Repeated exposure is an intervention that has people taste the same food repeatedly without a reward, aiming to change liking or intake of that food. Bitterness is cited as one major reason children reject vegetables, and becoming accustomed to vegetables, including bitter ones, underlies this analysis. A meta-analysis mainly of infants and young children, covering 43 papers and 117 comparisons, found higher vegetable liking (standardized mean difference 0.35) and intake (0.23) with repeated exposure than with no exposure. This corresponded to about 0.6 points on a 9-point liking scale. However, the authors themselves say 「効果量は小さい」 (in translation: "the effect sizes are small"); many included studies had high risk of bias, and comparisons were not stratified by bitterness intensity.
In the same analysis, the effect on liking for a novel vegetable did not persist to the longest follow-up (standardized mean difference 0.15, not statistically significant). In contrast, a study of 95 infants who had just begun complementary feeding reported that improved liking after 10 repeated exposures to artichoke purée remained stable for three months after exposure. Because the age groups differ, this may indicate age-related differences in how long an effect persists rather than a contradiction.
In one adult study, after a commercially available bitter-sweet drink was consumed at home once daily for 7 days, liking for that drink rose 68%, while a control drink tasted only in the laboratory did not change. In another experiment, repeated consumption for two weeks of EGCG, a tea catechin dissolved in water, reduced ratings of bitterness for that same EGCG solution compared with repeated consumption of water alone. These results concern a particular compound and a particular drink; they cannot be generalized to dishes or foods as a whole.
Appleton KM; Hemingway A; Rajska J; Hartwell H / 2018 / review / full text checked
Checked for this overview: Checked effect sizes for liking and intake after repeated exposure. / Checked the authors' summary that effect sizes were small. / Checked that the effect did not persist to the longest follow-up.
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repeated dietary exposure increases Liking of vegetables (SMD 0.35 [0.17, 0.53]) and intake (SMD 0.23 [0.07, 0.39]), compared with no exposure. About 0.6 points on a 9-point liking scalesupported
Scope: A meta-analysis of RCT comparisons (43 papers, 117 comparisons). Mainly infants and young children
Note: The effect size is small, and many of the original studies are at high risk of bias. Bitterness is mentioned as a major factor in vegetable rejection, but there is no analysis stratified by bitterness intensity.
Repeated exposure raised post-intervention liking compared with no exposure (SMD 0.35 [0.17, 0.53]; 17 comparisons, 820 participants), and also raised intake (SMD 0.23 [0.07, 0.39]; 21 comparisons, 1130 participants). This corresponds to about 0.6 points of liking on a 9-point scale, and about 10 g of vegetable.
Limitations: Most of the included studies are small and at high risk of bias. Heterogeneity is low (I2 25-29%), but the effect size is small.
Limits: The authors state verbatim: "Effect sizes, however, are small, and limited evidence suggests long-term benefits." Follow-up after the intervention was often limited.
Limitations: Follow-up periods were inconsistent; the authors themselves acknowledge limitations in adherence and reporting.
8 daily tasting sessions of sweet red pepper significantly increased liking (P = 0.006) and intake (P = 0.03) compared with a no-treatment control. The sticker-reward condition was intermediate, not differing significantly from either group.
Limitations: A single vegetable (sweet red pepper, not strongly bitter). Small sample. Abstract-level reading only.
Show dataprocess-repeated-exposure —increases→ "Liking of vegetables (SMD 0.35 [0.17, 0.53]) and intake (SMD 0.23 [0.07, 0.39]), compared with no exposure. About 0.6 points on a 9-point liking scale"
repeated dietary exposure has limited long-term persistence of Improvement in liking: at the longest follow-up point, no effect remained on liking for novel vegetables (SMD 0.15 [-0.21, 0.50], P = 0.41)supported
Scope: A follow-up analysis within the same meta-analysis. Follow-up periods were often short and inconsistent
Note: The authors' overview: 『Effect sizes, however, are small, and limited evidence suggests long-term benefits』. A counter/limiting claim showing the limits of the durability of acquired liking. Durability may be age-window dependent: in 95 infants, the effect was stable up to 3 months later (Remy 2013, linked as contradicts).
At the longest follow-up point, no effect remained on liking for novel vegetables (SMD 0.15 [-0.21, 0.50], P = 0.41), and when measured at follow-up, effects had generally decreased.
Limitations: Few studies measured long-term outcomes, and the range of follow-up periods is wide (up to several years at most).
