Metabolic regulation of brain response to food cues

Abstract

Identification of energy sources depends upon the ability to form associations between food cues and nutritional value. As such, cues previously paired with calories elicit neuronal activation in the nucleus accumbens (NAcc), which reflects the reinforcing value of food. The identity of the physiological signals regulating this response remains elusive. Using fMRI, we examined brain response to noncaloric versions of flavors that had been consumed in previous days with either 0 or 112.5 calories from undetected maltodextrin. We report a small but perceptually meaningful increase in liking for the flavor that had been paired with calories and find that change in liking was associated with changes in insular responses to this beverage. In contrast, NAcc and hypothalamic response to the calorie-paired flavor was unrelated to liking but was strongly associated with the changes in plasma glucose levels produced by ingestion of the beverage when consumed previously with calories. Importantly, because each participant ingested the same caloric dose, the change in plasma glucose depended upon individual differences in glucose metabolism. We conclude that glucose metabolism is a critical signal regulating NAcc and hypothalamic response to food cues, and that this process operates independently from the ability of calories to condition liking.

Figure 1

Study Design and Behavioral Protocol (A) Schematic depiction of the overall study design. For full details, please see “Experimental Design” in the Supplemental Experimental Procedures. Pretest: subjects participated in a pretest, four exposure days, a posttest, and an fMRI scanning session. In the pretest, subjects were trained on how to use the scales to rate overall intensity, sweetness intensity, liking, and wanting for each of ten noncaloric flavored beverages developed in-house. After training, subjects rated each of the flavor stimuli three times. Average ratings were calculated, and two beverages were selected to be close to neutral in liking and equally well liked. One of the beverages was designated as destined to be the 112.5 calorie beverage, and the other was designated as the 0 calorie beverage. Caloric content was achieved by adding maltodextrin, which is relatively tasteless and odorless. To confirm that subjects could not detect the presence of the maltodextrin, we performed a triangle test with a separate flavored beverage (see B). Finally, subjects were trained on the fMRI procedures. Exposure sessions: subjects participated in four exposure days (two per stimulus). In each exposure day, subjects drank one of the beverages three times. Thus, each stimulus (0 calorie-paired flavored beverage and 112.5 calorie-paired flavored beverage) was exposed six times (three drinks × two days). Blood was sampled once for each beverage. Posttest: subjects rated all ten flavored beverages from the pretest (including the noncaloric version of the two exposed flavors) and filled out questionnaires. Subjects participated in one fMRI scan session within three weeks of beginning the experiment. The figure on the right depicts the setup in our mock scanner where they receive training in the procedure prior to scanning. During scanning, subjects sampled the noncaloric versions of the 0 calorie-paired flavor, the 112.5 calorie-paired flavor, as well as one of the nonexposed flavors as a control stimulus and a tasteless and odorless solution. (B) To verify that study participants were unable to detect the presence of maltodextrin, they participated in triangle tests in which they indicated which of three cups was different. All cups contained the same flavor, but for each trial, either one or two cups also contained maltodextrin. Eight trials were conducted. We used the binomial distribution to set our criteria for maltodextrin detection; specifically, the minimum number of correct judgments to establish significance for the triangle test (one-tailed, α = 0.05, z = 1.64, probability of guessing p = 1/3) was calculated according to the formula $$\mathrm{X}={0.4714}^{\star }\mathrm{z}\surd \mathrm{n}+\left[\frac{(2\mathrm{n}+3)}{6}\right],$$ where n = number of trials, X = minimum number of correct judgments, and z = 1.64 if α is set to 5% [15]. For n = 8 tests, the minimum numbers of correct judgments is six. If they were unable to detect maltodextrin, they were given a lunch menu and asked to select items that would be provided as their lunch (sandwiches, chips, and fruit) in the exposure sessions. Each subject could select as many items as he or she wanted, but he or she was required to eat exactly the same thing for all lunch sessions. The number of correct trials for qualified subjects ranged from 2 to 5 (out of 8 possible), with an average of 3.58 and a SD of 1.16. Thus, no subject performed significantly differently from chance, allowing us to conclude that subjects were unable to detect the presence of maltodextrin in the flavored beverage. (C) Cartoon depicting the timing of stimulus delivery. There are two event types: “flavor” and “tasteless.” During flavor events, one of the three flavored beverages (control, 0 calorie-paired flavor, and 112.5 calorie-paired flavor) is delivered over 4 s. Note that no maltodextrin is added to the 112.5 calorie-paired flavor during scanning. Therefore, all flavors are noncaloric during scanning. Following delivery of the flavors, subjects are instructed to swallow and exhale through their nose so that the volatiles escape the oral cavity to reach the olfactory epithelium and induce retronasal olfaction, which is a key component of flavor perception. A variable period of rest (6–10.5 s) then follows. This “jitter” is important for aiding deconvolution of the hemodynamic response. Following the jitter, deionized water is delivered using the same procedure as the flavors, to rinse the mouth. During the tasteless event, the tasteless and odorless control solution is delivered over 4 s.

