Canadian DBCP 1997.

Methods	RCT
Participants	Women with mammographic densities > 50% breast area (Canada) CVD risk: low Control: randomised 448+, analysed 401 Intervention: randomised 448+, analysed 388 Mean years in trial: control 2.0, randomised 2.0 (note, papers suggest a 10‐year follow‐up overall) % male: 0% Age: mean control 45.9 (SD unclear), intervention 46.5 (SD unclear) Baseline BMI: mean control 23.6, intervention 23.4, no variance reported
Interventions	Reduced fat vs usual diet Control aims: usual diet Intervention aims: total fat 15%E, protein 20%E, CHO 65%E, isocaloric diet Control methods: encouraged to continue usual diet, interviewed by dietitian every 4 months during first year, then every 3 months in the second year Intervention methods: dietary prescription using food exchange (fat calories replaced by CHO), met with dietitian monthly during first year, then every 3 months. Scales, recipes, shopping guide provided Weight goals: "calories derived from fat were replaced by isocaloric exchange with carbohydrate" Total fat intake (at 2 years): intervention 21.3 (SD 6.2), control 31.8 (SD 6.7) %E Saturated fat intake (at 2 years): intervention 7.1 (SD 2.5), control 11.5 (SD 3.3) %E Style: diet advice Setting: community
Outcomes	Stated trial outcomes: incidence of breast cancer Available outcomes: weight
Notes	Weight data available for 1 and 2 years, 2‐year data used in main analysis
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomly allocated by telephone to Dept. of Biostatistics at Ontario Cancer Institute, stratified by centre
Allocation concealment (selection bias)	Low risk	As above
Blinding (performance bias and detection bias) All outcomes	High risk	Participants knew what arm they were in
Incomplete outcome data (attrition bias) All outcomes	High risk	At least 107 of at least 896 (12%) lost over 2 years (> 5% per year)
Selective reporting (reporting bias)	Unclear risk	No protocol found
Other bias	Low risk
Free of systematic difference in care?	High risk	Minor difference in attention for participants in intervention and control in first year
Free of dietary differences other than fat?	Low risk	Focus on dietary fat