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 flavored 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, protein-restriction induced a conditioned preference for protein, relative to carbohydrate, and this was associated with increased palatability.

Age dependent effects of protein restriction on dopamine release

“…When performed at adulthood, protein restriction did not affect rats' weight, consistent with our previous results [11]. Adult animals have lower protein requirements compared to adolescents, who are rapidly growing and developing.…”

“…We first investigated the impact of protein restriction (PR) during either adolescence or adulthood on weight and weight gain ( Figure 1C). As we previously observed [11], protein restriction at adulthood did not significantly affect either the weight of rats (two-way repeated measures ANOVA: Diet F(1,13) = 0.5, p = 0.5; Day F(11, 143) = 85.5, p < 0.001; Diet x Day F(11, 143)= 0.4, p = 1.0) or weight gain (NR: +48 g ± 6 / PR: +40 g ± 6; Diet t(13) = 0.9, p = 0.4). In contrast, protein restriction during adolescence significantly decreased weight gain, relative to control diet (Diet F(1,11) = 19.8, p < 0.001; Day F(13, 143) = 478.7, p < 0.001; Diet x Day F(13, 143)= 234.0, p < 0.001).…”

“…Previous studies have shown that the impact of protein restriction on weight is highly dependent on the degree of restriction and the physiological state of the animal [9,10,53]. Accordingly, we previously reported a slight increase in food intake in these diet conditions [11], which may explain the absence of effect on animals' weight as they attempted to compensate protein deficiency with a general hyperphagia. An alternative explanation is that low protein diet may change energy expenditure, as previously observed [64].…”

“…All rats were initially maintained on standard laboratory chow diet (Teklad global #2918, Envigo) containing 18% protein. One week after arrival rats either continued on standard laboratory chow diet (Non Restricted group; Adolescents-NR n = 6, Adults-NR n = 8) or were switched to a modified AIN-93G diet containing 5% protein from casein (#D15100602, Research Diets; Protein Restricted group: Adolescents-PR n = 7, Adults-PR n = 7) [11]. Rats had ad libitum access to their assigned diet.…”

“…Furthermore, protein deficiency and severe protein malnutrition are especially detrimental during development and early life when demand is highest [5][6][7]. Numerous species, including humans and rodents, regulate their food intake and food-related behaviors to avoid protein deficiency [8][9][10][11][12][13]. Increasing evidence implicates broad hypothalamic and limbic circuits in the regulation of protein appetite [10,[13][14][15].…”