Summary of findings for the main comparison. Total fat intake 30% or less of total energy compared to usual fat intake for body weight in children (RCTs)^a.

Total fat intake ≤ 30% of total energy compared to usual fat intake for bodyweight in children (RCTs) A comprehensive table including data for all time points for each outcome can be found in Appendix 2
Patient or population: boys and girls aged 24 months to 18 years Setting: paediatric practices, schools and health maintenance organisations in high‐income countries Intervention: lower total fat intake ≤ 30%TE Comparison: usual or modified fat intake
Outcomes (at time point ranges where data were reported)	No of participants (No of studies)	Illustrated comparative effect (95% CI)		Quality	What happens
		Usual fat intake1	Effect difference with total fat ≤ 30% of total energy2
Weight‐for‐age z‐score Follow‐up: range 6 to 12 months	151 (1 RCT)	The mean weight‐for‐age z‐score in control group was 0.29	MD 0.18 lower (0.51 lower to 0.15 higher)	⊕⊝⊝⊝ Very low3,4,5,6	We were uncertain whether lower total fat intake (≤ 30%TE) had an effect on weight‐for‐age in children over a 12‐month period (1 study).
Weight (kg) Follow‐up: range 6 to 12 months	620 (1 RCT)	The mean weight (kg) in control group was 38.2	MD 0.5 lower (1.78 lower to 0.78 higher)	⊕⊕⊝⊝ Low4,5,7,8	Lower total fat intake (≤ 30%TE) may have made little or no difference to weight in children over a 5‐year period (1 study).
Follow‐up: range 2 to 5 years	612 (1 RCT)	The mean weight (kg) in control group was 49.5	MD 0.6 lower (2.39 lower to 1.19 higher)	⊕⊕⊝⊝ Low4,5,7,8
BMI (kg/m2) Follow‐up: range 6 to 12 months	620 (1 RCT)	The mean BMI (kg/m2) in control group was 18.5	MD 0.3 lower (0.75 lower to 0.15 higher)	⊕⊕⊝⊝ Low4,5,7,8	Lower total fat intake (≤ 30%TE) may have made little or no difference to BMI in children over a 1‐year period (1 study).
Follow‐up: range 1 to 2 years	191 (1 RCT)	The mean BMI (kg/m2) in control group was 24.8	MD 1.5 lower (2.45 lower to 0.55 lower)	⊕⊕⊕⊝ Moderate4,9,10	Lower total fat intake (≤ 30%TE) probably reduced BMI in children over a period of 1 to 2 years (1 study).
Follow‐up: range 2 to 5 years	541 (1 RCT)	The mean BMI (kg/m2) in control group was 21.7	MD 0 (0.63 lower to 0.63 higher)	⊕⊕⊝⊝ Low4,5,7,8	Lower total fat intake (≤ 30%TE) may have made little or no difference to BMI in children over a 2 to 5‐year period and > 5‐years (1 study). Please see Appendix 2 for Data for > 5 years.
Total cholesterol (mmol/L) Follow‐up: range 6 to 12 months	618 (1 RCT)	The mean total cholesterol (mmol/L) in control group was 5.1	MD 0.15 lower (0.24 lower to 0.06 lower)	⊕⊕⊕⊝ Moderate4,5,7,11	Total fat intake ≤ 30%TE probably slightly reduced total cholesterol in children over a 12‐month period (1 study).
Follow‐up: range 2 to 5 years	522 (1 RCT)	The mean total cholesterol (mmol/L) in control group was 4.6	MD 0.06 lower (0.17 lower to 0.05 higher)	⊕⊕⊝⊝ Low4,5,7,8	Lower total fat intake (≤ 30%TE) may have made little or no difference to total cholesterol in children over a 2 to 5‐year period and > 5‐years (1 study). Please see Appendix 2 for Data for > 5 years.
LDL‐C (mmol/L) Follow‐up: range 6 to 12 months	618 (1 RCT)	The mean LDL‐C (mmol/L) in control group was 3.29	MD 0.12 lower (0.2 lower to 0.04 lower)	⊕⊕⊕⊝ Moderate4,5,7,11	Lower total fat intake (≤ 30%TE) probably reduced LDL‐C in children over a 12‐month period (1 study) and over a 2 to 5‐year period (1 study). Please see Appendix 2 for Data for > 5 years.
Follow‐up: range 2 to 5 years	623 (1 RCT)	The mean LDL‐C (mmol/L) in control group was 3.07	MD 0.09 lower (0.17 lower to 0.01 lower)	⊕⊕⊕⊝ Moderate4,5,7,11
HDL‐C (mmol/L) Follow‐up: range 6 to 12 months	618 (1 RCT)	The mean HDL‐C (mmol/L) in control group was 1.47	MD 0.03 lower (0.08 lower to 0.02 higher)	⊕⊕⊕⊝ Moderate4,5,7,12	Lower total fat intake (≤ 30%TE) probably made little or no difference to HDL‐C in children over a 6 to 12‐month period (1 study) and over a 2 to 5‐year period (1 study). Please see Appendix 2 for Data for > 5 years.
Follow‐up: range 2 to 5 years	522 (1 RCT)	The mean HDL‐C (mmol/L) in control group was 1.32	MD 0.01 lower (0.06 lower to 0.04 higher)	⊕⊕⊕⊝ Moderate4,5,7,12
Triglycerides (mmol/L) Follow‐up: range 6 to 12 months	618 (1 RCT)	The mean triglycerides (mmol/L) in control group was 0.98	MD 0.01 lower (0.08 lower to 0.06 higher)	⊕⊕⊕⊝ Moderate4,5,7,12	Lower total fat intake (≤ 30%TE) probably made little or no difference to triglycerides in children over a 6 to 12‐month period (1 study). Please see Appendix 2 for Data for > 2 years.
Height‐for‐age z‐score Follow‐up: range 6 to 12 months	151 (1 RCT)	The mean height‐for‐age z‐score in control group was 0.05	MD 0.05 lower (0.08 lower to 0.02 lower)	⊕⊝⊝⊝ Very low3,4,5,13	We were uncertain whether lower total fat intake (≤ 30%TE) reduced height‐for‐age in children over a 12‐month period (1 study).
Height (cm) Follow‐up: range 6 to 12 months	642 (1 RCT)	The mean height (cm) in control group was 143.1	MD 0 (1.11 lower to 1.11 higher)	⊕⊕⊝⊝ Low4,5,7,8	Lower total fat intake (≤ 30%TE) may have made little or no difference to height in children over a period > 5 years (1 study).
Follow‐up: range 2 to 5 years	540 (1 RCT)	The mean height (cm) in control group was 167.4	MD 0.10 lower (1.54 lower to 1.34 higher)	⊕⊕⊝⊝ Low4,5,7,8
%TE: percentage of total energy; BMI: body mass index; CI: confidence interval; HDL‐C: high‐density lipoprotein cholesterol; LDL‐C: low‐density lipoprotein cholesterol; MD: mean difference; RCT: randomised controlled trial. aNotes: For all outcomes, there were too few studies to assess publication bias.
GRADE Working Group grades of evidence High quality: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate quality: we are moderately confident in the effect estimate: the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low quality: our confidence in the effect estimate is limited: the true effect may be substantially different from the estimate of the effect. Very low quality: we have very little confidence in the effect estimate: the true effect is likely to be substantially different from the estimate of effect.

