Purpose: To investigate the short- and long-term effects of different combinations of dietary instructions on cariogenic food intake and salivary cariogenic bacteria (Streptococcus mutans [SM] and Lactobacillus [LB]).

Materials and Methods: In this randomised 2-arm parallel study, 75 6-year-old subjects were assigned to repeated (group A; 19M/19F) or isolated (group B; 17M/20F) verbal and/or written dietary advice (VWDA), with foods classified by cariogenic potential. Both groups underwent a baseline salivary test for SM/LB, kept a monthly food diary, and attended 4 weekly visits (T1–T4). At T1–T2; group A only received VDA. At T3, both groups received VWDA. At T4, participants handed in their food diaries and underwent another salivary test. After 1 year (T5), subjects were recalled for weekly food diary monitoring and salivary testing. Relative risk (RR) of high-to-low SM/LB density was calculated at T4 and T5.

Results: Comparing groups A and B, VDA determined an increase in the intake of weakly cariogenic food (p < 0.05) and a decrease in that of intermediately cariogenic food (p < 0.05). After VWDA, a statistically significant increase in intake of weakly cariogenic food and a statistically significant decrease in the RR of high-density SM/LB colonies occurred in both groups. At T5, group A showed less intake of highly cariogenic food than did group B (p = 0.05) and persistent, although non-significant, reduction in the RR of high-density SM/LB colonies.

Conclusions: Reinforcement measures on behavioural changes towards a noncariogenic diet not only help maintain long-lasting, healthier eating habits, but also decrease the cariogenic bacterial load in the short term, which tends to persist over time.