also instantaneously adjust their FI meant that we inferred a slight increase in total energy absorption during the switch to HFD and the reverse on the return to chow. In reality, it would be unlikely that the changes in apparent absorption efficiency would be instantaneous, and the slow responses of the individuals in their change in FI may have occurred because the animals were actually matching exactly their intake and expenditure during this transition phase to account for the changing absorption efficiency. The fact that body mass showed no parallel changes to these inferred changes in total energy absorption during these transitional phases after the diet switches supports this interpretation. The different rates of response between the lines, with the low-intake line adjusting their intake more slowly, suggests that the adjustment of absorption efficiency in this mouse in response to the diet was slower, which is understandable because the magnitude of the change in absorption efficiency was greater.
When exposed to HFDs, different strains of mice show a diversity of responses. From a study of nine inbred strains of mice 6 to 10 weeks old, the majority (n = 6) of the strains readily gained weight when presented with a diet containing 32.6% energy as fat. In most cases, however, this was generally due not to hyperphagia, because most strains reduce intake on high-fat feeding, but rather to a failure to accurately match the adjusted intake to expenditure (33). Other studies have also pointed to effects of sex and age on the responses to HFDs (34), but we found no differences due to sex in either strain studied here. The primary result of our study was that differences in RMR did not affect the likelihood of a strain being able to match intake with expenditure. In particular, we found that the strain with low RMR was not particularly susceptible to weight gain, compared with the strain that had a much higher RMR.
Since the discovery of leptin (35), considerable progress has been made in our understanding of the physiological control of FI (36,37). Nevertheless, despite these advances, our knowledge of the precise signaling mechanisms, which allow individuals to match their energy intake to their caloric requirements, is still rudimentary. Defects in this system are likely to underpin the inability of some mouse strains to resist fat gain when exposed to an HFD. In both of the lines we studied, this signaling system appears to be intact, suggesting that in both of these lines, there was good coupling between the level of RMR and the energy intake. However, the fact that some studies have highlighted that low RMR is a risk factor for developing obesity (5,6,7,8,38) suggests that close coupling of RMR to intake does not always occur. In the two mouse lines we studied, the close linkage of RMR to FI that we have revealed may be a function of the nature of the original selection process on the lines we used. In the selection process, animals were selected on the basis of FI corrected for body mass (22,23). The RMR change was a correlated response to this selection. Perhaps this selection process preserves the basic connection between the energy expenditure on RMR and energy intake, meaning that when we manipulated the macronutrient content of the diet, the animals were able to rapidly adjust their intake to match expenditure. This interpretation does not imply that the responses of the mice could have been predicted a priori from the process of selection. During selection, the high line increased both FI and RMR, whereas in the low line, both these traits were decreased. When exposed to the HFD, both lines responded by decreasing intake, almost preserving a match of intake to expenditure. If this matching was enabled by the presence of an intact linkage between RMR and intake, it would be interesting to explore the associations between RMR and diet in mice selected for RMR independently of FI differences to evaluate this hypothesis.
In conclusion, the mice from both high- and low-FI lines adjusted their FI to compensate for the greater energy density and higher AEAE when fed an HFD. In the low line, the adjustment maintained energy absorption at a constant level, and in the high line, a reduction in energy absorption was observed. Neither group responded to the HFD by increasing their body mass because of these rapid adjustments in intake, which took ∼4 days to develop completely. We found no support for the hypothesis that mice with low RMR were susceptible to an HFD, but this may reflect the nature of the original selection that generated the lines we worked on.
This work was supported by Wellcome Grant 060086/Z/99/Z. We thank the animal house staff (Duncan Wood, Shona Flemming, and Jim Levine) for the care of the mice, Wendy Peacock for information on the diet composition, and Christine Horrocks from the Rowett Research Institute for assistance with bomb calorimetry.
Nonstandard abbreviations: RMR, resting metabolic rate; FI, food intake; HFD, high-fat diet; GE, gross energy content; ADMAE, apparent dry mass absorption efficiency; AEAE, apparent energy absorption efficiency; GLM, general linear model; MH, maintenance high; ML, maintenance low.