Flavour Liking and Preference Conditioned by Caffeine in Humans

Effects of front-of-package caffeine and sweetener disclaimers in Mexico: cross-sectional results from the 2020 International Food Policy Study

OBJECTIVE

Front-of-package warning labels introduced in Mexico in 2020 included disclaimers that caution against allowing children to consume products with non-sugary sweeteners and caffeine. We examined the awareness and use of the disclaimers among Mexican adults and youth 1 month after the regulation was implemented. We also investigated their impact on the perceived healthfulness of industrialised beverages designed for children.

DESIGN

Data on the awareness and use of the disclaimers were analysed. Two between-subjects experiments examined the effect of a sweetener disclaimer (Experiment 1, youth and adults) or a caffeine disclaimer (Experiment 2, only adults) on the perceived healthfulness of industrialised beverages. Interactions between experimental conditions and demographic characteristics were tested.

SETTING

Online survey in 2020.

PARTICIPANTS

Mexican adults (≥18 years, n 2108) and youth (10-17 years, n 1790).

RESULTS

Most participants (>80 %) had seen the disclaimers at least rarely, and over 60 % used them sometimes or frequently. The sweetener disclaimer led to a lower perceived healthfulness of a fruit drink (adults: 2·74 ± 1·44; youth: 2·04 ± 0·96) compared with the no-disclaimer condition (adults: 3·17 ± 1·54; youth: 2·32 ± 0·96) (t's: >4·0, P values: <0·001). This effect was larger among older adults and male youth. The caffeine disclaimer did not affect adult's perceived healthfulness of a caffeinated drink (t = 0·861, P value = 0·3894).

CONCLUSIONS

There were high awareness and use of the sweeteners and caffeine disclaimers shortly after the warning labels were implemented. The sweetener disclaimer appears to be helping consumers modify their perceptions regarding industrialised beverages for children. Findings may help decision-makers improve the regulation and better target communication strategies.

Genetic determinants of liking and intake of coffee and other bitter foods and beverages

Coffee is a widely consumed beverage that is naturally bitter and contains caffeine. Genome-wide association studies (GWAS) of coffee drinking have identified genetic variants involved in caffeine-related pathways but not in taste perception. The taste of coffee can be altered by addition of milk/sweetener, which has not been accounted for in GWAS. Using UK and US cohorts, we test the hypotheses that genetic variants related to taste are more strongly associated with consumption of black coffee than with consumption of coffee with milk or sweetener and that genetic variants related to caffeine pathways are not differentially associated with the type of coffee consumed independent of caffeine content. Contrary to our hypotheses, genetically inferred caffeine sensitivity was more strongly associated with coffee taste preferences than with genetically inferred bitter taste perception. These findings extended to tea and dark chocolate. Taste preferences and physiological caffeine effects intertwine in a way that is difficult to distinguish for individuals which may represent conditioned taste preferences.

Caffeine-based flavor preference conditioning in the rat

The massive consumption of caffeine-containing beverages has prompted many studies involving human participants that have obtained caffeine-based increases in liking for a flavor. However, few studies have succeeded in obtaining caffeine-based flavor preference learning in rats. The main aim of the present study was to examine the conditions under which such learning can be detected. Three experiments differed mainly in terms of the base solution to which caffeine was added. Using a base of maltodextrin and saccharin, Experiment 1 found modest increases in flavor preferences in both food- and fluid-restricted rats. Experiment 2 found a strong caffeine-based flavor preference when water, but not saccharin, was used as the base. Whereas the first two experiments used a within-subject design, in which one flavor was paired with caffeine and a second flavor was not, Experiment 3 used a between-subject design with fluid-restricted rats given almond-flavored water containing caffeine in the Paired condition but not in the Unpaired condition; caffeine-based flavor preference learning was again found. In Experiments 1 and 2 post-conditioning exposure to the flavor alone produced a decrease in preference. In summary, the main achievements of this study were to extend the conditions under which caffeine-based flavor preferences can be detected in rats and demonstrate that such learned preferences are subject to extinction.