Contradicts: In 95 infants at the start of weaning (6.4 +/- 0.8 months), 10 repeated exposures to a basic artichoke puree raised intake (+63%) and liking (+21%), and verbatim, 'Learning of artichoke acceptance was stable up to 3 mo postexposure.' An adequately powered infant trial incorporating follow-up at 2 weeks, 3 months, and 6 months records persistence of the exposure effect over several months, conflicting with the meta-analysis finding that no effect remains at the longest follow-up point.
Limitations: The stability described explicitly in the abstract covers up to 3 months. Since the abstract does not report the result at 6 months, persistence beyond 3 months is untested here. Infants at the start of weaning are a different population from the older children who form the core of the meta-analysis, so this may reflect an age window rather than a genuine contradiction.
Show dataprocess-repeated-exposure —has_limited_long_term_persistence_of→ "Improvement in liking: at the longest follow-up point, no effect remained on liking for novel vegetables (SMD 0.15 [-0.21, 0.50], P = 0.41)"
repeated dietary exposure increases Adult hedonic ratings of a bittersweet beverage (+68% after once-daily consumption at home for 7 days). Meanwhile, a control stimulus tasted only in the laboratory did not changesupported
Scope: Adult volunteers, one commercially available bittersweet beverage (Stein 2003)
Note: A single study. The beverage's composition (presence or absence of caffeine) and n are not reported in the abstract, and the mechanistic attribution (post-ingestive association) remains the authors' speculation.
After consuming a commercially available bittersweet beverage once daily at home for 7 days, hedonic ratings of that beverage rose by 68%, while ratings of a control stimulus tasted only in the laboratory did not change; health-related information did not affect the change in hedonic ratings.
Limitations: The abstract shows neither the beverage's composition (caffeine content) nor the sample size; a single study; the mechanistic attribution is the authors' speculation.
Context: In 20 moderate caffeine consumers, pleasantness ratings of a novel flavored beverage paired with caffeine (CS+) rose gradually across 8 breakfast conditioning trials, while the pleasantness of a beverage paired with placebo (CS-) did not change. The authors conclude that this change 'are best explained in terms of Pavlovian associations between drink flavour and the postingestive effects of caffeine', providing direct experimental support for the post-ingestive mechanism that Stein 2003 could only speculate about.
Limitations: Since the abstract does not describe this beverage as bitter, this does not reproduce the result for bittersweet beverages, but rather contextualizes the acquisition mechanism for caffeinated beverages in general. Despite the change in pleasantness, overall preference did not change.
Limits: Health-related information did not affect the perceptual (hedonic) change accompanying exposure itself, but tended to somewhat increase the behavioral measure of acceptability. The authors interpret this as information potentially acting more strongly on behavior than on hedonic evaluation.
Limitations: Stops at a 『傾向』 (in translation: "tendency"), and the abstract does not report statistical significance. The definition and measurement method of the behavioral measure (acceptability) are also absent from the abstract. Full text not reached (Elsevier/Appetite, Unpaywall closed, no repository copy).
Show dataprocess-repeated-exposure —increases→ "Adult hedonic ratings of a bittersweet beverage (+68% after once-daily consumption at home for 7 days). Meanwhile, a control stimulus tasted only in the laboratory did not change"
repeated dietary exposure can decrease bitternesssupported
Scope: An evaluation after repeatedly taking EGCG, a type of tea catechin, dissolved in water for 2 weeks
Note: This is a comparison with repeating water alone, and does not show a mechanism of becoming accustomed to all foods.
After repeatedly taking EGCG dissolved in water for 2 weeks, the bitterness rating for the same EGCG solution was lower than after repeating water alone.
Limitations: This is a result from examining a single compound dissolved in water over a short period, and cannot be extended to cooking or food in general.
repeated dietary exposure does not significantly influence A test conducted after 14 days to distinguish saltiness, sweetness, sourness, and bitterness, and an intensity evaluationpreliminary
Scope: A small pilot study of 10 healthy women using an oral gel containing capsaicin
Note: This is a result in which no difference was found in a trial with few participants, and is not proof that there is no effect at all.
Before and after using the capsaicin-containing gel in the mouth for 14 days, there was no statistically clear change in the performance of distinguishing saltiness, sweetness, sourness, and bitterness, or in the perceived intensity.
Limitations: This is a small pilot study, and there are also limitations in how the comparison group was set up. It is not proof that there is no effect at all.
Show dataprocess-repeated-exposure —does_not_significantly_influence→ "A test conducted after 14 days to distinguish saltiness, sweetness, sourness, and bitterness, and an intensity evaluation"