1 July 2008
The Question:
Do rats have a preference for Fruit Loops over unsalted, dry roasted peanuts?
Background:
In other experiments investigating sensory specific satiety, we have assumed that
Froot Loops are more palatable than peanuts to laboratory rats. However, the
results of those experiments called in to question our assumption. Therefore, we
designed this experiment to determine whether rats have a preference for one of
these foods.
Hypothesis: Hungry rats will consume more Froot Loops than peanuts when
offered both simultaneously.
Experimental design:
Eleven large (661 ± 58 g, mean ± S.D., max = 784, min = 609 g) adult male
Sprague-Dawley rats previously used for several studies (Spring 2008 eating rate
studies and Summer 2008 pilot studies with Hoodia gordonii) housed individually
in shoebox cages were food deprived 24 hours prior to the start of the
experiment which was conducted at 1400 hours (2 hours after lights ON at 1200).
The experiment began following a standard one hour acclimation period to the
laboratory. Rats were placed into a new empty cage with an empty food pan of
the same type that would contain the test foods. Froot Loops and unsalted dry
roasted peanuts were weighed separately for each rat. Food was delivered in
separate bowls (2 bowl experiment, n= 5 rats) or Froot Loops and peanuts were
mixed and placed in the cage in a single bowl (1 bowl experiment, n= 6 rats).
Tipping and spilling from the bowls was prevented by double-sided tape which
attached the bowls to the cage floor.
For all rats, latency to consume the first bite was recorded to the nearest second
using a stopwatch. For all rats, food bowls were removed from each cage 30
minutes after the animal had taken its first bite. Froot Loops and peanuts were
weighed separately for each rat. In cases where food had been mixed in the same
bowl, Froot Loops and peanuts were manually separated and weighted
separately. Furthermore, each of the 3 observers watched two rats who received
a single bowl of mixed foods and recorded each food item removed from the
bowl and whether that food item was consumed or discarded. Thus, for all the “1
bowl” experiments, the number and sequence of each food item picked up and
eaten or discarded was recorded. Each observer also monitored one or two “2
bowl” experimental rats, but made no effort to record the number or sequence in
which food items were consumed or discarded.
Rationale for “1 bowl” and “2 bowl” experiments: If Froot Loops and peanuts
are placed in separate bowls, rats might return only to the bowl containing Froot
Loops once they discover this food item. Thus, by returning consistently to the FL
bowl, they avoid actively choosing between the Froot Loops and peanuts. By
mixing the Froot Loops and peanuts in the same bowl, rats are faced with a choice
each time they return to the bowl for another item of food.
Termination of the experiment: It would have been ideal to terminate the meal
using our most recent criterion (a cessation of intake lasting 10 minutes.)
However, it was not reliably possible to monitor the time at which each food item
had been consumed since each of the three observers was responsible for
monitoring three or four rats, two of which were the “1 bowl” rats that required
undivided attention. Therefore, the rats were arbitrarily allocated 30 minutes to
eat once they started eating. In actuality, none of the rats were still eating at the
end of the 30 minute interval and in fact most meals could be estimated to have
lasted for less than 15 minutes.
Parameters measured: Intake in grams of Froot Loops and Peanuts during 30
minutes for all rats (n=11), latency to eat the first bite for all rats, and number and
sequence of food items consumed for “1 bowl” rats (n=6).
Results (from food_choice_data.xls spreadsheet): In the 30 minutes allocated
for the meal, rats (n=11) consumed 3.2 ± 1.6 grams of Froot Loops and 0.2 ± 0.3 g
of peanuts. The difference is highly significant (p < 0.001, Mann- Whitney Rank
Sum test). Rats (n = 6) in the “1 bowl” study ate 17 ± 5 Froot Loops and 1 ± 1
peanuts, again a significant difference (p = 0.002, Mann- Whitney Rank Sum test).
When eating from one bowl where FL and peanuts were mixed, rats (n=6)
consumed 3.9 ± 1.3 g of FL. Rats (n=5) ate 2.4 ± 1.6 g of FL from a bowl of FL only
(no significant difference, p. 0.119, t-test.) Likewise, there was no difference in
the grams of peanuts eaten from one bowl (n=6) or two (n =5)(p = 0.052, MannWhitney Rank Sum Test.)
Other observations: Rat tended to ignore peanuts. The rats in the “1 bowl” group
picked up a total of 15 peanuts but actually ate only 6.5 peanuts. Thus, they
discarded the peanut 57% of the time. Those same rats consumed 101 Froot
Loops of the 107 Froot Loops they picked up (94% of those picked up were
consumed; 6% were discarded.)
Latency to eat the first bite ranged from 4 seconds to more than 10 minutes for
two rats. The median latency was 15 seconds.
Conclusions: Rats showed a strong preference for Froot Loops over peanuts
regardless of whether the foods were offered in the same or separate bowls.
Discussion: These results support the hypothesis that rats have strong preference
for Froot Loops when peanuts are provided simultaneously.