Feasibility and acceptability of a randomized controlled trial to investigate withdrawal symptoms in response to caffeinated sugary drink cessation among children

Background

Sugary drinks (SDs) are key contributors to excess added sugar intake and the predominant source of caffeine among children. Chronic caffeine intake causes dependence, and evidence for sugar dependence is emerging. Development of withdrawal symptoms may pose an obstacle to SD cessation among children. We examined the feasibility and acceptability of a three-arm randomized controlled trial (RCT) designed to investigate withdrawal symptoms resulting from replacement of children's usual caffeinated SD intake with either caffeine-free alternatives or caffeine-free and sugar-free alternatives, compared with continued consumption of caffeinated SDs.

Methods

Twenty-nine children 8–12 years old, who consumed ≥12 ounces caffeinated SDs daily, enrolled. The two-week RCT required three in-person meetings and daily completion of electronic questionnaires to assess withdrawal symptoms and intervention adherence. Children were randomized to replace their usual caffeinated SD consumption with 1) caffeine-free alternatives, 2) caffeine-free and sugar-free alternatives, or 3) caffeinated SDs (control), provided by the study team. Feasibility and acceptability were assessed quantitatively and qualitatively.

Results

Twenty-eight participants (97%) completed the study. Adherence was high, with 73% reporting compliance with beverage assignments, and 76% completing all questionnaires. In qualitative interviews at follow-up, children described feelings of importance and commitment, and parents did not find the procedures to be overly burdensome. While challenges to adherence were reported (e.g., child wanting other SDs, time commitment), participants described innovative strategies (e.g., designating a place for study drinks in the refrigerator) to maintain adherence.

Conclusion

Results indicated high levels of RCT feasibility and acceptability. The reported barriers and strategies for adherence will inform modifications required to design a larger and longer-term trial investigating withdrawal symptoms after SD cessation in children.

Effects of gastrointestinal delivery of non-caloric tastants on energy intake: a systematic review and meta-analysis

Purpose

Taste receptors are expressed throughout the gastrointestinal tract. The activation of post-oral taste receptors using tastants could provide a non-invasive treatment option in combating the obesity epidemic. The aim of this review was to examine the effect of post-oral delivery of non-caloric tastants on eating behavior reflected by primary outcome energy intake and secondary outcomes GI symptoms and perceptions and potential underlying mechanisms. This review was conducted according to the PRISMA guidelines for systematic reviews.

Methods

A systematic literature search of the Cochrane, PubMed, Embase, and Medline databases was performed. This systematic review and meta-analysis was registered in the PROSPERO database on 26 February 2020 (ID: CRD42020171182). Two researchers independently screened 11,912 articles and extracted information from 19 articles. If at least two studies investigated the effect of the same taste compound on primary outcome energy intake, a meta-analysis was performed to determine pooled effect sizes.

Results

Nineteen papers including healthy volunteers were included. In the 19 papers analyzed, effects of various tastants were investigated in healthy volunteers. Most extensively investigated were bitter tastants. The meta-analysis of effects of bitter tastants showed a significant reduction in energy intake of 54.62 kcal (95% CI − 78.54 to − 30.69, p = 0.0014).

Conclusions

Bitter stimuli are most potent to influence eating behavior. Energy intake decreased after post-oral delivery of bitter tastants. This highlights the potential of a preventive role of bitter tastants in battling the obesity epidemic.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00394-021-02485-4.

