# Onion Shipment Volume and Colorectal Cancer Mortality in Japan: An Ecological Study Using a Nationwide Health Insurance Claims Database

## TL;DR

This study explores the link between onion shipment volume and colorectal cancer mortality in Japan's elderly population using objective data instead of self-reported diets.

## Contribution

The study introduces onion shipment volume as a novel proxy for consumption opportunity in ecological dietary assessments.

## Key findings

- Prefectures with higher onion shipment volumes tended to have lower colorectal cancer mortality rates.
- Spearman's correlation showed a weak negative association between onion shipments and mortality.
- No consistent associations were found for other sulfur-containing vegetables like garlic or green onions.

## Abstract

Aim: Colorectal cancer is a leading cause of cancer-related mortality in Japan, particularly among older adults. While vegetable intake has been implicated in cancer prevention, most previous studies relied on self-reported dietary surveys, which are prone to recall bias. We aimed to examine the association between onion shipment volume, a novel proxy for consumption opportunity, and age-standardized colorectal cancer mortality among elderly populations in Japan.

Materials and methods: This nationwide ecological cross-sectional study analyzed all 47 prefectures of Japan. Shipment data for onions and other sulfur-containing vegetables (garlic, Chinese chives, green onions) in 2022 were obtained from the Ministry of Agriculture, Forestry and Fisheries. Age-standardized colorectal cancer mortality among individuals aged ≥75 years was obtained from the National Cancer Center. Data were evaluated using visual inspection, box plot analysis, Kruskal-Wallis tests, effect size (Cohen's d), and Spearman's correlation coefficients.

Results: Prefectures with higher onion shipment volumes tended to have lower colorectal cancer mortality rates. Although the difference across prefectures was not statistically significant (Kruskal-Wallis p = 0.643), the effect size was moderate (Cohen's d = 0.524). Spearman's correlation indicated a weak negative association between onion shipments and mortality (r = -0.205, p = 0.168). No consistent associations were observed for other sulfur-containing vegetables.

Conclusion: By using objective shipment data instead of self-reported dietary surveys, this study provides a novel methodological approach to ecological dietary assessment. The findings suggest a possible inverse relationship between onion consumption and colorectal cancer mortality in older adults. Future individual-level studies are warranted to validate these ecological trends and investigate underlying biological mechanisms.