Dietary proteins and food-related reward signals

Food protein-originating peptides as tastants - Physiological, technological, sensory, and bioinformatic approaches

Taste is one of the factors based on which the organism makes the selection of what to ingest. It also protects humans from ingesting toxic compounds and is one of the main attributes when thinking about food quality. Five basic taste sensations are recognized by humans: bitter, salty, sour, sweet, and umami. The taste of foods is affected by some molecules of some specific chemical nature. One of them are peptides derived from food proteins. Although they are not the major natural compounds originating from food sources that are responsible for the taste, they are in the area of scientific research due to the specific composition of amino acids which are well-known for their sensory properties. Literature data implicate that sweet, bitter, and umami are the tastes attributable to peptides. Moreover, the bitter peptide tastants are the dominant among the other tastes. Additionally, other biological activities like, e.g., inhibiting enzymes that regulate the body functions and acting as preventive food agents of civilization diseases, are also associated with the taste of peptides. The advance in information technologies has contributed to the elaboration of internet archives (databases) as well as in silico tools for the analysis of biological compounds. It also concerns peptides - namely taste carriers originating from foods. Thus, our paper provides a summary of knowledge about peptides as tastants with special attention paid to the following aspects: a) basis of taste perception, b) taste peptides detected in food protein sequences with special emphasis put on the role of bitter peptides, c) peptides that may enhance/suppress the taste of foods, d) databases as well as bioinformatic approaches suitable to study the taste of peptides, e) taste-taste interactions, f) basis of sensory analysis in the evaluation of the taste of molecules, including peptides, and g) the methodology applied to reduce/eliminate the undesired taste of peptides. The list of taste peptides serving some biological functions is presented in the Supplement file. The information provided includes database resources, whereas peptide sequences are given with InChiKeys, which is aimed at facilitating the Google® search. Our collection of data regarding taste peptides may be supportive for the scientists working with the set of peptide data in the context of structure-function activity of peptides.

The Macronutrients, Appetite, and Energy Intake

Each of the macronutrients-carbohydrate, protein, and fat-has a unique set of properties that influences health, but all are a source of energy. The optimal balance of their contribution to the diet has been a long-standing matter of debate. Over the past half century, thinking has progressed regarding the mechanisms by which each macronutrient may contribute to energy balance. At the beginning of this period, metabolic signals that initiated eating events (i.e., determined eating frequency) were emphasized. This was followed by an orientation to gut endocrine signals that purportedly modulate the size of eating events (i.e., determined portion size). Most recently, research attention has been directed to the brain, where the reward signals elicited by the macronutrients are viewed as potentially problematic (e.g., contribute to disordered eating). At this point, the predictive power of the macronutrients for energy intake remains limited.

A randomized crossover, pilot study examining the effects of a normal protein vs. high protein breakfast on food cravings and reward signals in overweight/obese "breakfast skipping", late-adolescent girls

Background

This pilot study examined whether the addition of a normal protein (NP) vs. high protein (HP) breakfast leads to alterations in food cravings and plasma homovanillic acid (HVA), which is an index of central dopamine production, in overweight/obese ‘breakfast skipping’ late-adolescent young women.

Methods

A randomized crossover design was incorporated in which 20 girls (age 19 ± 1 y; BMI 28.6 ± 0.7 kg/m²) consumed 350 kcal NP (13 g protein) breakfast meals, 350 kcal HP (35 g protein) breakfast meals, or continued breakfast skipping (BS) for 6 consecutive days/pattern. On day 7 of each pattern, a 4 h testing day was completed including the consumption of breakfast (or no breakfast) followed by food craving questionnaires and blood sampling for HVA concentrations throughout the morning.

Results

Both breakfast meals reduced post-meal cravings for sweet and savory foods and increased HVA concentrations vs. BS (all, p < 0.05). Between breakfast meals, the HP breakfast tended to elicit greater reductions in post-meal savory cravings vs. NP (p = 0.08) and tended to elicit sustained increases in HVA concentrations prior to lunch vs. NP (p = 0.09). Lastly, HVA concentrations were positively correlated with the protein content at breakfast (r: 0.340; p < 0.03).

Conclusions

Collectively, these findings suggest that the addition of breakfast reduces post-meal food cravings and increases homovanillic acid concentrations in overweight/obese young people with higher protein versions eliciting greater responses.

Acute hormonal response to glucose, lipids and arginine infusion in overweight cats

In cats, the incidence of obesity and diabetes is increasing, and little is known about specific aspects of the endocrine control of food intake in this species. Recent data suggest that ghrelin has an important role in the control of insulin secretion and vice versa, but this role has never been demonstrated in cats. Here we aimed to improve our understanding about the relationship between insulin, amylin and ghrelin secretion in response to a nutrient load in overweight cats. After a 16 h fast, weekly, six overweight male cats underwent randomly one of the four testing sessions: saline, glucose, arginine and TAG. All solutions were isoenergetic and isovolumic, and were injected intravenously as a bolus. Glucose, insulin, acylated ghrelin (AG), amylin and prolactin were assayed in plasma before and 10, 20, 40, 60, 80 and 100 min after the nutrient load. A linear mixed-effects model was used to assess the effect of bolus and time on the parameters. A parenteral bolus of glucose or arginine increased insulin and ghrelin concentrations in cats. Except for with the TAG bolus, no suppression of ghrelin was observed. The absence of AG suppression after the intravenous load of arginine and glucose may suggest: (1) that some nutrients do not promote satiation in overweight cats; or that (2) AG may be involved in non-homeostatic consumption mechanisms. However, the role of ghrelin in food reward remains to be assessed in cats.

Gut-brain nutrient signaling. Appetition vs. satiation

Multiple hormonal and neural signals are generated by ingested nutrients that limit meal size and suppress postmeal eating. However, the availability of sugar-rich and fat-rich foods can override these satiation/satiety signals and lead to overeating and obesity. The palatable flavor of these foods is one factor that promotes overeating, but sugar and fat also have postoral actions that can stimulate eating and increase food preferences. This is revealed in conditioning studies in which rodents consume flavored solutions paired with intragastric sugar or fat infusions. The significant flavor preferences and increased intake produced by the nutrient infusions appear to involve stimulatory gut-brain signals, referred to here as appetition signals, that are distinct from the satiation signals that suppress feeding. Newly developed rapid conditioning protocols may facilitate the study of postoral appetition processes.

Dietary proteins and food-related reward signals

Proteins play a crucial role in almost all biological processes. Dietary proteins are generally considered as energy yielding nutrients and as a source of amino acids for various purposes. In addition, they may have a role in food-related reward signals. The purpose of this review was to give an overview of the role of dietary proteins in food-related reward and possible mechanisms behind such effects. Dietary proteins may elicit food-related reward by several different postprandial mechanisms, including neural and humoral signals from the gastrointestinal tract to the brain. In order to exert rewarding effects, protein have to be absorbed from the intestine and reach the target cells in sufficient concentrations, or act via receptors ad cell signalling in the gut without absorption. Complex interactions between different possible mechanisms make it very difficult to gain a clear view on the role and intesity of each mechanism. It is concluded that, in principle, dietary proteins may have a role in food-related reward. However, the evidence is based mostly on experiments with animal models and one should be careful in drawing conclusions of clinical relevance.