The model of sugar bingeing has been developed in Sprague Dawley rats, with studies conducted in both male (29, 30, 33, 34) and female (31, 32) rats, ranging in age from 6 to 12 weeks at the onset of the study. This is not to say that binge consumption cannot be evaluated in the same manner in other rat breeds or species at ages outside those presented here; however, this protocol has only been validated within these parameters.

Rats should be divided into experimental and control groups (at least n  =  8–10 per group) of similar body weight (<10% variation between groups) and individually housed. There is some variability in chow and palatable food intakes between rats, so it is advisable to use at least 8–10 rats per group. Some potential control groups will be discussed in Sect. 2.4.

Rats should be housed in a rodent vivarium with a 12-h light/dark cycle, maintained at 21 °C. All experimental procedures (including handling, housing, husbandry, etc.) must be conducted in accordance with National and Institutional Guidelines for the Care and Use of Laboratory Animals and University Institutional Animal Care and Use Committee protocols.

Animals should be individually housed in order to allow for the accurate measurement of food intake throughout the experiment. Wire bottom cages (or wire inserts added to solid bottom cages) are preferred because solid bottom cages retain the animals’ feces and urine, which introduces confounding factors into the experiment. If using solid bottom cages, use a noncaloric bedding. Consumption of bedding material, which is often caloric in nature, and fecal boli make it difficult to truly food deprive the rat, which is necessary in this paradigm. Further, gastric distension that results from filling the stomach with bedding or other substances collected in the bottom of the cage can cause the release of feeding peptides and neurotransmitters (41), potentially confounding the study results. Cages should be outfitted with removable food hoppers. It is important that hoppers can be easily removed from the cage, allowing for easy facilitation of the 12-h deprivation period without too much disturbance to the rat’s environment.

When animals arrive to the housing facility, they should be allowed a minimum of 5 days to acclimate to the new environment prior to experiment onset. Animals should be maintained on standard rodent chow until the start of the experimental paradigm. Water access must be ad libitum for the duration of the study. Water can be provided using bottles with steel-ball tip valves or it can be made available using automatic watering systems.

Binge behavior has been observed with various sugar solutions, including glucose (31, 37) and sucrose (28, 30, 31, 33, 36, 42). To prepare the 10%  w/v sucrose solution, slowly dissolve 100 g of sucrose (table sugar) in approximately 800 mL of tap water while using a stir bar to stir it. Then, fill the container to 1,000 mL with tap water. Prepare only enough sugar solution for each day. Store extra solution at 4 °C for a maximum of three days, otherwise bacteria and mold can begin to form. Drinking bottles for sugar solutions should be emptied, rinsed, and refilled with new solution each day to avoid bacterial growth. Each week bottles and drinking tubes should be sterilized using a laboratory dishwasher or commercial cage-washing device.

Sugar solution should be presented to animals in 100-mL, graduated (in 1-mL increments) drinking tubes (e.g., glass drinking tubes (Lab Products, Inc.) or tubes made from 100-mL polyethylene graduated cylinders (Fisher Scientific) by cutting off the flange and filing the top flat). Tubes should be sealed with rubber stoppers with steal ball tips (e.g., Lab Products, Inc.). These are preferred for providing fluid to the rats because they prevent unintentional fluid spillage. Drinking tubes can be mounted to the outside of a wire cage using a spring. Mounting bottles on the outside of the cage allows the researcher to take frequent readings of the fluid volume without disturbing the rat or risking fluid spillage.

Each rat is typically given 100 mL per day of the sugar solution, and those rats that drink almost all of it are given more on subsequent days. Ample amounts of chow should also be provided. Male Sprague Dawley rats consume about 30–35 g of chow each day, nearing 100 Kcal, with fluctuation depending on body weight and age. The goal is to always provide more sugar solution and chow than the rats will consume. By the end of one month, some rats may increase sucrose consumption to a degree that larger drinking tubes (e.g., 250 mL) are required.

