The reinforcing value of vegetables does not increase with repeated exposure during a randomized controlled provided vegetable intervention among overweight and obese adults
Authors: Jahns Lisa , Johnson LuAnn K , Temple Jennifer , Keim Nancy L , Casperson Shanon , Roemmich James N 
Published: November 2023

Objective

The primary aim of this randomized controlled trial is to determine whether the relative reinforcing value (RRV) of vegetables compared to a snack food can be increased through repeated exposure (incentive sensitization) to amounts of vegetables recommended by the Dietary Guidelines for Americans. The primary hypothesis is that repeated exposure to recommended amounts of vegetables will increase vegetable RRV more than exposure to the amount currently consumed.

Methods

Subjects were overweight and obese adults who reported usually consuming < 1 cup of vegetables/day. The experimental design was a two‐arm, randomized, controlled, community‐based, 8‐wk feeding intervention trial. The two arms of the study included 1) active intervention‐ during which participants came to the research center twice each week to pick‐up a variety of pre‐packaged, minimally prepared fresh or frozen vegetables and have a skin carotenoid scan as a compliance measure and 2) attention control arm – participants received no active intervention, but attended a similar number of visits at the research center. Intervention participants received recommended amounts of vegetables based upon energy needs measured by resting metabolic rate. The control group received no vegetables but came to the center twice each week to have a skin carotenoid scan as an attention control. The RRV of vegetables was tested at baseline and after 8 weeks. Follow‐up RRV tests occurred at 12 and 16 weeks. During the RRV task, “work” was defined as how many times a person was willing to click a mouse button while playing a computer game to win points for the “reward” of a portion of a vegetable or an alternative snack cracker. Differences in groups across time in the primary outcome, RRV of vegetables, was tested using a mixed linear model with time (baseline, 8 weeks) as a within‐subject factor, treatment (intervention or control) as a between‐subjects factor and subject as a random effect.

Results

Preliminary results include data from 27 overweight and obese adults (22 women and 5 men), with a mean age of 41 ± 13y (SD) and BMI of 36.6 ± 8.7 kg/m2. At baseline, intervention participants pressed the mouse button a total of 450 ± 50 (SE) times for vegetables and 507 ± 100 times after the intervention. Control participants at baseline responded 867 ± 260 button presses and 631 ± 244 presses after 8 weeks. There was no significant effect of treatment for total responses. An RRV of > 0.5 favors vegetables; the RRV of total responses was 0.74 ± 0.05 at baseline for intervention participants and 0.71 ± 0.06 after the intervention. Among controls, the RRV was 0.76 ± 0.07 at baseline and 0.70 ± 0.07 after 8 weeks. There were no significant differences between treatments for RRV.

Conclusions

These preliminary data indicate that it is difficult to change the reinforcing value of vegetables among adults.

Support or Funding Information

USDA‐ARS 3062‐51000‐051‐00D.