Knowing too much: Knowledge of energy content prevents liking change through flavour-nutrient associations

Associations between flavours and the consequences of ingestion can lead to changes in flavour liking depending on nutrient content, an example of flavour-nutrient learning (FNL). Expectations about the consequences of ingestion can be modified by information at the point of ingestion, such as nutritional labelling. What is unknown is the extent to which these label-based expectations modify FNL. Since nutrient information can alter expectations about how filling a product would be, we hypothesised that labels predicting higher energy (HE) content would enhance satiety and so promote more rapid flavour learning. To test this, participants consumed either a lower energy (LE: 164 kcal) or HE (330 kcal) yoghurt breakfast on four separate days, either with no product label or with labels displaying either the actual energy content (Congruent label) or inaccurate energy (Incongruent label). Participants rated liking on all four days: on Days 1 and 4, they could also consume as much as they liked, but consumed a fixed amount (300 g) on Days 2 and 3. Both liking and intake increased with exposure in the HE, and decreased in the LE, condition when unlabelled in line with FNL. In contrast, no significant changes were seen in either the Congruent or Incongruent label conditions. Contrary to predictions, these data suggest that FNL occurs when there is an absence of explicit expectations of actual nutrient content, with both accurate and inaccurate information on nutrient content disrupting learning.

Caffeine Deprivation State Modulates Expression of Acquired Liking for Caffeine-Paired Flavours

Previous studies found that caffeine consumers acquired a liking for the flavour of novel caffeinated drinks when these drinks were consumed repeatedly in a caffeine-deprived, but not nondeprived, state. Expression of this acquired liking appeared acutely sensitive to current caffeine deprivation state, but the use of between-subjects designs confounded interpretation of those studies. The present study evaluated these findings further using a within-subject design, with one flavour paired with caffeine (CS +) and the second with the absence of caffeine (CS–). During four CS + and four CS– training days, 32 moderate caffeine consumers alternatively consumed a novel flavoured drink a CS + paired with caffeine and a CS– flavour paired with placebo. Participants evaluated both drinks before and after training in two motivational states: caffeine deprived and nondeprived. As predicted, pleasantness ratings for the caffeine-paired flavour increased overall. However, this acquired liking was only significant when tested in a caffeine-deprived state. These data are consistent with a conditioned-flavour preference model and imply that expression of acquired liking for a novel caffeinated flavour depends on the need for the effects of caffeine at the time when the drink is evaluated.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

Can energy drinks increase the desire for more alcohol?

Energy drinks, the fastest growing segment in the beverage market, have become popular mixers with alcohol. The emerging research examining the use of alcohol mixed with energy drinks (AmEDs) indicates that the combination of caffeine-containing energy drinks with alcohol may be riskier than the use of alcohol alone. The public health concerns arising from AmED use are documented in different research domains. Epidemiologic studies reveal that the consumption of AmEDs is frequent among young and underage drinkers, demographic groups that are more likely to experience the harms and hazards associated with alcohol use. In addition, for all consumers, elevated rates of binge drinking and risk of alcohol dependence have been associated with AmED use when compared to alcohol alone. Results from laboratory studies help explain why AmED use is associated with excessive intake of alcohol. When an energy drink (or caffeine) is combined with alcohol, the desire (or urge) to drink more alcohol is more pronounced in both humans and animals than with the same dose of alcohol alone. The experience of drinking alcohol appears to be more rewarding when combined with energy drinks. Given that caffeine in other foods and beverages increases preference for those products, further research on AmEDs may elucidate the underlying mechanisms that contribute to alcohol dependence.

Factors that influence the reinforcing value of foods and beverages

Behavioral economic principles state that as the cost of a product increases, purchasing or consumption of that product will decrease. To understand the impact of behavioral economics on ingestive behavior, our laboratory utilizes an operant behavior paradigm to measure how much work an individual will engage in to get access to foods and beverages. This task provides an objective measure of the reinforcing value. We have shown that consumption of the same high fat snack food every day for two weeks reduces its reinforcing value in lean individuals, but increases its reinforcing value in a subset of obese individuals. This increase in the reinforcing value of food predicts future weight gain. Similarly, we have shown that repeated intake of caffeinated soda increases its reinforcing value in boys, but not in girls. This increase in reinforcing value is not related to usual caffeine consumption, but may be associated with positive, subjective effects of caffeine that are more likely to be reported by boys than by girls. Because food and beverage reinforcement relates to real-world consumption, it is important to determine factors that increase or decrease the reinforcing value and determine the consequences of these responses. We are especially interested in determining ways to shift the behavioral economic curve in order to develop novel strategies to decrease the reinforcing value of less healthy snack foods and beverages, such as soda, potato chips and candy and to increase the reinforcing value of healthier foods and beverages, such as water, fruits, and vegetables.