The main experimental group with binge access to sugar will have a 12-h deprivation period (no food, water only), followed by 12-h access to a 10% sucrose or 25% glucose solution in addition to standard pelleted rodent chow (e.g., LabDiet #5001, PMI Nutrition International, Richmond, IN; 10% fat, 20% protein, 70% carbohydrate, 3.01 Kcal/g) and water. The feeding schedule should be timed such that the access starts 4 h into the dark cycle. Animals typically engage in a large meal at the onset of the dark cycle. By delaying access to chow and sugar until 4 h into the dark period, the rat engages in a binge when food is presented. Water is always provided ad libitum, which ensures that sugar solution consumption is not driven by dehydration, but rather palatability and motivation. Maintain rats on this binge-feeding schedule for 21–28 days to elicit the dependence-like signs (28).

At the same time, maintain control groups of rats, which may include:

Record the 1-h intake of sugar solution after the first hour of access and record the daily intake before removing the solution at the end of the 12-h access period. Intake data can also be converted into calories. For reference, 1 mL of 10% sucrose solution has 0.4 kcal. 1 mL of 25% glucose solution has 0.97 kcal.

While the time frame for completing an experiment is about one month, daily time commitments for routine preparation, administration, and removal of the sugar solution will vary depending on the number of subjects being tested, but generally requires approximately 1 h per day.

Corwin and colleagues have a well-developed model of binge consumption of vegetable fat (see Chap. 4). We have adapted this paradigm slightly by offering the rats a longer deprivation/feeding period. The main experimental group with binge access to fat has a 12-h deprivation period (no food, water only), followed by 12-h access to a vegetable shortening, in addition to standard pelleted rodent chow and water. Further, as previously described, food should be reintroduced each day 4 h into the dark period. Diets should be maintained for 21–28 days and intake should be recorded and analyzed as described above. Vegetable shortening contains approximately 9 kcal/g. Control groups, as described above, should also be considered (43).

Another variation is to obtain or prepare a nutritionally complete sweet-fat diet (a pelleted diet can be purchased from Research Diets (New Brunswick, NJ, #12451; 45% fat, 20% protein, 35% carbohydrate, 4.7 kcal/g)). Again, we use a 12-h deprivation, 12-h feeding model. As described above, the main experimental group will have a 12-h deprivation period (no food, water only), followed by 12-h access to sweet-fat chow and water. Food should be reintroduced each day 4 h into the dark period. Diets should be maintained for 21–28 days and intake should be recorded and analyzed as described above. Control groups, as described above, should be considered (43).

Various sources of fat can be used. Vegetable shortening can be provided in glass jars (100–125 mL) and cages should be outfitted with springs to hold the jars in place. Vegetable shortening should be replaced every third day or more frequently if needed. Always provide more shortening than the rat will consume in the access period. Intake will increase over the course of the study, potentially requiring more frequent refilling. Our laboratory has studied various sweet-fat diets, including those that are solid (rodent pellets), emulsions (sugar and oil), and semisolid diets (sugar and butter) (43).

Although validated only using the presented parameters, this method of inducing binge eating is open to a great deal of modification. For example, we have used a variety of palatable foods, ranging in consistency, caloric content, and macronutrient composition with the 12-h deprivation, 12-h access period, and have successfully initiated binge-eating behavior. Some of the diets investigated have been nutritionally complete, such as the pelleted diet suggested above, while others have been sugar or fat supplements to a nutritionally complete rodent chow. Both have clinical relevance, with a great deal of individuals reporting binge eating on “snack”- or “dessert”- type foods (supplements to their diets), while others report binge eating on foods that are calorically dense, but nutritionally complete (24, 44). As previously mentioned, we have also explored a variety of food consistencies, including liquids (sugar solutions), semisolids (vegetable shortening and sugar, “cake frosting”-like diet), emulsions (rich in either fat or fat and sugar), and solid diets (commercially available sweet-fat, pelleted chow). It is of interest to explore varying consistencies, given differences that have been identified in the consumption of liquids as compared to solid foods (45). Further, we find it crucial to include a chow-fed control group in order to have a balanced nutritive diet to compare binge-eating groups to. Lastly, we have studied various macronutrient compositions. While studying pure sugar or pure fat binges is advantageous in the laboratory and has allowed for the understanding of the specific effects of different macronutrients (46), it is not representative of the human condition, as people tend not to consume foods that contain only one macronutrient. For this reason, combination diets are also of importance when exploring binge-eating behaviors. The flexibility of this paradigm allows for further exploration of a variety of foods.