The REBOST project was divided in three studies, two with animals and one with human participants. In the animal studies, rats were divided in three groups, each with their own six-week diet: healthy control (CTR), high-fat, high sugar (HFHS), or junk food (Cafeteria diet, or CAF). The CAF diet consisted of a choice of the same unhealthy, but tasty products as humans eat (e.g. bacon, muffins, cookies, etc.), mimicking overeating behaviour seen in humans. After the diet, the rats were tested in a reward behaviour test in which they had access to either a food or sexual reward. During the reward behaviour test, fibre photometry was used to measure real-time calcium fluctuations (as indicator of neural activity) in the behaving rats on a millisecond time scale in the ventral tegmental area (VTA, study 1) and the nucleus accumbens (NAc, study 2). In the third study, on the other hand, the effects of junk food consumption on the human brain was investigated. The participants were placed on a diet of either junk food or healthy food for 48h, after which their neural activity during a monetary reward test was assessed using fMRI.
In study 1, long-term exposure to CAF-diet reduced the reward interest towards junk food, shown by a decreased amount of time spent sniffing and eating the junk food reward during the reward behaviour test compared to CTR and HFHS. Interestingly, this reduction in behaviour was accompanied by lower neural activity in the VTA. On the other hand, there was not a clear effect of CAF-diet exposure on interest towards a sexual reward. In fact, all three groups showed similar levels of sexual behaviour (in female rats: paracopulatory behaviour and lordosis responses). However, the neural activity of the VTA was also reduced in CAF-rats during sexual activity. When exposed to dissatisfactory standard chow, CAF-rats also ate significantly less chow compared to CTR- and HFHS-rats. Again, this reduction in reward interest corresponded with lower neural activity in the VTA. Finally, after a period in which all rats were on a healthy food (standard chow) diet, only the behavioural effects towards a CAF reward were partially reversed, while the neural activity in the VTA remained reduced.
Study 2 was a repetition of study 1, but now the effects in the NAc-shell and NAc-core were assessed. Analysis of the data is ongoing, but preliminary results revealed that CAF-diet does not change the neural activity patterns in the NAc-shell and NAc-core compared to CTR-diet during the expectation phase of the reward behaviour test. This could potentially be explained by the existence of D1- and D2- medium spiny neurons in the NAc which play opposite roles upon reward responses. Further data analysis of the other reward phases is needed to understand our preliminary data.
Lastly, analysis of the data from the study 3, the human study, is still ongoing. It has been predicted that junk food exposure will lead to a desensitization of the reward circuit seen during a monetary reward test. Self-reports, however, have already revealed that 48h of junk food intake already increased the negative internal states, such as a reduction of concentration compared to the control group on a healthy food diet.
The preliminary results of the project were presented at the 31st annual meeting of the International Behavioral and Neuroscience Society (IBNS) in Glasgow, June 2022. In addition, we plan to publish the outcomes of study 1 and 2 together in one publication, while the findings of study 3 will be split into 2 publications. All publications will be submitted to high-impact peer-review journals with open access in 2023.
Finally, the various aspects of the project, including the methodology, translational design, and results, have been communicated to a wide range of audiences. The REBOST project featured on national radio and local newspapers, and it was disseminated to students from elementary schools to universities. We also presented the project at the popular science festival "Pint of Science”.
