This method aims to model overeating and subsequent obesity by providing access to a wide variety of palatable foods high in sugar and saturated fat. The variety and palatability of the diet ensures that overeating is reliably promoted, leading to substantial metabolic disturbance within weeks. Providing a cafeteria diet allows for the metabolic and neural changes associated with obesity to be examined in great detail.
Visual demonstration of this method is important because logistical constraints can be demonstrated, as well as how planning and organization can make the feeding more efficient. Secondly, as the model involves greater interaction with rats, it's also important to demonstrate how to distribute and collect the food appropriately without stressing them. Demonstrating the procedure will be Sarah-Jane Leigh, a grad student, and Mike Kendig, a postdoc researcher from my laboratory.
To begin this procedure, obtain an appropriate amount of cafeteria food, and thaw it 24 hours prior to use. Food can be defrosted using a microwave but should not be presented while hot. Changing the type of cafeteria diet foods offered daily ensures that variety is maximized and cage soiling is minimized.
As rats eat most of their daily intake in the initial portion of the dark cycle, schedule cafeteria food replenishment close to the onset of the dark schedule so that food is fresh at this time. Replenish cafeteria diet every day. To feed the chow groups, turn the water bottles around so the spouts are up, and place the cage on the workspace.
Remove the lid, and lightly disturb the bedding for approximately 20 seconds, to simulate the process of collecting food items from the bedding of cafeteria diet cages. Then, place a small handful of chow pellets from the food hopper into the bedding, to equate exposure to food on the cage bedding. Replace the cage lid, and return the cage to the rack.
When the cage is settled, return the water bottles to their original orientation. To feed the cafeteria groups, place the cafeteria diet items into a labeled container for each cage. Turn the water bottles around so the spouts are up, place the cage on the workspace, and remove the lid.
Remove as much of the old cafeteria diet from the bedding as possible, and place fresh cafeteria diet into the cage. After this, close the cage and return it to the rack. When the cage is settled, replace the water bottles and the sucrose bottles.
Top up the chow, water, and sucrose bottles as necessary. When commencing the cafeteria diet, measure body weight and 24-hour food intake at least twice per week to monitor the effectiveness of the diet. Begin food intake measurements close to the onset of the dark phase.
Aim to present the same set of cafeteria foods on food intake measurement days, as equating for flavor and energy densities will permit accurate monitoring of changes over time. First, prepare the cafeteria diet and place into a labeled container for each cage, making sure to weigh and record each component. Refill the water, sucrose, and chow levels as required.
For control cages, turn the water bottles around so the spouts are up, place the cage on the workspace, and remove the lid. Record the weight of the water bottles and chow. Weigh the rats, and transfer them to a cage with fresh bedding, along with environmental enrichment.
Then, replace the cage lid, return the cage to the rack, and turn the water bottles around. Small signs stating Food Intake can be added to the cage to notify attendants and researchers not to touch bottles and chow. For the cafeteria cages, perform this same process, but also scatter the pre-weighed container of cafeteria foods around the cage with fresh bedding before the rats are transferred.
Leave the rats for a 24-hour period with minimal disturbance, and record any unanticipated disruptions. After this, record the weight of the water bottles and chow for the chow cages, and carefully search the bedding for pieces of chow. Once this is complete, remove the Food Intake signs from the cages.
Next, prepare fresh cafeteria diet for the cafeteria cages. Perform the following process for each cafeteria cage. Record the weight of the water, chow, and sucrose.
Remove the cage lid and environmental enrichment. Carefully remove cafeteria fragments from bedding and place into separate containers, making sure to remove the largest pieces first, and then systematically sift all of the cage bedding from one end to the other using gentle sweeping motions. Distribute the new cafeteria diet around the cage, return the environmental enrichment, close the cage lid, and return the cage to the rack.
Remove the Food Intake sign, and record the weight of each cafeteria food to the nearest 10th of a gram. Rats are given palatable foods to model the so-called Western diet. Cafeteria diet increases energy intake 2.5-fold relative to chow control.
Cafeteria feeding leads to a 20%increase in body weight compared with controls after three to four weeks. After six weeks diet, the mean difference in weight gain between groups is 67%and adiposity is approximately doubled. Macronutrient intakes on cafeteria diet are consistent across sexes and ages, with approximately 8%of energy consumed as protein, 34%as fat, and 58%as carbohydrate.
Our maintenance chow provides 22%protein, 13%fat, and 65%carbohydrate. Cafeteria-fed rats overeat all three macronutrients relative to controls. The dramatic hyperphagia is driven predominantly by carbohydrate and fat intake.
Female rats may be particularly vulnerable to the obesogenic effects of cafeteria diet. In this cohort of older rats, cafeteria-fed females were 20%heavier and ate 3.8 times more energy after only two weeks relative to controls. After six weeks of diet, body weight gain was greater, and fat mass was doubled in cafeteria-fed rats.
Providing insufficient cafeteria foods may constrain consumption, so check that there is food left in cafeteria cages. Additionally, different sets of cafeteria foods may alter macronutrient intake. Conducting food intake procedures in a consistent manner will ensure accurate measurement.
Careful planning of the diet to ensure foods are palatable and varied is key. Effective administration of the cafeteria diet can be time-consuming and requires careful planning and pilot work to establish steady hyperphagia and resulting weight gain. We can monitor metabolic function via glucose tolerance tests, as well as cognitive function through recognition memory tests, while exposing animals to the cafeteria diet.
We can then analyze many peripheral metabolic tissues and brain regions postmortem. This method can be used in rats and mice and has been occasionally extended into nonhuman primates. The cafeteria diet can be used to study dysregulated feeding behavior in the presence of hyperpalatability, as well as the impact of obesity on all organ systems.
This cafeteria diet method has added to our understanding of the etiology and consequences of overeating and obesity, as well as potential lifestyle and pharmacological approaches to improve health outcomes.
