Purpose <p>Malnutrition and dysphagia in older adults complicate the maintenance of independent living. This study aimed to evaluate a dietary observation and assessment model by non-specialized staff and a consultation and support model by specialized staff in adult day services (ADS).</p> Methods <p>This non-randomized controlled trial was conducted across 31 ADS facilities in Japan. Participants were care-dependent older adults (<i>n</i> = 590; median age, 85&#xa0;years; 69% female) for whom a 6-month follow-up assessment was completed, including body weight measurement, diet texture documentation, and Functional Oral Intake Scale (FOIS) scoring. The intervention group (<i>n</i> = 317) received 3&#xa0;months of monthly mealtime observation by non-specialized staff, with targeted consultation from specialists (dental hygienists, dietitians) for individuals at risk, whereas the control group (<i>n</i> = 273) received usual care. The primary outcome was weight loss of ≥ 5% at 6&#xa0;months, analyzed using a mixed-effects model. Sensitivity analyses were performed to assess the impact of attrition.</p> Results <p>The incidence of ≥ 5% weight loss was significantly lower in the intervention group than in the control group (10.1% vs 17.3%; adjusted odds ratio 0.52, 95% confidence interval 0.31–0.89). This finding was upheld in a realistic sensitivity scenario. The intervention group showed a significantly greater increase in the use of texture-modified diets (<i>p</i> = 0.039) without a change in oral intake ability.</p> Conclusion <p>An interprofessional model combining observation by non-specialized staff with targeted specialist support offers a promising, efficient, and scalable strategy to mitigate the risk of weight loss in older adults who attend day-care settings.</p> Trial registration <p>UMIN000056668, 2025/01/08, retrospectively registered.</p>