The next experiment: The rats used in this trial have had no prior exposure to
peanuts and thus may have exhibited neophobia. Therefore, each of these rats
will be provided peanuts per daily for 5 days prior to a repeat of this experiment
so as to eliminate the possibility that neophobicity toward peanuts is responsible
for the pattern of consumption observed.
Pilot Study # 2
Background: The results of Pilot Study # 1 showed that hungry rats had a very
strong preference for Froot Loops when both Froot Loops and dry, unsalted
roasted peanuts were simultaneously available. However, these rats could have
been reluctant to eat peanuts having had no prior experience with this food.
Therefore the experiment was repeated identically one week later with the only
modification being that in the 5 days leading up to the experiment, each rat was
provided peanuts (approximately xx gram) at about 0900 hrs daily to overcome
whatever neophobicity might have been present.
Hypothesis: Hungry rats will consume more Froot Loops than peanuts when
offered both simultaneously.
Experimental design: The same rats and same protocol described above was
used. Thus, the only difference for this study was that rats had become familiar
with peanuts during the week prior to testing.
Results (from food_choice_data.xls spreadsheet): A summary of the results
from Pilot Studies 1 and 2 are show in the Table 1.
Table 1.
Condition
n
g FL eaten
3.1 ± 1.6
cal FL
eaten
11.9 ± 5.9
g PNT
eaten
0.2 ± 0.3
cal PNT
eaten
1.1 ± 1.8
% cal
from FL
94 ± 9
Unfamiliar
with
Peanuts
(Pilot #1)
Familiar
with
Peanuts
(Pilot #2)
No daily
Peanuts
Pilot #3)
11
11
2.3 ± 1.9
8.8 ± 7.1
4.4 ± 2.7
26.6 ± 16.6
30 ± 27
11
1.3 ± 0.8
4.9 ± 2.9
6.5 ± 3.0
39.5 ± 18.0
13 ± 9
After becoming familiar with peanuts, rats that had eaten mostly Froot Loops and
very few peanuts (as grams or calories) in Pilot Study 1 now showed a reversal of
this pattern. Rats unfamiliar with peanuts in Pilot Study 1 had obtained 94% of
their total caloric intake from Froot Loops during the 30 minute meal whereas in
Pilot Study 2, Froot Loops accounted for only 30% of caloric intake.
Table 2 shows that in terms of absolute numbers of items eaten or rejected, there
was a dramatic shift in behavior after exposure to peanuts. Rats unfamiliar with
peanuts ate a total of 101 Froot Loops during Pilot Study 1 and in Pilot Study 2 the
number had dropped to 55. Rats unfamiliar with peanuts had eaten only 5 and a
half peanuts during Pilot Study 1 but in Pilot Study 2 they ate 84 peanuts which is
even more than the number of Froot Loops eaten (55.) During the first study, 56
% of the peanuts that were picked up by the rats were not consumed yet nearly
all Froot Loops picked up were eaten (only 6.5% rejected). In Pilot Study 2,
whichever food item was picked up was almost always consumed (3.5% of FL
were rejected and 1.2% of peanuts were rejected.) Altogether rats picked up 84
peanuts and rejected only 1, thus the caloric intake of peanuts was much greater
than for Froot Loops in Pilot Study 2.
Table 2. Total numbers of items eaten for all 6 animals in the “1 bowl test” for
which eat item picked up and either eaten or rejected was recorded.
Condition
n
FL eaten
101
PNT
eaten
5.5
% FL
rejected
6.5
% PNT
rejected
56.0
Unfamiliar
with
Peanuts
(Pilot # 1)
Familiar
with
Peanuts
(Pilot # 2)
No daily
Peanuts
(Pilot #3)
6
6
55
84
3.5
1.2
11
42
120
8.7
0.8
Table 3 shows the first food item eaten as well as the last item eaten for six rats in
the “1 bowl” experiment. In every case, rats selected and ate a Froot Loop first.
In Pilot Study 1, 4 of 5 rats for which data was recorded concluded the meal with
a Froot Loop. In Pilot Study 2, all rats began by eating a Froot Loop and all rats
concluded the meal with a peanut.
Table 3.
Condition
Unfamiliar
with Peanuts
(Pilot # 1)
Familiar with
Peanuts
(Pilot # 2)
No daily
Peanuts
(Pilot #3)
n
6
First food eaten
FL = 6 of 6
Last food eaten
FL =4, PNT = 1,
1 rat not recorded
6
FL = 6 of 6
PNT = 6 of 6
11
FL = 8 of 11
PNT = 9 of 11
Conclusions: In Pilot Study 2, rats showed a strong preference for peanuts over
Froot Loops regardless of whether the foods were offered in the same or
separate bowls and by all measures used (number of items eaten, number of
grams consumed, number of calories consumed.)