Caffeine increases liking and consumption of novel-flavored yogurt

RATIONALE

Caffeine has been shown to increase preference for beverages with which it is paired; however, it is not known if caffeine alters liking for foods with which it is paired indirectly.

OBJECTIVES

The purpose of the current experiment was to test the hypothesis that a caffeinated beverage paired with a novel-flavored yogurt will increase preference for that yogurt compared to one paired with placebo. We also tested the hypothesis that liking would increase more when caffeine was paired with high energy density yogurt.

METHODS

Men and women (n = 62) were randomized to receive a beverage containing placebo (PLA) or caffeine (CAF) and to consume a low (LED) or high energy density (HED), novel-flavored yogurt. Participants rated, ranked, and consumed seven novel-flavored yogurts and then had a target yogurt paired with either PLA or CAF over four consecutive days.

RESULTS

In general, yogurt liking increased over time, the HED yogurt was liked more than the LED yogurt, and yogurt paired with caffeine was liked more than yogurt paired with placebo. Participants showed a significant increase in liking of LED yogurt paired with caffeine compared to those with LED yogurt paired with placebo.

CONCLUSIONS

Caffeine administration may increase liking and consumption of novel-flavored foods, particularly if the food is not highly liked at baseline. This suggests that caffeine pairing may be a way to increase liking of LED foods, such as vegetables and fruit.

Obesity: lessons from evolution and the environment

The 9th Stock Conference acknowledged the complex background of genetic, cultural, environmental and evolutionary factors of obesity. Gene-environment interactions underlie the flexibility in body-weight and body-fat regulation, illustrated by the hunter-gatherers' feast and famine lifestyle, the variation in physical activity over the lifespan being highest at reproductive age, the variation in energy intake through 'eating in the absence of hunger', while running the risk of exceeding the capacity of triacylglyceride storage, leading to lipotoxicity and metabolic problems. Perinatal metabolic programming for obesity via epigenetic changes in response to a 'Western diet' results in production of lipid-poor milk and metabolically efficient pups, contributing to the perpetuation of obesity throughout generations. Evolutionary insight from comparative physiology and ecology indicates that over generations activity-induced energy expenditure has remained the same compared to wild mammals, that energy balance might be dependant on protein balance, while the function of taste changed from detection of poison or energy to social drinking and social behaviour. At present, the impact of assortative mating on obesity prevalence is unambiguously positive. The complexity that appeared can only be fully appreciated by setting the data into the context of our evolutionary history.

Influence of caffeine on the liking of novel-flavored soda in adolescents

BACKGROUND

Soda manufacturers claim that caffeine is added to soda as a flavor enhancer, but many researchers have speculated that caffeine is added to increase the hedonic and reinforcing properties of the soda. Studies in adults have demonstrated that caffeine can condition flavor preferences when added to novel-flavored beverages.

OBJECTIVES

The purpose of this study was to test the hypothesis that caffeine added to novel-flavored drinks would increase liking and preference in adolescents.

METHODS

Adolescents (n = 99) between the ages of 12 and 17 rated and ranked seven novel soda drinks. They were then randomly assigned to consume one of these beverages paired with either caffeine (1 or 2 mg/kg) or placebo over four consecutive days and rate liking. On the final visit, participants retasted the seven beverages and provided hedonic ratings and rankings.

