We tested whether subjects who showed > 2.5% increase in BMI over the 6-month follow-up (N = 8, M %BMI change = 4.41, range = 2.6 to 8.2) exhibited a reduction in caudate activation in response to milkshake intake relative to those who showed < 2% change in BMI (N = 12, M %BMI change = .05, range = -0.64 to 1.7) to provide a direct test of the a priori hypothesis that weight gain would be associated with a reduction in striatal response to palatable food relative to weight stable participants. Exploratory analyses also tested whether participants who showed a > 2.5% decrease in BMI (N = 6, M %BMI change = -4.7, range: -3.1 to -6.8) exhibited differential change in striatal response to palatable food than participants who remained weight stable or gained weight. In terms of raw weight change, this translated into a mean weight change of 6.4 lbs for the weight gain group, a mean weight change of 0.5 lbs for the weight stable group, and a mean weight change of −6.8 lbs for the weight loss group. Although groups did not differ on BMI at baseline, we controlled for this variable. Because there was some variation in the time of day at which the baseline and follow-up scans were conducted across subjects that might have influenced results, we also controlled for the difference in time of the two scans (in hours). Parameter estimates from milkshake - tasteless contrasts were entered into a second-level 2×2×2 random effects ANOVA (e.g., weight gain - weight stable) by (milkshake receipt - tasteless receipt) by (6-month follow-up - baseline).
As hypothesized, the weight gain group showed significantly less activation in right caudate in response to milkshake intake (12, -6, 24, Z = 3.44, FDR corrected p =.03, r = -.35; 9, 0, 15, Z = 2.96, FDR corrected p = .03, r = -.26) at 6-month follow-up compared to baseline relative to changes observed in weight stable participants (). The weight loss group did not show significant changes in activation in the caudate in response to milkshake intake compared to the weight gain group or the weight stable group (). To illustrate the relation between the continuous measure of degree of weight gain and the magnitude of the reduction in striatal responsivity to palatable food, we regressed change in BMI against change in right caudate (12, -6, 24) activation for all participants in SPSS, controlling for baseline BMI and scan time difference (). To determine whether change in the right caudate for those who gained weight compared to those who maintained weight was significantly greater than in the mirror-region of the left caudate, we compared the activation in the right and left caudate using ROI analysis. We conducted an ANOVA testing the interaction between hemisphere, time, and group for the contrast between activation in response to receipt of milkshake versus tasteless solution. There was no significant interaction (F (1, 18) = 0.91, p = 0.35). Thus, although our analyses revealed a significant time by group interaction in the right caudate, but not the left caudate, we cannot conclude that the observed effect was significantly lateralized.
Fig 3 Coronal section showing less activation in the right caudate (12, -6, 24, Z = 3.44, pFDR =.03, P <.05) in the weight gain group (N = 8; ≥2% BMI gain) versus the weight stable group (N = 12; ≤2% BMI change) during milkshake receipt (more ...)
Scatter plot showing change in right caudate activation during milkshake receipt – tasteless receipt at 6-month follow-up compared to baseline as a function of change in % BMI.