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Citation:

Rumawas ME, Meigs JB, Dwyer JT, McKeown NM, Jacques PF. Mediterranean-style dietary pattern, reduced risk of metabolic syndrome traits, and incidence in the Framingham Offspring Cohort. Am J Clin Nutr. 2009; 90(6): 1,608-1,614.


PubMed ID: 19828705
Study Design:
Prospective Cohort Study
Class:
B - Click here for explanation of classification scheme.
POSITIVE: See Research Design and Implementation Criteria Checklist below.
Research Purpose:

To examine the prospective association between a diet consistent with a Mediterranean-style dietary pattern and metabolic syndrome traits and its incidence in non-diabetic US men and women.

Inclusion Criteria:

The Framingham Heart Study Offspring Cohort is a longitudinal community-based study of CVD among the offspring of the participants of the Framingham Heart Study Original Cohort.

Exclusion Criteria:
  • Excluded participants with diabetes (N=378) on the basis of previous clinical diagnosis, the use of insulin or oral hypoglycemic medication, or a fasting blood glucose higher than 126mg per dL
  • Among the remaining eligible participants, 318 participants were excluded because of missing or invalid food frequency questionnaire (FFQ) data (i.e., reported energy intakes of less than 600kcal per day for all or more than 4,000kcal per day for women and more than 4,200kcal per day for men or less than 12 blank food items), which left 3,103 non-diabetic participants with valid dietary data.
  • Further exclusion of participants who did not attend the follow-up (N=349) and those who were missing covariates (N=24); 2,730 participants were followed over a mean of seven years for the longitudinal analyses of metabolic syndrome traits
  • For the analysis of the incidence of metabolic syndrome, 1,019 participants with prevalent metabolic syndrome at baseline (the fifth examination) were excluded from the 3,103 non-diabetic participants with valid dietary data
  • After further exclusion of those with incomplete information to define metabolic syndrome incidence at follow-up (N=143) or with missing covariates (N=23), 1,918 participants were followed over a mean of seven years to assess the incidence of metabolic syndrome.
Description of Study Protocol:

Study Description and Duration

  • The fifth examination cycle (1991 to 1995) of the Framingham Heart Study Offspring Cohort, was used as the baseline for the current analyses
  • Participants were followed over a mean of seven years for the longitudinal analyses of metabolic syndrome traits
  • The Mediterranean-style dietary pattern score (MSDPS) was calculated as a mean score obtained from the dietary data of the fifth (1991 to 1995), sixth (1995 to 1998) and seventh (1998 to 2001) examinations, and this mean score was applied to the longitudinal analysis of metabolic syndrome traits.
  • For the analysis of metabolic syndrome incidence, the MSDPS was calculated as a mean score calculated from the dietary data of the fifth examination (baseline) up to the examination at which metabolic syndrome incidence was ascertained.

Location

US.

