Increase Font Size Decrease Font Size View as PDF Print
Citation:

Gopinath B, Rochtchina E, Flood VM, Mitchell P. Diet quality is prospectively associated with incident impaired fasting glucose in older adults. Diabet Med. 2013 Jan 10. doi: 10.1111/dme.12109.


PubMed ID: 23301551
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 temporal association between diet quality  and the 10-year incidence of impaired fasting glucose (IFG) and Type 2 diabetes.

Inclusion Criteria:
  • Participants of the Blue Mountains Eye Study (BMES), a population-based study 
  • Living in two post codes of the Blue Mountains region, west of Sydney, Australia
  • Aged 49 years or older.
 
 

 

Exclusion Criteria:

None listed.

Description of Study Protocol:

Study Description

The current report uses data collected in the Blue Mountains Eye Study (BMES), a population-based study of participants aged 49 years, living in two post codes of the Blue Mountains region, west of Sydney, Australia, which has studied age-related eye diseases and other health outcomes in an older urban Australian population. 

  • Between 1992 and 1994, 3,654 participants 49 years or older were examined (82.4% participation; BMES-1). Surviving baseline participants were invited to attend five-year follow-up examinations (1997 to 1999, BMES-2), at which 2,335 (75.1% of survivors) were examined.
  • At the 10-year follow-up (2002 to 2004, BMES-3), 1,952 participants (75.6% of survivors) were re-examined
  • At baseline, dietary data were collected using a 145-item self-administered modified Willett food frequency questionnaire (FFQ), modified for Australian diet and vernacular and including reference portion sizes
  • All participants were asked to undertake a fasting blood test within a month of the interview to assess fasting blood glucose.

Study Duration

Between 1992 and 1994, 3,654 participants 49 years or older were examined (82.4% participation; BMES-1). Surviving baseline participants were invited to attend five-year follow-up examinations (1997 to 1999, BMES-2), at which 2,335 (75.1% of survivors) were examined. At the 10-year follow-up (2002 to 2004, BMES-3), 1,952 participants (75.6% of survivors) were re-examined.

Location

Australia.

Data Collection Summary:

Dietary Assessment Method

A modified 145-item self-administered Willett food frequency questionnaire modified for Australian diet and vernacular and including reference portion sizes.

Brief Description of Dietary Pattern(s)

  • Participants used a nine-category frequency scale to indicate the usual frequency of consuming individual food items during the past year
  • A modified version of the Australian diet quality index based on the Dietary Guidelines for Australian Adults and the Australian Guide to Healthy Eating was used to establish the Total Diet Score (TDS) assessing adherence to the Australian dietary guidelines
    • Briefly, scores were allocated for intakes of selected food groups and nutrients for each participant as described in the Dietary Guidelines for Australian Adults
    • The TDS is divided into 10 components, and each component has a possible score ranging from zero to two:
      • A maximum score of two was given to subjects who met the recommendations with prorated scores for lower intakes
      • These were then summed providing a final score ranging between zero and 20, with higher scores indicating closer adherence to the dietary guidelines.
    • The TDS accounts for both food intake and optimal choice  with scores allocated to reflect intake characteristics from both sources:
      • Food intake scores were based on total intakes of vegetables, fruit, cereals and breads, meat, fish, poultry and alternatives and dairy, as well as sodium, alcohol, sugar and extra foods intakes
      • Optimal choices scores determined intakes of foods with greater dietary benefits including servings of whole grain cereals, lean red meat, low or reduced fat milk vs. whole milk, low saturated fat intake and fish consumption
      • Cut-points for scores were determined from recommended number of serves given in the  Australian Guide to Healthy Eating with the following exceptions:
        • Australian Guide to Healthy Eating’s recommended two servings per day of fruit was replaced with three servings per day and the number of vegetables consumed per day from five servings to seven servings to allow for self-reported FFQ over-estimation.

Outcomes Measured

  • Dietary intake
  • Healthy Eating Index
  • Total diet score
  • Incident diabetes.

Methods of Outcome Assessment

Type 2 diabetes-related mortality and incidence: 

  • Criteria for a diagnosis of diabetes were:
    • Self-reported diabetes history and current use of diabetic medications
    • A fasting plasma glucose concentration of 7.0mmol per L
    • Incident diabetes was defined in participants who were free of diabetes at baseline but who developed diabetes, before or at the five- or 10-year follow-up examination
    • Incident impaired fasting glucose (IFG) was defined as participants who were free of diabetes or IFG at baseline but who developed IFG, diagnosed before or at the five- or 10-year follow-up examination.
  • Glucose intolerance: Measured. 
Description of Actual Data Sample:

Sample Size

Of the 3,654 participants examined at BMES-1, 2,564 participants were examined at either or both the five-year and 10-year follow-up.

