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Database of Abstracts of Reviews of Effects (DARE): Quality-assessed Reviews [Internet].

Long-term effects of low-fat diets either low or high in protein on cardiovascular and metabolic risk factors: a systematic review and meta-analysis

L Schwingshackl and G Hoffmann.

Review published: 2013.

CRD summary

This review compared the long-term effects of high- versus low-protein low-fat diets on cardiovascular and metabolic risk factors, concluding that these factors were not affected by changes in dietary protein content. Given the limitations of the evidence, and uncertainty in the reliability of the findings, the authors' recommendations for practice seem appropriate.

Authors' objectives

To compare the long-term effects of high- versus low-protein low-fat diets, on cardiovascular and metabolic risk factors.

Searching

MEDLINE (from 1966), EMBASE (from 1980), and Cochrane Central Register of Controlled Trials (CENTRAL) were searched up to August 2012, without language restrictions. One search term was used. Reference lists of retrieved articles were manually searched.

Study selection

Eligible for inclusion were randomised controlled trials (RCTs) that compared a high-protein diet with a low-protein diet (defined in the review), and reported the outcomes after at least 12 months of follow-up. Both treatment groups had to receive a low-fat diet. The outcomes of interest included weight, blood pressure, and tests for blood levels of, for example, cholesterol, triglycerides, or glucose when fasting.

The included trials had more female than male participants. Where reported, body mass index at baseline ranged from 27 to 40kg per m². Some trials included participants with type 2 diabetes. The percentage of carbohydrates in the high- and low-protein diets varied, with low-protein groups receiving higher percentages. Some trials restricted energy intake, but calorie intake was generally similar for both protein groups. Trials lasted from 12 to 24 months.

The two authors independently screened studies for inclusion.

Assessment of study quality

Both authors independently assessed the trial methods, according to Jadad criteria; scores of 0 to 2 out of 5 were considered low quality, scores of 3 or more were considered high quality. Methodological quality was also assessed using the Cochrane risk of bias tool.

Data extraction

The means and standard deviations, for after, or before and after intervention, were extracted to calculate mean differences between high- and low-protein groups or change in means between two time points in the two protein groups. The results from the longest follow-up were extracted, and where two types of high- or low-protein diet were assessed, the treatment groups were combined.

Both authors independently extracted the outcome data. Any discrepancies were resolved by consensus.

Methods of synthesis

A random-effects model was used to combine the mean differences, to calculate weighted mean differences and their 95% confidence intervals. Statistical heterogeneity was assessed using X² and I², and I² of over 50% indicated substantial heterogeneity.

Sensitivity analyses were performed by including only high-quality RCTs on the Jadad scale, and by excluding RCTs that enrolled patients with type 2 diabetes.

Publication bias was assessed using funnel plots.

Results of the review

Fifteen RCTs (1,990 participants) were included in the review. Eight RCTs were considered high quality and seven were considered low quality on the Jadad scale. The Cochrane risk of bias tool suggested that most trials were at a high risk of bias due to a lack of blinding, and more than half the trials were at an unclear risk of bias for randomisation and allocation concealment. Where reported, participant drop-out rates ranged from 7% to 55%.

Participants receiving a high-protein diet had significantly greater reductions in fasting insulin, compared with participants on a low-protein diet (WMD -0.71 micro international units per mL, 95% CI -1.36 to -0.05; 11 RCTs; I²=0). No significant differences between protein groups were reported for any other outcomes.

The results of the sensitivity analyses were reported. There was evidence of some asymmetry on the funnel plots, indicating the potential for bias.

Authors' conclusions

The biomarkers predicting obesity, cardiovascular disease and glycaemic control were not affected by changes in dietary protein content.

CRD commentary

The review question and supporting inclusion criteria were clearly defined. The literature search was adequate, but no attempts were made to locate unpublished evidence, which means that potentially relevant data may have been missed. Funnel plots indicated that relevant data may have been missed. Each stage of the review was conducted in duplicate, minimising the potential for reviewer error and bias. Appropriate criteria were used to assess the trial methods, and just over half the trials were considered high quality; the main issues were a lack of blinding and unclear sequence generation and allocation concealment.

Appropriate methods were used to pool the data and assess statistical heterogeneity. There was evidence of statistical heterogeneity between trials, with wide confidence intervals, for some outcomes. The authors acknowledged the variability between trials in the diets, populations, interventions, and outcome measures.

The trial samples were generally small, and most trials had high drop-out rates. Given the limitations of the evidence, and uncertainty in the reliability of the findings, the authors' recommendations for practice seem appropriate.

Implications of the review for practice and research

Practice: The authors stated that the unanimous recommendation of a high-protein diet was not supported by the evidence.

Research: The authors stated that further research was needed before recommendations could be changed to a diet with a higher percentage of daily protein.

Funding

Not stated.

Bibliographic details

Schwingshackl L, Hoffmann G. Long-term effects of low-fat diets either low or high in protein on cardiovascular and metabolic risk factors: a systematic review and meta-analysis. Nutrition Journal 2013; 12: 48. [PMC free article: PMC3636027] [PubMed: 23587198]

Indexing Status

Subject indexing assigned by NLM

MeSH

Biological Markers /blood; Blood Glucose /analysis; Blood Pressure; C-Reactive Protein; Cardiovascular Diseases /prevention & control; Cholesterol, HDL /blood Cholesterol, LDL /blood; Databases, Factual; Diet, Fat-Restricted; Dietary Proteins /administration & dosage; Glycemic Index; Hemoglobin A, Glycosylated /analysis; Humans; Metabolic Syndrome X /prevention & control; Obesity /prevention & control; Randomized Controlled Trials as Topic; Risk Factors; Time Factors; Triglycerides /blood; Waist Circumference; Weight Loss

AccessionNumber

12013020694

Database entry date

28/10/2013

Record Status

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.

CRD has determined that this article meets the DARE scientific quality criteria for a systematic review.

Copyright © 2014 University of York.

PMID: 23587198

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