Data Collection Summary:
  • Dietary intake was assessed by using the Harvard semi-quantitative FFQ, mailed to the participants before the examination. All participants were asked to bring the completed questionnaire with them to their appointment.
  • The FFQ consisted of 126 items, including a list of foods together with a standard serving size and a selection of nine frequency categories ranging from “never or less than once per month” to “six or more a day.” The questionnaires also allowed participants to add up to three additional foods usually consumed that were not listed on the FFQ, as well as types of cold breakfast cereal and cooking oil usually used.
  • Participants were asked to report their frequency of consumption of each food item during the past year
  • Nutrient intakes were calculated by multiplying the frequency of consumption of each unit of food from the FFQ by the nutrient content of the specified portion. The nutrient values were calculated on the basis of the US Department of Agriculture food composition database and supplemented by other published sources and personal communications from laboratories and manufacturers.
  • The Mediterranean-style dietary pattern score (MSDPS) was developed to assess the conformity of an individual’s diet to a traditional Mediterranean-style dietary pattern as defined by the Mediterranean diet pyramid, based on the recommended intakes of 13 food groups in the Mediterranean diet pyramid (i.e., whole-grain cereals, fruit, vegetables, dairy, wine, fish, poultry, olives/legumes/nuts, potatoes, eggs, sweets, meat and olive oil)
  • With the exception of olive oil, each food group was scored from zero to 10 depending on the degree of correspondence with the recommendation (e.g., consuming 60% of the recommended servings would result in a score of six)
  • Exceeding the recommendations resulted in a lower score proportional to the degree of over-consumption (e.g., exceeding the recommendation by 60% would result in a score of four). A negative score due to this over-consumption penalty was defaulted to zero.
  • Olive oil’s scoring was categorical in nature on the basis of the exclusive use of olive oil (score 10), the use of olive oil along with other vegetable oils (score five) or no olive oil (score zero). The sum of the 13 component scores was standardized to a zero-to-100 scale and weighted proportionally by a continuous factor from zero to one, which reflected the proportion of energy intake attributed to the consumption of foods included in the Mediterranean diet pyramid. The total MSDPS ranged from zero to 100. Six biomarkers were used to identify metabolic syndrome traits, including homeostasis model assessment-insulin resistance.
  • [HOMA-IR = (fasting glucose · fasting insulin)/22.5], fasting plasma glucose, waist circumference, plasma triglycerides, HDL cholesterol and systolic and diastolic blood pressures, measured at the fifth and seventh examinations
  • Metabolic syndrome was defined according to the modified definition of the National Cholesterol Education Program (NCEP) Adult Treatment Panel III guidelines as more than three of any of the following:
    • Hyperglycemia (fasting plasma glucose higher than 100mg per dL) or current use of insulin or oral hypoglycemic medication
    • Increased waist circumference (higher than 102cm in men or higher than 88cm in women)
    • Hypertriglyceridemia (higher than 150mg per dL)
    • Low HDL cholesterol (less than 40mg per dL in men or less than 50mg per dL in women) or current use of lipid lowering treatment
    • Elevated blood pressure (higher than 130/85mm Hg) or current treatment of hypertension
  • Height (to the nearest 0.25 inch) and weight (to the nearest 0.25 lb) were measured with the subject standing, with shoes off, wearing only a hospital gown, and BMI was calculated.
  • Other covariates included:
    • Age (in years)
    • Sex
    • Smoking within the past year
    • Multivitamin use
    • Estrogen-replacement therapy (ERT) in post-menopausal women only
    • Physical activity level (PAL) assessed as a weighted average of the proportion of a typical day spent sleeping and performing sedentary, slight, moderate or heavy physical activities (expressed in metabolic equivalents)
    • Energy intakes (kcal per day) as an average intake calculated from the dietary data of the fifth, sixth, and seventh examinations (for the analysis of metabolic syndrome traits) or of the fifth examination (baseline) up to the examination at which the metabolic syndrome incidence was ascertained (for the analysis of metabolic syndrome incidence)
  • Because self-reported dietary intake tends to under-estimate energy intake relative to energy expenditure measured by the doubly labeled water (DLW) method, authors used the Schofield equation to predict a participant’s resting metabolic rate and identified under-reporters as those in whom the ratio of reported energy intake to predicted resting metabolic rate was less than the Goldberg cutoff for a sedentary PAL of 1.35.
Description of Actual Data Sample:
  • Sample size:
    • 2,730 participants were followed over a mean of seven years for the longitudinal analyses of metabolic syndrome traits
    • 1,918 participants were followed over a mean of seven years to assess the incidence of metabolic syndrome
  • Age: 95% CI ranges from 51.6 to 55.4 across gender-adjusted quintiles in results table
  • Gender: Percent of women women ranged from 43% to 70% across age-adjusted proportions across quintiles
  • Baseline weight status: Not provided. Table indicates BMI 95% CI: 26 to 27.4 across quintiles
  • Baseline distribution of dietary patterns: The median quintile MSDPS ranged from 15.0 to 31.9 In discussion, authors state that of the possible maximum MSDPS of 100, the highest score achieved in this study population was 49.5.
Summary of Results:
  • The MSDPS was positively associated with total energy intakes only among those whom we classified as under-reporter participants, but not among those classified as accurate reporters
  • Participants with a higher MSDPS had significantly lower waist circumference (P=0.001) than those with lower scores, after adjustment for their corresponding baseline values and the other risk factors for T2DM, including age, sex, energy intake, smoking dose, BMI and change in BMI
  • In a model adjusted for age, sex, energy intake, smoking dose, BMI and change in BMI among participants without metabolic syndrome at the baseline, those in the highest quintile category of MSDPS had the lowest cumulative incidence of metabolic syndrome over seven years of follow-up [(cumulative incidences (95% CI) from the lowest to the highest quintile categories were 38.5% (33.9%, 43.2%), 33.4% (29.0%,37.8%), 36.0% (31.5%, 40.4%), 31.9% (27.5%, 36.2%) and 30.1% (25.8%, 34.4%), respectively]. Adjustment for additional covariates, as described above for the analysis of metabolic syndrome traits, did not affect the observed associations. Also, no significant interaction was shown between the MSDPS and the accuracy of reporting energy intake for the metabolic syndrome incidence (P=0.49).

 

Confounders
Total Energy Intake Physical Activity Baseline BMI Sex Age Alcohol Intake Other
x x x x x    

 

 

Author Conclusion:

Consuming a diet consistent with the principles of the Mediterranean-style dietary pattern was favorably associated with avoiding metabolic syndrome traits that were assessed longitudinally, specifically, less abdominal obesity, less insulin resistance and less atherogenic dyslipidemia. The authors also showed that a diet consistent with a Mediterranean-style dietary pattern was associated with a lower incidence of metabolic syndrome.

Strengths and Limitations

  • The prospective design, by which dietary information was measured before the outcome occurred, strengthens the causal inference on the association between the Mediterranean-style dietary pattern and the study’s outcomes
  • Because the MSDPS assigned an equal weight to each of its food components, the associations in this study were also based on the assumption that each food group composing the Mediterranean-style dietary pattern contributed equally to the outcomes
  • A potential limitation of this work concerns the nature of dietary pattern analysis on the basis of a diet score. Of the possible maximum MSDPS of 100, the highest score achieved in this study population was 49.5.
  • The use of dietary data that derived from an FFQ to calculate the MSDPS is a potential limitation of this research. However, earlier validation studies of the Harvard FFQ showed that many of the foods included in the MSDPS were adequately captured on the FFQ on the basis of correlations with diet records.
  • Although the apparent protective association of a Mediterranean-style diet with metabolic syndrome traits and incidence persisted after adjustment for lifestyle and T2DM risk factors, residual confounding cannot be ruled out.
Reviewer Comments:

None.


Research Design and Implementation Criteria Checklist: Primary Research
Validity Questions
1. Was the research question clearly stated?
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?
Yes
  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?
Yes
3. Were study groups comparable?
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 used? (Concurrent preferred over historical controls.)
N/A
  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?
Yes
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?
???
5. Was blinding used to prevent introduction of bias?
N/A
  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.)
N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded?
N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described?
Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described?
Yes
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?
Yes
  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?
No
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups?
Yes
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?
Yes
  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)?
Yes
  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?
No
9. Are conclusions supported by results with biases and limitations taken into consideration?
Yes
  9.1. Is there a discussion of findings?
Yes
  9.2. Are biases and study limitations identified and discussed?
Yes
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