Do rats have an upper limit for caloric intake over a 24 hour period?
Background
The results from the 1st week of data gather for the binge eating study of Summer 2007 suggest that rats are limiting their daily intake of food to about 100 kcal. This is in contrast to the results of Corwin 2004 (whose protocol we are attempting to replicate) who reports that control animals consistently consume about 70 kcal per 24 hour period and when given access to Crisco consume up to about 100 kcal per 24 h. Corwin’s rats were maintained on a commercial rodent diet (Laboratory Rodent Diet 5001, PMI Feeds, Richmond, IN; percent of calories as protein: 28.05%, as fat: 12.14%, as carbohydrate: 59.81%; 3.3kcal/g). Our rats are maintained on Harlan Teklad 8604 (by weight: crude protein ≥ 24%, crude fat ≥ 4%, fiber ≤ 4.5%). Thus, it appears that daily caloric intake depends at least in part on the type of commercial rat chow provided. If rats actually do regulate caloric intake on a daily basis, and if that intake is homeostatically maintained at about 100 kcal/day, then our rats eating Teklad 8604 ad libitum are already consuming at the upper limit. Thus providing FL (which are of similar energy density) simply results in a reduction of chow intake commensurate with the intake of FL such that a daily intake of ~100 kcal is maintained.
Why might Spraguy Dawley rats consume more of one type of commercial chow than another? Perhaps palatability differs amongst the two types of chow. One might test for differences in RC palatability by simultaneously offering both types of chow and measuring relative consumption (much like the preference tests we have already conducted with FL, Funyuns, Crisco, and Oreos.)
One might also examine closely the nutrient and caloric composition of various lab chows, and extend our investigation to other published studies which monitored daily food intake as grams or kcal.
An important issue is the relative energy densities of Froot Loops (3.8 kcal/g) and Crisco (9.2 kcal/g). Consuming equivalent masses of FL and Crisco would result in a 2-3 fold increase in caloric intake! To account for 25- 30 kcal per day difference in caloric intake in Corwin’s R7 and R3 groups, rats would need to consume only 2.7 to 3.3 g of Crisco (25 or 30 kcal/9.2kcal/g) in two hours. In our preference pilot study (old rats offered simultaneous access to FL, Crisco, and Oreo) rats (n=22, body mass 664 ± 70 g) consumed 1.7 ± 1.5 g; mean ± SD (20.3 ±16.1 kcal) of Crisco in only 1 hour (the experiment was terminated at 60 minutes.) Our rats had only one prior overnight exposure to Crisco to eliminate neophobicity so they could not have developed a tendency to binge. Therefore it seems quite possible that our rats could have consumed a total of 30 kcal given an additional hour.
If energy intake is regulated homeostatically via negative feedback about some set point value, then set point might vary across individuals over a normal range. Considering Hagan’s notion of binge-eating prone (BEP) and binge-eating resistant (BER) classifications, one might expect BEPs to have a higher set point for daily caloric intake than BERs.
To replicate Corwin’s 2004 protocol, it was necessary to create groups of animals based on body mass and average intake of a highly palatable food over 24 hours. Therefore, we conducted a pilot study using aged rats that had a history of large food intake relative to their peers. We chose 8 rats that had consistently consumed the largest quantities of FL in several previous experiments and measured their FL intake over 24 hours and compared these values to intake from 7 rats (of 80 in this series) who had the highest caloric intake for combined FL and RC in the pretest study for the summer 2007 binge eating study. (see t-test below.) Aged, experienced rats (707 ± 53 g body mass; mean ± SD) consumed 135.7 ± 8.9 kcal/24hr and the younger (380 ± 16 g body mass) inexperienced rats consumed 141.3 ± 3.8 kcal/24 hr. There was no difference in the 24 hr caloric intake for these groups (p=0.596; t-test.) Thus it appears that selecting only for rats which eat the largest quantities, there is an upper limit of about 140 kcal/day regardless of age or food history.
A description of Harlan Teklad rodent diets is on-line at
t-testMonday, July 02, 2007, 7:55:51 AM
Data source: Data 1 in Notebook 1
Normality Test:Passed(P > 0.050)
Equal Variance Test:Passed(P = 0.230)
Group Name N MissingMeanStd DevSEM
Col 1 7 0 141.257 10.036 3.793
Col 2 8 0 135.708 25.199 8.909
Difference5.549
t = 0.544 with 13 degrees of freedom. (P = 0.596)
95 percent confidence interval for difference of means: -16.486 to 27.584
The difference in the mean values of the two groups is not great enough to reject the possibility that the difference is due to random sampling variability. There is not a statistically significant difference between the i nput groups (P = 0.596).
Power of performed test with alpha = 0.050: 0.050
The power of the performed test (0.050) is below the desired power of 0.800.
You should interpret the negative findings cautiously.
col 1 = 24 hr kcal intake (RC and FL total) for new rats during pretest selecting only the 7 rats with highest kcal intake for this period.
col 2 = 24 hr kcal intake of FL only during the pilot study on old, experienced rats who where selected based on their history of greater intake relative to peers in their cohort (of old animals used in spring 2007 for binge eating study.)
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