Discussion: These results do not support the hypothesis that rats have strong
preference for Froot Loops when peanuts are provided simultaneously. The rats
used in this trial had been recently and frequently exposed to peanuts. They
clearly overcame any neophobicity toward that food and surprisingly show a
reversal of their food preference.
Based on sensory specific satiety, one might predict that as an animal approaches
satiety when eating a meal consisting of more than one type of food, the last item
eaten would be that of the higher palatability. Rats in the 2nd Pilot Study
consistently concluded the meal with a peanut, which suggests that peanuts are
more palatable than Froot Loops. Given that Froot Loops are sweeter, one might
expect them to be more palatable to a rat. However, this does not appear to be
the case in Pilot Study 2.
What accounts for the reversal of food preference in Pilot Study 2? Since these
rats have been 24 food deprived, perhaps they are choosing the food that
provides the greatest energy density (Peanuts 6.07 cal/g vs. Froot Loops 3.75
cal/g) to quickly remediate their caloric deficiency. It is possible that Froot Loops
are indeed more palatable but when there is a need to restore energy balance, a
less palatable food will be eaten if that food provides greater caloric and/or
nutritional value. To test this idea, one might conduct the experiment in nonfood deprived rats, thereby eliminating the possible influence of caloric
deficiency. Perhaps rats would eat more Froot Loops when not food deprived,
which would suggest that 1) Froot Loops are more palatable and 2) support the
hypothesis that foods are selected based on energy demands.
Another possible explanation can be formulated. Perhaps in the Pilot Study 2,
rats are simply continuing to eat the food (peanuts) that they became
accustomed to eating in addition to the rat chow that was available ad libitum
except during the 24 hour food deprivation. Thus, hungry rats would choose to
eat peanuts because they have recent experience with this food item, it is
palatable, and it provides high energy content. Perhaps their memory of Froot
Loops, which is by now one week in the past, is insufficient to drive them to eat
Froot Loops. To test this idea, one might conduct a third pilot study by providing
rats with a daily allocation of Froot Loops in the week prior to test day. On test
day after the standard 24 hour food deprivation, Froot Loops and peanuts would
again be offered simultaneously. If rats switch their preference to back to Froot
Loops in this condition, it would support the hypothesis that the food type with
which rats have the most recent familiarity is the food preferred when hungry
rats are simultaneously offered two palatable foods.
Another possible explanation exits. Perhaps daily exposure to a limited amount
of peanuts triggers binge eating of peanuts on test day when peanuts are
available ad libitum. It has been shown that binge eating can be elicited from
laboratory rats under certain conditions of stress and/or limited access to highly
palatable, energy-dense foods. In our previous studies using limited access to
Froot Loops with or without Chronic Mild Stress, we found evidence of overeating only when Chronic Mild Stress was combined with access to Froot Loops
restricted to 3 days per week (Mackechnie et al., 2008, Govind et al., 2008). In
those studies, it may have been challenging to detect binge eating because the
difference in energy density between rat chow (3.3 cal/gram) and Froot Loops
(3.75 cal/gram) is small. In one of the earliest studies of binge eating in
laboratory rats, Corwin (xx) used 100% fat (Crisco, 9.2 kcal/g) as the test food and
expressed food intake in terms of calories per 24 hours. Since Crisco has such a
high energy density, rats would need to consume very little of this food in order
to obtain significantly elevated caloric intake. Perhaps using peanuts (with their
intermediate energy density (6.07 kcal/g) due to fat and protein content) in
combination with Chronic Mild Stress would permit us to detect greater changes
in daily caloric intake and thus generate results more similar to those reported by
Corwin.
Yet another explanation can be offered. Froot Loops are physically less dense
than peanuts (grams/unit volume). It is possible that the ingestion of Froot Loops
requires more saliva for swallowing and more gastric secretions upon arrival in
the stomach than do peanuts. It is possible that gastric distention associated with
Froot Loops is greater than gastric distention with peanuts. If so, one might
predict that a meal of Froot Loops would be terminated sooner (expressed as
either time in minutes to meal termination or volume consumed). If, by
experience, rats “learn” that eating peanuts permits a greater caloric intake per
unit volume of stomach distention, then in order to overcome the energy deficit
produced by a 24 hour food deprivation, rats may compensate for the deficiency
most effectively by choosing to eat peanuts.
The next experiment:
The several possibilities listed above deserve careful deliberation. The ideal
experiment would permit us to address several of these hypotheses at once.
Pilot Study # 3
For this continuing study conducted on 15 July, 2008, the protocol will be as
before (24 hour food deprivation, etc.) with the only change being that neither
Froot Loops nor peanuts will be given daily in the days prior to the experiment.
Thus, we will address the possibility that food choice is governed primarily by
recent frequent exposure. Furthermore, all 11 animals will be tested using the “1
bowl” protocol since those results provide information about selection and
rejection of individual food items as well as first and last item eaten. To collect
this data from all 11 rats will require the three observers to conduct the
experiment in two rounds on the same afternoon with one or two rats per round.