This review aimed to determine the effects of intentional weight loss on proteinuria and kidney function and found that weight loss was associated with decreased proteinuria and microalbuminuria, which suggested a beneficial effect on surrogate outcomes of the decrease of urinary protein excretion. Overall the review appeared well conducted and the conclusions are likely to be reliable.

To determine the effects of intentional weight loss on proteinuria and kidney function.

MEDLINE, Cochrane Central Register of Controlled Trials (CENTRAL) and Scopus were searched for English-language papers published between 1985 and May 2009. Search terms were reported. Reference lists of papers the reviewers considered relevant were searched for further papers.

Eligible studies were of patients who were at least 18 years old, were overweight (body mass index of more than 25kg/m²) and had proteinuria or other markers of renal dysfunction. Randomised and non-randomised controlled clinical trials, including single-arm trials, were eligible for inclusion. Studies were excluded if urinary protein measurements were taken less than 24 hours after exercise, patients had chronic conditions associated with non-intentional weight loss or were elite athletes or observational study designs were used.

Within included studies (where reported) mean age ranged from 31 to 60 years. The proportion of participants with diabetes, proteinuria and albuminuria ranged from 0% to 100%. Interventions included: regular exercise; dietary interventions (included regular low calorie diets and regular diets); surgical interventions (included gastroplasty and gastric bypass); and pharmaceutical interventions (drugs included topiramate, metformin and orlistat). Comparisons within studies included before and after and treatment and control. Outcome measures included weight, body mass index, proteinuria and albuminuria and creatinine clearance. Follow-up duration ranged from four to 104 weeks (median 40 weeks).

Two reviewers performed the study selection.

It appeared that all included studies were assessed according to criteria of level of loss-to-follow-up, whether intention-to-treat (ITT) analysis was used and whether eligibility criteria indicated that results were generalisable. Additionally, randomised controlled trials (RCTs) were assessed according to the adequacy of randomisation, allocation concealment and blinding.

It appeared that study validity was assessed by one reviewer and checked by a second reviewer.

Means and standard deviations (SDs) were extracted in order to calculate effect sizes. Where possible, data was retrieved by contacting authors; otherwise they were extracted from the papers or estimated from medians and ranges reported in papers using the Hozo method. Where patient characteristics were more general, only subgroups of patients with proteinuria were included in the analysis.

One reviewer extracted data and another reviewer checked extracted data.

Random-effects meta-analyses were performed that compared the difference between urinary protein or albumin before and after intervention. I² values were calculated to assess heterogeneity. Outcomes were pooled as mean differences.

The influence of variables such as weight (at baseline and change after treatment), mean arterial pressure, age and duration of intervention on proteinuria, microalbuminuria and creatinine clearance were investigated using meta-regression. Subgroup analyses were performed to identify differences in effectiveness between weight loss strategies.

One reviewer performed the analysis and another reviewer checked this analysis.

Thirteen studies (n=522 patients, range eight to 94 patients) were included in the review: four RCTs, one CCT and eight prospective cohort studies.

All four RCTs had explicit eligibility criteria. Three were open-label rather than double-blinded. There was no loss to follow-up in three studies and 15.4% loss to follow-up in the fourth study, which did not use intention to treat. The CCT had explicit eligibility criteria, was non-randomised and open-label and had no reported loss to follow-up. Four of the eight prospective cohorts had explicit eligibility criteria. All were open label. No loss to follow-up was reported in six studies; for the other studies loss to follow-up was 3.3% and 40.2%

Proteinuria: Three studies (n=69 patients, range 17 to 30 patients) measured proteinuria. They included patients with nephrotic range proteinuria and used caloric restriction to try to reduce weight loss and produced a pooled mean difference in overt proteinuria of -1.66 g/day (95% CI -2.63 to -0.69 g/day, I²=59.5%), which suggested a statistically significant association between weight loss and proteinuria levels for this patient group.

Microalbuminuria: Eight trials (n=391 patients, range eight to 94 patients) produced a statistically significant pooled mean difference of -13.87mg/day (95% CI -17.12 to -10.61, I²=50%) that favoured weight loss interventions

Creatinine clearance: Seven trials (n=203 patients, range eight to 61 patients) produced a statistically significant pooled mean difference of -23.73 mL/minute (95% CI -36.12 to -11.35, I²=0%) that favoured weight loss interventions.

A number of meta-regressions and subgroup analyses were presented.

Weight loss was associated with decreased proteinuria and microalbuminuria. Evidence supported the beneficiary effect of weight loss on surrogate outcomes of the decrease of urinary protein excretion.

This review addressed a clear research question using well-specified and appropriate study selection criteria. Evidence was identified from a number of different sources. Validity was assessed with appropriate methods. It appeared that attempts were made to minimise error and bias throughout the review process. Only studies published in English were considered for inclusion, so the possibility of language bias could not be ruled out. The methods of synthesis appeared thorough and appropriate, although not all details were clearly reported; similarly. The methods and data used within the subgroup analyses were not clearly stated. Overall the review appeared well conducted and the conclusions are likely to be reliable.

Research: The authors stated that further research was required to determine the impact of weight loss on clinical renal outcomes.

Practice: The authors did not state any implications for practice.

None stated.

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.