**Background:** Japan has a high aging rate (28% aged ≥65 years), and semi-mountainous rural regions experience even greater aging. Single older adults living alone face risks of malnutrition due to reduced social interaction, decreased appetite, and food access problems. Limited research exists on nutritional status and associated factors specifically among single older residents in these regions.
**Methods:** This cross-sectional study was conducted in a semi-mountainous rural area of western Kochi Prefecture, Japan (population 1,222; aging rate ~44%). Participants were recruited by Social Welfare Council representatives. Inclusion criteria: aged ≥65 years, living alone, adequate verbal literacy. Fifty-three participants (7 male, 46 female; mean age 83.9 ± 6.8 years; 86.8% aged ≥75 years) completed an anonymous self-administered questionnaire. Nutritional status was assessed using the Mini Nutritional Assessment-Short Form (MNA-SF; scores 0–14; 12–14 = normal, 8–11 = risk of malnutrition, 0–7 = malnourished). Additional measures included Instrumental Activities of Daily Living (IADL), social situation (community activities, social interactions, meal sharing), environment (grocery access, transport), and psychological state (subjective health/happiness, Kessler Psychological Distress Scale [K6] with ≥5 points indicating risk of depression/anxiety). Binomial logistic regression analysis (forward selection) identified factors associated with malnutrition risk.
**Key Results:** The mean MNA-SF score was 12.1 ± 1.5. Thirty-eight participants (71.7%) had normal nutritional status, and 15 (28.3%) were in the risk of malnutrition group (none were malnourished). Most participants were independent in IADL (median: males 4 [IQR 3–5], females 7 [IQR 7–8]). Twenty-two of 50 participants (44.0%) had K6 scores ≥5, indicating risk of depression/anxiety. Socially, 47.2% participated in seniors' clubs, 45.3% in hosted events, 26.4% in local residents' association gatherings; 18.9% participated in no community activities. Social interactions included neighbors (54.1%), relatives (49.1%), friends (39.6%), and community health workers (22.6%); 28.3% had no social interaction. Only 32.7% had someone to share a meal with. Chi-square tests showed significant associations between MNA-SF categories and participation in local residents' association gatherings, social interaction with friends, and risk of depression/anxiety. Binomial logistic regression revealed: participation in local residents' association gatherings was associated with increased risk of malnutrition (OR 7.42, 95% CI 1.17–47.01); risk of depression/anxiety was associated with increased risk of malnutrition (OR 12.77, 95% CI 1.99–81.94); social interaction with friends was associated with decreased risk of malnutrition (OR 0.11, 95% CI 0.02–0.76). The model had a deterministic rate of 82.0% and Hosmer–Lemeshow test p = 0.850.
**Clinical Implications:** Despite predictions of malnutrition in this vulnerable population, nutritional status was generally good, likely reflecting that residents who remain independent in daily activities and meal preparation can maintain normal nutrition. The counterintuitive finding that participation in local residents' association gatherings was associated with malnutrition risk likely reflects proactive targeting of at-risk individuals by community health workers. Social interaction with friends was protective, suggesting that actively maintained social connections support nutritional health. The strong association between depression/anxiety risk and malnutrition (OR 12.77) underscores the need for integrated mental health and nutritional support. Community health workers should share health information across networks and promote social events to help single older adults maintain healthy lifestyles. Limitations include small sample size (n=53), potential selection bias (participants recruited through community events), self-reported BMI, and regional specificity limiting generalizability.