The addition of breakfast led to reductions in food cravings which were accompanied by increases in Homovanillic Acid concentrations, with slight improvements observed following the higher vs. normal protein meals. Collectively, these data suggest that the daily addition of breakfast, particularly one rich in protein, might serve as a beneficial strategy to reduce food cravings and modulate food reward in overweight/obese young people.
Much of the animal and human work concerning the dietary components that stimulate food reward, food cravings, and/or drive food preferences have centered around highly palatable foods which typically consist of high fat and high sugar foods. These energy dense, yet nutrient-void foods can lead to increased appetite, overeating, and weight gain over the long term . Thus, identifying equally palatable, but nutrient-rich, healthier replacements might reduce some of these negative effects.
Dietary protein has been well documented to improve appetite control and food intake regulation through physiological increases in satiety and subsequent reductions in intake . Although dietary fats and sugars are generally thought to be more rewarding and craved compared to other foods, similar cravings have been shown for protein-rich foods . Further, as discussed in the review by Journel, et al. , dietary protein has also been proposed to modulate food hedonics. However, the mechanism of action by which the consumption of dietary protein elicits these responses remains unclear but has been postulated to be involved with dopamine production. Specifically, the rate of dopamine synthesis is sensitive to local substrate concentrations, primarily the amino acid tyrosine, which is influenced by the protein content of a single meal as well as within the overall diet . Since tyrosine is the substrate required in the rate-limiting step of dopamine synthesis [38, 39], the consumption of higher protein meals containing increased tyrosine potentially leads to increases in dopamine synthesis. This has been demonstrated in an animal study in which rats fed a higher protein diet exhibited a substantial increase in central tyrosine which was accompanied by an increase in dopamine synthesis compared to rats fed lower protein diets . In the current study, we showed that HVA concentrations were higher following the higher protein meal and were related to protein content and tyrosine content of the breakfast meal. Together, these data further support that dietary protein might be an important regulator of both physiological and hedonic food intake.
Another dietary factor which has significant effects on both aspects of food intake regulation is the common practice of skipping breakfast. In a pilot study from our lab, we showed that breakfast skipping adolescents exhibit greater appetite and reduced satiety throughout the morning, leading to increased energy intake; however, the addition of breakfast, particularly breakfast meals containing increased dietary protein, reversed these outcomes . In a subsequent pilot study, we incorporated functional MRI to identify the neural responses to visual food stimuli prior to lunch in overweight/obese ‘breakfast skipping’ teen girls . We found that breakfast skipping led to increased neural activation in brain regions controlling food motivation and food reward (i.e., hippocampus, amygdala, anterior cingulate, and parahippocampus) prior to lunch; however, the addition of a protein-rich breakfast led to reduced activation in these regions. The current study extended our previous fMRI findings to include HVA concentrations which serve as another marker of central food motivation and reward. We found that the addition of breakfast led to increased HVA concentrations throughout the morning with a trend for greater increases in HVA following the high protein breakfast vs. normal protein version. These data, along with the positive correlation between HVA concentrations and breakfast protein quantity, suggest that the consumption of increased dietary protein potentially stimulates the formation, secretion, and/or utilization of dopamine. The increased HVA concentrations throughout the morning were also accompanied by reduced food cravings, further supporting the role of dopamine on food reward.
Although central dopamine regulates a number of pathways in the body that impact cognition, motor control, mood, pain perception, and sexual behavior , it has also been shown to mediate food motivation and reward through a variety of learning and motivational pathway . See Meye & Adan review . Although dopamine is typically secreted in response to high fat foods, chronic exposure to these foods, particularly in obese individuals, leads to neural adaptations including reductions in dopamine receptor expression and decreased dopamine secretion . In diet-induced obese rodents, this reduced dopamine response leads to an overcompensation of foods high in fat to potentially re-establish normal dopamine concentrations . However, unlike dietary fat, the chronic consumption of protein-rich foods appears to elicit a more balanced, sustain influence on food reward as illustrated by the reductions in high fat and high sugar, evening snacking following the chronic (i.e., 7-d) consumption of high protein vs. normal protein breakfast meals .
In humans, psychostimulant drugs such as amphetamines and cocaine increase dopamine secretions and are known to have anorexigenic effects . Further, the administration of dopamine agonists such as bromocriptine and methylphenidate have been shown to significantly reduce body fat and body weight in obese humans [47, 48]. Although the exact mechanisms behind this phenomenon have not been determined, these studies report reduced daily energy intake with the administrations of these agonists. Taken together, these data suggest that dopamine appears to play a critical role in modulating the reinforcing value and reward of food. Further, the dopamine pathway is blunted in obesity due to chronic exposure to highly palatable foods but can be re-established with pharmaceutical agents and potentially dietary factors including breakfast and increased dietary protein.
This study was likely to have been limited by the small sample size (n = 16). Although this sample size was adequate to detect differences between skipping breakfast vs. breakfast, a larger sample may have led to an increased ability to detect differences, particularly in HVA concentrations, between breakfast meals. However, further research, involving increased sample sizes, longer testing durations, and assessments of subsequent food intake are key in assessing the role of increased dietary protein at breakfast on dopamine, food motivation, and reward in overweight/obese teens.