Restriction of dietary protein leads to conditioned protein preference and elevated palatability of protein-containing food in rats

Abstract

The mechanisms by which intake of dietary protein is regulated are poorly understood despite their potential involvement in determining food choice and appetite. In particular, it is unclear whether protein deficiency results in a specific appetite for protein and whether influences on diet are immediate or develop over time. To determine the effects of protein restriction on consumption, preference, and palatability for protein we assessed patterns of intake for casein (protein) and maltodextrin (carbohydrate) solutions in adult rats. To induce a state of protein restriction, rats were maintained on a low protein diet (5% casein) and compared to control rats on non-restricted diet (20% casein). Under these dietary conditions, relative to control rats, protein-restricted rats exhibited hyperphagia without weight gain. After two weeks, on alternate conditioning days, rats were given access to either isocaloric casein or maltodextrin solutions that were saccharin-sweetened and distinctly flavoured whilst consumption and licking patterns were recorded. This allowed rats to learn about the post-ingestive nutritional consequences of the two different solutions. Subsequently, during a preference test when rats had access to both solutions, we found that protein-restricted rats exhibited a preference for casein over carbohydrate whereas non-restricted rats did not. Analysis of lick microstructure revealed that this preference was associated with an increase in cluster size and number, reflective of an increase in palatability. In conclusion, proteinrestriction induced a conditioned preference for protein, relative to carbohydrate, and this was associated with increased palatability.

Investigating the Effect of Physiological Need States on Palatability and Motivation Using Microstructural Analysis of Licking

“…In these 'forced choice' trials, no difference in cluster number or cluster size was seen, similar to the pattern of results during conditioning in our previous study when only one bottle was available ( Figure 3B, left). These results demonstrate that, in absence of choice and a comparison between the two nutrientcontaining solutions, rats do not differ in their evaluation of each solution, which explains the similar amount of licking (Chiacchierini, et al, 2019;Murphy, et al, 2018).…”

“…Circles are data from individual rats and bars are mean. Results of this study have previously been published inMurphy et al (2018). † † p<0.01 (2-way ANOVA followed by Sidak's post hoc tests casein vs. maltodextrin).…”

“…Our group recently showed that this shift to a low protein diet increases the preference for a distinctly-flavored solution that contains the dairy protein, casein, relative to a solution containing the carbohydrate, maltodextrin (Murphy et al, 2018). Analysis of lick microstructure showed that the increased casein preference in protein-restricted rats was associated with both an increase in cluster number and cluster size.…”

Protein appetite drives macronutrient-related differences in ventral tegmental area neural activity

“…Recently, we developed a rodent model of protein appetite in which animals rodents maintained on a protein-restricted diet developed a strong preference for a protein-rich solution, relative to a carbohydrate-rich solution (Murphy et al, 2018; see also Hill et al, 2019), indicating that animals can specifically direct feeding and food-seeking behavior towards protein sources in times of need. However, the neural mechanisms by which diets that are low in protein might shift behavior are not understood.…”

“…Here, we use in vivo fiber photometry to record the activity of VTA neurons during consumption of isocaloric protein-and carbohydrate-containing solutions in an animal model of protein preference (Murphy et al, 2018;Naneix et al, 2019Naneix et al, , 2020. We find that, in protein-restricted animals, protein consumption is associated with elevated neural activation, relative to carbohydrate consumption.…”