RESULTS

Participants in the 2-mg/kg caffeine group increased the liking of the beverage over the exposure period after an initial decrease, but there was no change in liking for those in the placebo group or in the 1-mg/kg group. The increase in liking in the 2-mg/kg group was accompanied by a decrease in perceived bitterness, but no change in beverage ranking or consumption during the post-test.

CONCLUSIONS

Caffeine added to novel beverages results in a decrease in liking followed by an increase in liking with repeated exposures that may result from habituation to the bitterness of caffeine. Change in bitter perception may be the mechanism by which adolescents establish regular caffeine use.

Caffeine increases the motivation to obtain non-drug reinforcers in rats

BACKGROUND

Caffeine is widely considered to be a reinforcer in humans, but this effect is difficult to measure in non-human animals. We hypothesized that caffeine may have dual reinforcing effects comparable to nicotine--limited primary reinforcing effects, but potent reinforcement enhancing effects. The present studies tested this hypothesis by investigating the effect of caffeine on responding for non-drug rewards.

METHODS

In two experiments, rats were shaped to respond on a progressive ratio (PR) schedule for sucrose solution (20%, w/v; experiment 1) or a fixed ratio 2 (FR2) schedule for a moderately reinforcing visual stimulus (VS; experiment 2). Pretreatment with various doses of caffeine (0-50 mg/kg, intraperitoneal injection) were administered prior to tests over successive week days (M-F). In experiment 1, acute administration of low-moderate caffeine doses (6.25-25 mg/kg) increased responding for sucrose under the PR schedule. This effect of caffeine declined over the initial 15 test days. In experiment 2, only acute pretreatment with 12.5mg/kg caffeine increased responding for the visual stimulus and complete tolerance to this effect of caffeine was observed over the 15 days of testing. In follow up tests we found that abstinence periods of 4 and 8 days resulted in incomplete recovery of the enhancing effects of caffeine.

CONCLUSION

The findings suggest that caffeine enhances the reinforcing effects of non-drug stimuli, but that the pharmacological profile of these effects may differ from other psychomotor stimulants.

Genetics of taste and smell: poisons and pleasures

Eating is dangerous. While food contains nutrients and calories that animals need to produce heat and energy, it may also contain harmful parasites, bacteria, or chemicals. To guide food selection, the senses of taste and smell have evolved to alert us to the bitter taste of poisons and the sour taste and off-putting smell of spoiled foods. These sensory systems help people and animals to eat defensively, and they provide the brake that helps them avoid ingesting foods that are harmful. But choices about which foods to eat are motivated by more than avoiding the bad; they are also motivated by seeking the good, such as fat and sugar. However, just as not everyone is equally capable of sensing toxins in food, not everyone is equally enthusiastic about consuming high-fat, high-sugar foods. Genetic studies in humans and experimental animals strongly suggest that the liking of sugar and fat is influenced by genotype; likewise, the abilities to detect bitterness and the malodors of rotting food are highly variable among individuals. Understanding the exact genes and genetic differences that affect food intake may provide important clues in obesity treatment by allowing caregivers to tailor dietary recommendations to the chemosensory landscape of each person.

Additive effects of flavour–caffeine and flavour–flavour pairings on liking for the smell and flavour of a novel drink

Previous research has established that caffeine consumption can reinforce changes in liking for caffeine-paired flavours, while pairing a novel flavour with a liked or dislike taste can also result in enduring changes in liking for the flavour. The present study examined how these two forms of flavour-learning interact. 72 habitual caffeine consumers who liked sweet tastes rated the odour and flavour of a novel tea drink before and after four training sessions where the flavour was paired with either 100 mg caffeine or placebo in one of three flavour contexts: added sweetness (aspartame), bitterness (quinine) or control. The liking for both the odour and flavour of the tea increased after pairing with caffeine regardless of flavour context, while pairing with bitterness reduced flavour liking regardless of the presence of caffeine. Pairing with quinine increased the rated bitterness of the tea odour, and reduced the rated sweetness of the tea flavour, post-training, independent of effects of caffeine. These data suggest that flavour-caffeine and flavour-flavour associations have additive effects on drink liking, while confirming that flavour-flavour associations can alter the immediate sensory experience of a flavour alone.