¹Mean change observed between baseline and follow‐up in the control group.

²Difference in intervention group (and its 95% confidence interval) was based on the assumed change in the comparison group (and its 95% confidence interval).

³Downgraded by 1 for risk of bias: unclear risk of bias across all domains.

⁴Only 1 study for this outcome, therefore we could not rate for inconsistency.

⁵Downgraded by 1 for indirectness: participants were children with raised blood lipids, thus results may not be directly generalisable to all children.

⁶Downgraded by 1 for imprecision: small sample size and confidence interval included no effect and important benefit or harm.

⁷Not downgraded for serious risk of bias; a well‐conducted trial (methods in place to minimise risk of selection, performance, detection, attrition and reporting bias).

⁸Downgraded by 1 for imprecision: confidence interval included no effect and important benefit or harm.

⁹Downgraded by 1 for risk of bias: allocation concealment not reported.

¹⁰Not downgraded for serious imprecision: both bounds of the confidence interval indicate benefit, and calculated optimal information size met (158 patients are required to have a 80% chance of detecting, as significant at the 5% level, an important decrease in BMI of 1.7 kg/m2 (the average of the change across the 50th to 97th percentiles in 12.5 year‐olds, as per BMI‐for‐age tables, Centers of Disease Control & Prevention, 2000).

¹¹Not downgraded for serious imprecision: both bounds of the confidence interval indicate benefit.

¹²Not downgraded for serious imprecision: precise estimate of no effect.

¹³Downgraded by 1 for imprecision: small sample size (optimal information size not met).