Figure 2

Hedonic Conditioning (A) The conditioning procedure produced a weak influence on overall liking ratings across the flavored beverages [two-way repeated-measures (RM) ANOVA, caloric load × time effect F(1,13) = 3.3; p = 0.05]. Liking ratings increased significantly in post- compared to preconditioning sessions for the 112.5 kcal-paired flavor (post hoc paired two-sample t test, p < 0.04, Bonferroni corrected), but not for the 0 kcal-paired flavor (p = 0.09). No effects were associated with a control flavor for which no conditioning sessions were performed. (B) A significant correlation is observed between the magnitude of hedonic conditioning to the CS+ flavor shown in (A) with the differential response in the anterior insula to the CS+ flavor versus response to the unexposed control flavor. The peak is at −39, 14, 2−5; z = 3.13, k = 16; p = 0.04, similar analyses were performed for CS−, yielding significant correlations at −42, 11, 1; z = 3.0; p = 0.03 false discovery rate (FDR) corrected across the voxels of the taste cortex region of interest. This and all subsequent images are thresholded at p < 0.005 and a cluster criterion of 5. The color bar represents the t value. The graph depicts the contrast estimate (y axis) extracted from the peak voxel plotted against the change in liking ratings posttest minus pretest.

Figure 3

Biological Utility Conditioning (A) Subject-normalized plasma glucose levels increased significantly 30 min following ingestion of the 112.5 kcal, but not the 0 kcal, flavored beverage [two-way RM ANOVA, caloric load × time effect F(1,13) = 157.7; p < 0.001]. Post hoc paired two-sample t test, p < 0.001; NS, statistically nonsignificant; paired two-sample t test, p = 0.39. 112.5 kcal, 112.5 kcal beverage; 0 kcal, 0 kcal beverage. (B) No overall decreases in hunger levels were observed after ingestion of the flavored beverages (two-way RM ANOVA, caloric load × time effect; p = 0.37); however, hunger levels increased significantly 30 min following ingestion of the 0 kcal, but not the 112.5 kcal, beverage (post hoc paired two-sample t test, p = 0.04). (C) Consistently, changes in hunger levels were not correlated with changes in glucose levels. Data are shown as standardized (z score) changes in glucose. D112.5 kcal, changes in glucose levels (post- versus preconditioning session) for the 112.5 kcal beverage. Similar definition applies for Δ0 kcal. (D) Coronal sections showing positive correlations between parameter estimates (PEs) associated with the statistical parametric map (SPM) contrast (112.5 kcal-paired [CS+] flavor minus 0 kcal-paired [CS−] flavor) and changes in plasma glucose levels (Δglucose, in mg/dl) following ingestion of the 112.5 versus the 0 kcal beverage during the conditioning sessions in the NAcc (MNI coordinates on the left are −6, 8, −5 [z = 3.9] and right = 9, 5, −8 [z = 3.5] [graph not shown for right NAcc]) and hypothalamus (MNI coordinates = 3, −1, −5; z = 3.7). Scatter plots displaying the relationship between changes in glucose levels versus PEs from these voxels are shown to the right of each coronal section. These peaks fall within a single cluster that was found to be significant at p < 0.05 FDR corrected at the cluster level across the whole brain (number of voxels in cluster k = 107). Color scale bars correspond to SPM t values. Changes in glucose are shown as standardized (z score). When liking was included as the first regressor and Δglucose as the second, minimal changes were observed in the results. In NAcc, the peak voxels were located at −3, 5, −5 (z = 3.81) and 9, 5, −5 (z = 3.35). In hypothalamus, the peak voxels were located at 3, −1, −5 (z = 3.7) and 3, −1, −5 (z = 3.9). As in the original analysis, these peaks fall within a single cluster that was found to be significant at p < 0.05 FDR corrected at the cluster level across the whole brain. See also Table S1.

Figure 4

Changes in Flavor Liking Fail to Regulate Calorie-Predictive Flavor Responses in Human NAcc and Hypothalamus (A) No significant associations in NAcc or hypothalamus were found between the parameter estimates (PEs) associated with the SPM contrast (CS+ minus CS−) and standardized (z score) postconditioning CS+ liking ratings minus post-conditioning CS− liking ratings. For (A) and (B), the z and p values are uncorrected for multiple comparisons. au, arbitrary units. (B) No significant associations in NAcc or hypothalamus were found between responses revealed by the SPM contrast (CS+ minus CS−) and standardized (z score) postconditioning liking ratings for the CS+. (C) No significant associations were observed between standardized (z score) changes in liking ratings and standardized (z score) changes in glucose or hunger levels. For statistically nonsignificant correlations, p > 0.5. ΔCS+ is change in ratings (post- versus preconditioning) for the CS+ flavor. Similar definition applies for ΔCS−.