  • Of these, 517 were excluded from incidence analyses as they had diabetes and did not have fasting blood and dietary data collected at baseline
  • A further 226 participants who did not have fasting blood at follow-up were excluded. The remaining 1,821 were included for incident diabetes analyses.
  • For the IFG incidence analyses,1,630 participants were included (excluded 191 participants who had baseline IFG or developed diabetes during follow-up).

Age

49 years or older; mean age 64 years.

Gender

  • 58% in IFG analysis
  • 42% in the diabetes analysis.

Baseline Health Status

  • More women than men reported a family history of diabetes and had higher incidence of hypertension
  • Men had higher serum triglycerides and fasting blood glucose.

Baseline Distribution of Dietary Patterns

Women appeared to have better diet quality than men at baseline.

Summary of Results:

Results Related to Type 2 Diabetes Incidence and Type 2 Diabetes-related Mortality

  • No significant associations were observed between tertiles of Total Diet Score (TDS) and 10-year incidence of Type 2 diabetes in the BMES cohort
  • In addition, no significant associations were observed between TDS as a continuous variable and incident diabetes (multivariable-adjusted P=0.88).

Results Related to Glucose Intolerance

  • Men compared with women included for incidence analyses were more likely to have higher serum triglycerides and blood glucose levels, but a lower Total Diet Score (TDS) and frequency of family history of diabetes
  • In men, when comparing the highest with the lowest tertile of TDS, a significant 75% decrease in the risk of incident IFG was observed (Ptrend=0.02)
  • Each two-point increase in TDS was associated with a 52% reduction in the 10-year incidence of IFG in men (OR, 0.48; 95% CI: 0.33 to 0.69)
  • No significant associations were observed between:
    • TDS and incident IFG among women
    • Tertiles of TDS and 10-year incidence of Type 2 diabetes in the BMES cohort
    • TDS as a continuous variable and incident diabetes (multivariable-adjusted P=0.88).

Model Adjustments

Confounders
Total Energy Intake BMI Sex Age Smoking Alcohol Intake Physical Activity
  x x x x    


  • Model 1: Adjusted for age and sex
  • Model 2: Additionally adjusting for confounders that were significantly associated with the incidence of IFG (current smoking, BMI, hypertension and serum triglycerides) and diabetes (current smoking, BMI, family history of diabetes, fasting blood glucose and serum triglycerides) in the BMES cohort.

 

Author Conclusion:

Greater compliance with published dietary guidelines (better diet quality) was associated with a reduced risk of prediabetes in men but not women.

Strengths and Limitations

Strengths 

  • Its representative population-based sample with relatively high participation and its prospective design, which minimizes reverse causality
  • Used a validated food questionnaire to collect dietary data and had available rich confounder information such as:
    • Smoking
    • BMI
    • Serum triglycerides
    • Hypertension.

Limitations

  • We cannot exclude the possibility of residual confounding as a result of unmeasured confounders or the presence of measurement error in those confounders adjusted for in multivariate models
  • FFQs for self-reported dietary intake can under-estimate or over-estimate both macronutrient and micronutrient intake
  • The assumption that the dietary guidelines used to define diet quality indexes are based on the best available scientific knowledge may not necessarily be true as it is difficult to keep dietary guidelines up to date.
  • The TDS includes a physical activity component that is not included in most diet quality indices; hence, this makes comparisons with other published studies difficult.

Reviewer Comments:
  • This is an interesting observational study looking at the relationship between dietary guidelines compliance and risk of type 2 diabetes in older adults
  • Overall the authors did not find that better adherence to dietary guidelines appreciably reduced the risk of diabetes in the long term among older Australians
    • However, a potential association between lack of compliance with established dietary compliance and increased risk of type 2 diabetes in men but not women
  • Although the researchers included physical activity, the the results were not sensitive to the removal of physical activity and a similar odds ratio and P-values were observed. For example, in men, each two-point increase in TDS was associated with a 50% reduction in incident IFG (OR, 0.50; 95% CI: 0.34 to 0.73).
  • Co-morbid conditions were not included, which may have an impact of diabetes risk
  • Exclusion criteria were not specified
  • Some study results (confounders in age/sex adjusted analyses, etc.) are presented in the Methods section instead of the results.

Research Design and Implementation 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)
???
  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 nutrition or dietetics practice?
Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies)
N/A
 
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?
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?
No
  2.2. Were criteria applied equally to all study groups?
N/A
  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
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)
N/A
  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
  3.5. If case control or cross-sectional 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. Criterion may not be applicable in some cross-sectional studies.)
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?
No
  4.1. Were follow-up methods described and the same for all groups?
N/A
  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%.)
No
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for?
No
  4.4. Were reasons for withdrawals similar across groups?
N/A
  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?
N/A
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?
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
  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?
N/A
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied?
N/A
  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?
???
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured?
N/A
  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?
N/A
  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?
???
  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?
???
  7.6. Were other factors accounted for (measured) that could affect outcomes?
Yes
  7.7. Were the measurements conducted consistently across groups?
N/A
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?
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