PDM: Metabolic Syndrome (2013)


Mukuddem-Petersen J, Stonehouse (Oosthuizen) W, Jerling JC, Hanekom SM, White Z. Effects of a high walnut and high cashew nut diet on selected markers of the metabolic syndrome: A controlled feeding trial. British J Nutr. 2007; 97: 1,144-1,153.


PubMed ID: 17381974
Study Design:
Randomized Controlled Trial
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To determine the effects of a high walnut diet and a high unsalted cashew nut diet on markers of metabolic syndrome compared to a control diet.


Inclusion Criteria:
  • ATP III criteria for the diagnosis of metabolic syndrome
  • Subject’s ability to comply with the controlled feeding conditions
  • Being willing and able to eat walnuts and cashew nuts
  • 21 years of age and younger than 65 years.
Exclusion Criteria:
  • Pregnancy or lactation
  • Taking thiazide medication (25mg per day)
  • β-blocker (non-specific, β1 and β2) use
  • Subjects having nut allergies
  • Diagnosed diabetes.


Description of Study Protocol:


68 white/Caucasian volunteers with metabolic syndrome were recruited from the Potchefstroom Campus of the North-West University and surrounding areas in Potchefstroom, South Africa. 


Randomized, parallel, controlled trial. 


  • Study protocol consisted of a three-week run-in period where the subjects consumed a control diet
  • After the run-in period, participants were grouped according to gender and age and then into three groups by randomly drawing numbers from a hat
  • Group one received walnuts (N=21), group two received unsalted cashew nuts (N=21), group three continued with the control diet without any nuts or nut-based ingredients (N=22)
  • The three intervention diets were followed for eight weeks
  • All the food was provided to the participants for the duration of the trial
  •  Fasting blood samples, oral glucose tolerance tests, anthropometric measurements and blood pressure measurements were taken before (after the three-week run-in period) and after the intervention period (eight-week controlled feeding)
  • BMI (kg/m2) was calculated
  •  Diet:
    • The proportion of total energy (ranging from 63 to 108g per day) from nuts was 20%
    • Except for the nuts, the diets were identical
    • The study featured a highly controlled feeding protocol. All subjects were required to have their lunch at the metabolic ward of the Department of Nutrition at the Potchefstroom Campus of the North-West University. Breakfast and dinner were provided in take-away format.
    • Using pre-packed food parcels and a variety of set menu options ensured compliance
    • A validated FFQ and physical activity questionnaire measuring activity index were analyzed in order to determine the correct energy intake requirements for the maintenance of body weight for each participant
    • A 14-day menu cycle was designed for five amounts of energy intake, ranging from 8,000 to 14,000kJ per day (1,905 to 3,333kcal per day)
    • The macronutrient profiles and fatty acid distribution of the three diets were analyzed chemically to validate the diet composition
    • Quality control and compliance with the protocol were ensured:
      • Foods were weighed to the nearest gram before being served to the participants
      • The principal investigator, a registered dietitian, supervised mealtimes and ensured the complete intake of all study foods
      • Participants kept food diaries of the additional points used and possible leftovers were collected and weighed (by researchers)
      • Participants were weighed twice weekly and the energy intake was adjusted (especially during the first three-week run-in period) in order to maintain body weight
      • Participants were urged to maintain the same activity level throughout the study.

*** Individuals who used chronic medication (e.g., lipid-lowering medication) at baseline were instructed to continue use and to maintain the same dosage for the duration of the trial.

Statistical Analysis

The statistical analysis was done in five steps:

  • Variables were tested for normality using the Shapiro–Wilk’s W-test
  • Descriptive statistics were done
  • Data that were normally distributed are expressed as mean and 95% CI
  • Data that are not normally distributed or logarithmic and square-root transformed are expressed as median (25th, 75th percentiles)
  • For changes within groups, from baseline to end, a T-test for dependent samples was used for parametric data and the Wilcoxon matched-pairs test was used for non-parametric data
  • Differences in baseline and D (change from baseline to end) between the three groups were determined by using the ANOVA for parametric data and the Kruskal–Wallis ANOVA for non-parametric data and the Tukey honest significant difference test was used for unequal N for parametric data
  • Weight-adjusted differences in baseline and D between the three groups were determined by using the analysis of covariance
  • With 80% power calculation, significance was set at P≤0.05.
Data Collection Summary:

Timing of Measurements   

Baseline and at the end of the eight-week controlled feeding period.

Dependent Variables

  • Lipid profiles
  • Serum fructosamine and plasma glucose
  • Blood pressure, uric acid and serum high-sensitivity C-reactive protein.

Independent Variables

  • Intervention diet high in walnuts
  • Intervention diet high in unsalted cashew nuts.

Control Variables 

Control diet.

Description of Actual Data Sample:
  • Initial N:  68 

Walnut Diet


Cashew Diet


Control Diet


Male:Female 10:11 8:13 11:11
  • Attrition (final N): N=64 (29 males, 35 women) 
  • Age: 21 years to 65 years (mean age: 45±10 years)
  • Ethnicity: White/Caucasian
  • Other relevant demographics: Mostly obese, sedentary and non-smokers
  • Anthropometrics: Baseline anthropometrics were similar between the groups
  • Location: Potchefstroom Campus of the North-West University, South Africa.



Summary of Results:

Other Findings

  • Weight, BMI and waist circumference remained unchanged during the study
  • Serum lipid concentrations showed no significant changes between walnut, cashew and the control groups
  • Results from both nut groups showed no significant change in high-density lipoproteins, low-density lipoproteins, triglycerides or total cholesterol after the eight-week nut diet intervention
  • There was no significant difference in serum fructosamine between the groups after the nut intervention
  • Plasma glucose concentration (T=zero) increased significantly by 0.70mmol per L (P=0.04) in the cashew nut group compared to the control group
  • No significant change was found in systolic and diastolic blood pressure, uric acid concentrations and C-reactive protein between the three groups after the eight-week nut diet intervention. 
Author Conclusion:

The authors concluded that individuals having metabolic syndrome showed no improvement in the markers of this syndrome after following a walnut diet or a cashew nut diet for eight weeks compared to a control diet while maintaining body weight.

Funding Source:
Government: The National Research Foundation and the South African Government’s Technology and Human Resources for Industry Program
Reviewer Comments:
  • The methodology was sufficiently detailed with a high degree on quality and compliance control
  • An eight-week nut intervention diet may not have been long enough to see significant change in the metabolic syndrome biomarkers
  • Specific detail on how the subjects were recruited was not identified
  • Drawing numbers out of a hat is not a gold-standard method to use for randomization.



Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? ???
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? No
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? ???
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? No
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? N/A
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? No
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? ???
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? ???
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes