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Effect of reducing total fat intake on body weight: systematic review and meta-analysis of randomised controlled trials and cohort studies

BMJ 2012; 345 doi: http://dx.doi.org/10.1136/bmj.e7666 (Published 06 December 2012) Cite this as: BMJ 2012;345:e7666
  1. Lee Hooper, senior lecturer in research synthesis and nutrition1,
  2. Asmaa Abdelhamid, research associate1,
  3. Helen J Moore, research associate2,
  4. Wayne Douthwaite, research associate2,
  5. C Murray Skeaff, professor3,
  6. Carolyn D Summerbell, professor of human nutrition2
  1. 1Norwich Medical School, University of East Anglia, Norwich NR4 7TJ, UK
  2. 2Obesity Related Behaviours Group, School of Medicine and Health, Wolfson Research Institute, Durham University, Stockton on Tees, UK
  3. 3Department of Human Nutrition, University of Otago, Dunedin, New Zealand
  1. Correspondence to: L Hooper L.hooper{at}uea.ac.uk
  • Accepted 7 November 2012

Abstract

Objective To investigate the relation between total fat intake and body weight in adults and children.

Design Systematic review and meta-analysis of randomised controlled trials and cohort studies.

Data sources Medline, Embase, CINAHL, and the Cochrane Central Register of Controlled Trials to June 2010.

Inclusion criteria Randomised controlled trials and cohort studies of adults or children that compared lower versus usual total fat intake and assessed the effects on measures of body fatness (body weight, body mass index, or waist circumference) after at least six months (randomised controlled trials) or one year (in cohorts). Randomised controlled trials with any intention to reduce weight in participants or confounded by additional medical or lifestyle interventions were excluded.

Data extraction Data were extracted and validity was assessed independently and in duplicate. Random effects meta-analyses, subgroups, sensitivity analyses, and metaregression were done.

Results 33 randomised controlled trials (73 589 participants) and 10 cohort studies were included, all from developed countries. Meta-analysis of data from the trials suggested that diets lower in total fat were associated with lower relative body weight (by 1.6 kg, 95% confidence interval −2.0 to −1.2 kg, I2=75%, 57 735 participants). Lower weight gain in the low fat arm compared with the control arm was consistent across trials, but the size of the effect varied. Metaregression suggested that greater reduction in total fat intake and lower baseline fat intake were associated with greater relative weight loss, explaining most of the heterogeneity. The significant effect of a low fat diet on weight was not lost in sensitivity analyses (including removing trials that expended greater time and attention on low fat groups). Lower total fat intake also led to lower body mass index (−0.51 kg/m2, 95% confidence interval −0.76 to −0.26, nine trials, I2=77%) and waist circumference (by 0.3 cm, 95% confidence interval −0.58 to −0.02, 15 671 women, one trial). There was no suggestion of negative effects on other cardiovascular risk factors (lipid levels or blood pressure). GRADE assessment suggested high quality evidence for the relation between total fat intake and body weight in adults. Only one randomised controlled trial and three cohort studies were found in children and young people, but these confirmed a positive relation between total fat intake and weight gain.

Conclusions There is high quality, consistent evidence that reduction of total fat intake has been achieved in large numbers of both healthy and at risk trial participants over many years. Lower total fat intake leads to small but statistically significant and clinically meaningful, sustained reductions in body weight in adults in studies with baseline fat intakes of 28-43% of energy intake and durations from six months to over eight years. Evidence supports a similar effect in children and young people.

Footnotes

  • We thank the members of the WHO NUGAG subgroup on diet and health for their work in setting up the question and the protocol for this review (agreed in outline at its first meeting in February 2010, but not published), offering further studies for examination and assessment of inclusion, and in ensuring robust analysis. WHO agreed with the publication of this systematic review in a scientific journal as it serves as the background evidence review for updating WHO guidelines on total fat intake and should therefore be available widely.

  • Contributors: The WHO NUGAG subgroup on diet and health discussed and developed the question for this review and included LH and CMS. The protocol was drafted by LH and approved by the NUGAG subgroup on diet and health. LH, WD, and HJM carried out the searches. LH, AA, WD, HJM, and CSE assessed the eligibility of the studies for inclusion, extracted data, and assessed trial validity. LH carried out the first GRADE assessment, which was refined by the NUGAG subgroup on diet and health. WD wrote the first draft of the report for the review by the NUGAG subgroup on diet and health. LH wrote the first draft of this paper. All authors contributed to the analysis, as did the NUGAG subgroup on diet and health in response to the first draft of the review. All authors agreed on the final draft of this review. LH is the guarantor.

  • Funding: WHO provided funding to Durham University towards the cost of carrying out this systematic review. No funding was received for the searching, analysis, or writing up of the data from randomised controlled trials in adults, but it was supported by the Norwich Medical School, University of East Anglia. The funders did not have any vested interests in the findings of this research.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: CMS, WD, and HJM had financial support from the WHO for the submitted work; LH and CMS received funding from WHO to attend meetings of the NUGAG subgroup on diet and health; LH received research funding to carry out a systematic reviews for Barry Callebaut (to assess the effects of chocolate on markers of antioxidant status, funding ceased in August 2010); no further financial relationships existed with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: Not required.

  • Data sharing: No additional data available.

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