Burmester, Victoria (2019) The effects of oxytocin on appetite in humans. (PhD thesis), Kingston University, .
Abstract
The nine-amino acid peptide oxytocin is synthesised mainly in the hypothalamus and secreted directly into the brain but also released via the posterior pituitary gland to regulate a range of physiologic and metabolic eating processes. Previous research suggests that oxytocin is an anorectic with selective effects on sweet-tasting food. The influence of oxytocin on palatable food intake not initiated by deprivation-induced hunger has not yet been investigated. There is evidence that oxytocin modulates attention from food to social stimuli in females with anorexia but, to date, this has not been researched in people without eating disorders. This thesis used the bogus taste-test paradigm but employed a more rigorously disguised experimental protocol than previous research in the field to examine palatable food intake in men and women individually. Using a similar protocol, it also tested the anorectic effects of oxytocin on group eating. To assess how oxytocin modulates attention in healthy adults not reporting disordered eating, a dot-probe test with food and neutral pictures was carried out. The dot probe also assessed the effects of oxytocin on romantic and social images as these domains are also influenced by oxytocin. The thesis results reported that in the male-only experiment showed that intranasal administration of 24 IU of oxytocin increased sweetness ratings for sweet food and attenuated both sweet and salty snack consumption, 15 minutes after lunch, with a particularly large reduction on sweet food of 63%. In fed females, 24 IU of intranasal oxytocin inhibited only sweet-snack intake but with a similarly large effect of 57%, but had no effect on taste ratings or cortisol, suggesting that reductions in snack eating may not be mediated by cortisol. In the group experiment, 24 IU of intranasal oxytocin facilitated increased self-reported sociality and social modelling of food intake, but there was no overall anorectic effect of the peptide on eating. In the dot probe task, an attentional bias to food was demonstrated and this was reduced by oxytocin. However, oxytocin did not affect reaction times to romantic or social stimuli, suggesting that effects were food specific. An additional online study in this thesis demonstrated that two domains related to oxytocin function, loneliness and parental bonding, predict higher scores on a measure of food “addiction”. High loneliness scores and low maternal care predicted high food “addiction”, but age, gender, hours of sleep, anxiety, and activity levels did not. The experiments of this doctoral thesis are the first experiments to demonstrate anorectic effects of oxytocin on eating not initiated by deprivation-induced hunger. This thesis also shows, for the first time, the anorectic capacity of oxytocin in females, and its independence from cortisol levels. The dot-probe experiment of this thesis provides important evidence that oxytocin reduces attentional bias to food across a range of body mass indexes but does not alter the attention paid to romantic or social stimuli. The final experiment provided evidence that oxytocin increases social modelling of food intake, which is consistent with previous research into oxytocin’s social effects. Future work could extend this thesis in a number of important research directions, for instance by using a wider range of snack foods and palatability measures to account for different tastes, improve generalisability and test oxytocin’s efficacy with different foods. Another important direction that this thesis suggests would be useful to investigate is oxytocin’s influence on the snack-food intake and sociability of different compositions of group, varied, for instance, by gender, size and familiarity. Taken together, the impact of these findings could have potential benefits for people with obesity or disorders of overeating and indicate that oxytocin might be useful to research as a therapeutic drug to target overeating.
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