Spillover effect of a dietary intervention on physical activity in a randomized controlled trial with colorectal cancer patients

TL;DR

A dietary intervention in colorectal cancer patients initially increased physical activity but the effect faded after one year.

Contribution

The study shows that spillover effects of dietary interventions on physical activity may be temporary and not sustained over time.

Findings

The diet intervention group showed a 0.18 h/day increase in MVPA at 6 months compared to controls.

The spillover effect on physical activity was not significant at 12 months (p = 0.24).

Physical function improved in both groups at 6 months but not at 12 months.

Abstract

Randomized controlled studies (RCTs) targeting dietary changes may also lead to other, untargeted changes in lifestyle habits, as spillover effects. In particular, the isolated impact of the dietary intervention may be difficult to separate due to spillover effects from changes in physical activity and physical function. Therefore, the aim of this study was to investigate the spillover effect of a one-year dietary intervention in post-surgery colorectal cancer patients by comparing the changes in physical activity and physical function between the diet intervention group and the control group in a randomized controlled trial, called the CRC-NORDIET study.

Men and women, aged 50–80 years were randomized into either the intervention group (n = 240) or the control group (n = 229). Both groups received similar incentives on physical activity. Activity sensors were used to collect data on physical activity at baseline, 6 months, and 12 months. Physical function was estimated by results from handgrip strength, 30 s sit-to-stand test and 6-min walking test. Anthropometric measurements and body composition were also measured.

We found a significantly higher increase in moderate-to-vigorous intensity physical activity (MVPA) of 0.18 h per day from baseline to 6 months in the diet intervention group compared to the control group, respectively. However, the spillover effect of the dietary intervention on physical activity diminished to 0.10 h per day at 12 months follow-up which was not statistically significantly different (p = 0.24) from the control group. All measures of physical function increased in both groups from baseline to 6 months with no further increase at the 12-month follow-up.

The dietary intervention did not induce a significant spillover effect on physical activity after 12 months of baseline, which was the main timepoint of the intervention. Providing identical physical activity guidance to both study groups during the 12-month intensive dietary intervention period, ensured comparable levels of physical activity across both study groups. This approach facilitated the isolation and analysis of the dietary intervention's effects on primary endpoints, as well as effects of behaviour interventions in secondary preventions, such as the CRC-NORDIET study.

The study is registered on the National Institutes of Health Clinical Trials website (www.ClinicalTrials.gov; Identifier: NCT01570010).

The online version contains supplementary material available at 10.1186/s12966-025-01757-0.

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