How much can hungry rats eat?
30 July2 August 2007
Background
Animals eat to satisfy a need for energy and nutrients (homeostatic drive) and because certain foods taste good (hedonic drive.) In response to either drive, meals eventually terminate as animals become satiated, as palatability declines (sensory specific satiety, a diminution of hedonic drive) or perhaps because of physical limitations of the stomach to accommodate additional food. In many experiments, food intake is measured in response to some experimental manipulation which might be expected increase or decrease food intake. However, there may be a ceiling effect such that animals do not consume additional food because intake is limited by the volume of the stomach. Since laboratory rats are often used for such studies in which food intake is measured, it is important to determine if there is indeed an upper limit to the volume of food that can be consumed in a finite time.
It is well documented that laboratory rats effectively regulate their daily caloric intake about a set point, and that when highly palatable, energy dense foods with high hedonic values are offered, rats will consume these foods on one day and autonomously limit their intake the subsequent day such that excessive energy intake is compensated for by under eating on the second day.
Binge eating has also been investigated in laboratory rats, but the most commonly cited studies have use foods comprised of fats or combinations of fats and sugars that are much more energy dense thant commercial rodent chow (3.3 kcal/g).
Test FoodEnergy DensityLab
100% Crisco9.3 kcal/gCorwin
Oreo cookies4.8 kcal/gBoggiano
Sugar Fat whip7.35 kcal/gRowland and Mathes
Froot Loops3.8 kcal/g
Rodent chow3.3 kcal/g
There is no clear conventional definition of binge eating other that than than the non-quantitative working definition: consumption of a larger meal than normal meal in a short period of time. If meal intake is expressed as kcal and a high calorie food is the test food, then the conditions are biased toward the “detection” of binge eating since animals would need to consume only small amounts of an energy dense food to produce large changes in energy intake.
In our experiments Froot Loops are the test food because they have been demonstrated to be highly palatable food to rats, yet have an energy density of 3.8 kcal/g,. whichThis is comparable to commercial rodent chow (3.3 kcal/g).; Tthus, rats might consume considerable amounts of Froot Loops in terms of grams which may not correspond with dramatic increases in kcal intake due to the relatively low energy density. In fact, the intake of FL may be limited by the dimensions of the stomach and the properties of the FL once they arrive in the stomach and mix with digestive juices to occupy an even larger volume.
Thus, we want to know that is the largest amount of Froot Loops that a rat can consume in two hours.
Methods
To create conditions intended to maximize that amount consumed, we placed 20 430 rats (ageborn 04/03/2007, tested on 07/20/2007 when average weight was 483 ± 8 grams (mean ± SEM) and then 07/27/2007 after 6 days of caloric restriction when average weight was XXX ± XX gram., average body weight that had been used in a prior study on binge eating; non-stressed R7, R3, and I animals from the Community of Scholars 2007 project ) on a restricted chow intake of 21.5 ± 0.2 g (70.1 kcal) per day provided just before lights off at 4 pm. This represents 67% of the daily caloric chow intake for these animals based on 28 prior days of daily caloric intake measurements. The restricted chow diet was maintained for for 67 days. On the 7th day, rats were offered FL ad libitum on plexiglass rods placed on cage lids for two hours starting at 2 pm (two hours before lights off). FL intake was measured twice during this interval ((at 30 min and 120 min). Thus, these animals would be consuming Froot Loops to satisfy both their caloric restriction and their hedonic drive for palatable foods.
Hypothesis
It is hypothesized that under these conditions, FL intake will be greater in animals modestly calorically restricted for one week than when the same animals had been maintained ad libitum on rat chow for the previous 4 weeks.
Results
There was no significant different in the number of grams of FL consumed in 2 hours for rats (n=30) maintained on chow ad libitum (6.7 ± 0.6 g) or for those same animals after 6 days of caloric restriction of chow intake limited to 67% of baseline intake (7.2 ± 0.4 g; p = 0.283, t-test). Futhermore, body weight had decreased significantly from 483 ± 8 grams to xxx ± x grams.
Conclusion
The observation that rats made chronically hungry by 6 days of moderate food restriction do not eat more FL in 2 hours that rats that were maintained on an ad libitum diet of chow suggests that the consumption of FL may be limited by a ceiling phenomenon. Rats that have been maintained on chow ad libitum eat FL presumably as a result of hedonic drive. Rats that have been chronically food deprived would be eating FL in response to both hedonic and homeostatic drives, and thus one might expect FL consumption to be greater under this condition. However, it there was not difference in FL intake. Thus it seems that even though quite hungry, rats are incapable of consuming beyond a certain fixed value, perhaps due to size limitation of the stomach.
Future studies
There remains a manipulation that could conceivably result in the greatest degree of hunger: one week of modest food deprivation (67% of baseline caloric intake) followed by 18 hours of complete food deprivation just before measuring the intake of FL during a two hour test interval.
There is yet another potential factor that may influences meal size. In the studies conducted thus far, 2 hr FL intake was measured in the two hours immediately preceding the onset of the dark period. It is possible that food intake is affected by the time of the photoperiod in which the experiment is conducted. For example, it is conceivable that food intake could be appreciably greater in the first two hours of the dark cycle. It would be relatively easy to design an experiment to investigate this possibility.