Regular physical exercise promotes health and prevents and treats multiple 
chronic diseases. Despite the well-acknowledged health benefits, many people 
remain physically inactive. Affective responses induced by exercise are believed 
to influence future exercise behaviour. Previous studies suggest that pleasurable 
sensations experienced in response to exercise are regulated by the endogenous 
opioid system. The opioid system is also involved in the reward processing, and 
may modulate food reward responses after exercise, possibly contributing to subsequent 
caloric intake and weight loss outcomes. 

In this thesis, affective responses to high-intensity interval training (HIIT) and 
moderate-intensity continuous training (MICT) were investigated over a two-week 
training intervention in untrained healthy subjects and subjects with type 2 diabetes 
or prediabetes. Positron emission tomography (PET) was used to explore endogenous 
opioid release after HIIT and MICT in young healthy subjects. The interaction 
between exercise-induced opioid activation and changes in food reward processing 
were assessed using functional magnetic resonance imaging (fMRI). 

HIIT generated a more negative overall affective experience in comparison with 
MICT; however, this lessened over the training period. Thus, HIIT appears as a 
tolerable exercise method for sedentary adults with and without diabetes. Furthermore, 
HIIT induced opioid release in key brain regions implicated in emotion 
and pain processing and the opioid release correlated with measures of negative 
emotionality. In contrast, MICT did not result in significant opioid release, although 
increased opioid activation correlated with increased euphoria after MICT 
as well as with increased neural responses to palatable foods. These results indicate 
that the intensity of the exercise regulates endogenous opioid release and 
concomitant changes in affect and reward processing. Taken together, these findings 
may have practical implications in developing more tolerable and likeable 
exercise programs to enhance physical activity participation in different population 
groups, as well as in optimising the efficient use of exercise in health care, 
for example in weight loss interventions and in the treatment of various affective 
disorders.