Associative learning and the control of human dietary behavior

Most of our food likes and disliked are learned. Relevant forms of associative learning have been identified in animals. However, observations of the same associative processes are relatively scarce in humans. The first section of this paper outlines reasons why this might be the case. Emphasis is placed on recent research exploring individual differences and the importance or otherwise of hunger and contingency awareness. The second section briefly considers the effect of learning on meal size, and the author revisits the question of how learned associations might come to influence energy intake in humans.

The genetics of tea and coffee drinking and preference for source of caffeine in a large community sample of Australian twins

AIMS

To investigate the genetic and environmental influences on tea consumption and their commonalities with coffee consumption; and to further examine the genetic and environmental aetiology of preference for tea/coffee.

DESIGN

A classical twin design was used in which the similarity of identical and non-identical twins is compared, enabling estimates of genetic, common environmental and unique environmental influence on the trait.

SETTING AND PARTICIPANTS

An Australian population-based sample of 1796 identical (i.e. monozygotic) and 2013 non-identical (i.e. dizygotic) twin pairs aged 16-87 years was studied, roughly three-fifths of whom were female. The sample represented approximately 70% of those approached for study participation.

MEASUREMENTS

As part of a Health and Lifestyle Questionnaire, respondents were asked how many cups of each tea and coffee they consumed per day. Additional measures of 'total tea and coffee consumption' and 'preference for coffee' were calculated.

FINDINGS

Age was positively associated with tea consumption but negatively associated with coffee preference; women consumed more beverages than men, but showed a lower preference for coffee. An inverse relation between tea and coffee consumption--larger in females (-0.41) than males (-0.34)--was supported. This association was mediated entirely by the unique environment in males, and by both the unique environment (68.3%) and genes (31.7%) in females. Tea and coffee drinking were shown to have similar heritabilities (0.46) in males, but tea consumption was influenced by common environmental factors whereas coffee consumption was not. Coffee preference was shown to be influenced by genes (0.42) and the unique environment (0.58).

CONCLUSIONS

As the patterns of genetic and environmental variation were shown to differ for tea and coffee consumption it may be more informative to retain them as separate measures of caffeine intake in future studies of stimulant use and taste perception.

Attentional bias for caffeine-related stimuli in high but not moderate or non-caffeine consumers

RATIONALE

Attentional bias for drug-related cues has been reported with a wide range of drugs, but to date the extent to which caffeine consumers show similar biases for caffeine-related stimuli has not been tested. The present study therefore examined this issue in terms of differences in attentional bias for caffeine-related words in High, Moderate and Non-caffeine consumers using a dot-probe word task following overnight caffeine abstinence.

OBJECTIVES

This study was conducted to test whether caffeine consumers show an attentional bias for caffeine-related words, and whether such biases relate to habitual levels of caffeine use.

METHODS

Sixteen High, Moderate and Non-consumers of caffeine were asked to complete a modified dot-probe task in order to measure attentional bias for caffeine-related relative to neutral control word groups. The task was completed following overnight caffeine abstinence, and participants also completed mood and caffeine-craving measures.

RESULTS

The High consumer group showed a significant attentional bias for the caffeine-related words, but no such bias was seen in Moderate or Non-consumer groups. As expected, craving for caffeine was strongest in the High consumers and weakest in the Non-consumers. Attentional bias in the High group correlated with self-reported caffeine consumption and with craving for caffeine, but neither effect was significant in the Moderate group.

CONCLUSIONS

These data confirm that High caffeine consumers show attentional bias for caffeine-related stimuli, consistent with current theories of drug addiction.