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AWM: High Calcium (2006)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
The purpose was to examine, prospectively, the independent associations between dairy consumption and IRS after taking into account physical activity level, macronutrient and fiber intake, and other potentially confounding variables.
Inclusion Criteria:
Black and white men and women, ages 18-30 years of age at baseline, who were enrolled in the Coronary Artery Risk Development in Young Adults (CARDIA) Study
Exclusion Criteria:

Excluded were individuals enrolled in the CARDIA study who:

  • had no year 0 or year 7 dietary data (n=1175)
  • had unusually high or low dietary intake values (n=707)
  • were pregnant at baseline or within 180 days of year 10 clinic examination (n=184)
  • were taking medications that affect blood lipid levels (n=87)
  • individuals who had 2 or more components of IRS at baseline (n=265)
  • individuals with missing IRS data (n=141)
Description of Study Protocol:

Recruitment No information provided

Design  The CARDIA study is a population-based prospective study. Participants have been followed for up to 15 years, with current analyses including the first 10 years an 5 clinic examinations beginning with baseline in 1985 and including 1987, 1990, 1992, and 1995.  

Blinding used (if applicable) NA

Intervention (if applicable) NA

Statistical Analysis General linear regression models were used to compare incidence of components of IRS and IRS itself across dietary intake categories. Multiple logistic regression was used to determine associations between dairy intake and the odds of developing IRS during the 10-year study after excluding those with IRS at baseline. Odds ratios (ORs) and their 95% confidence intervals were calculated for the 2nd through 5th category of the respective dairy food group with the first category (lowest intake) as reference group. Linear trend across categories was tested with contrast statements using orthogonal polynomial coefficients. Models were constructed to assess influence of potential confounding variables and to evaluate micronutrients and macronutrients as possible physiologic mediators of the relationship between dairy intake and IRS. Two-way interactions were tested using a prior hypotheses about potential differences in associations between dairy intake and IRS by race and sex, and by baseline overweight status. Statistical significance was set at P=0.05.   

 

Data Collection Summary:

Timing of Measurements Measurements were taken at baseline and 10 years after baseline. Participants were stratified by race and by overweight status (BMI>25) at baseline.  

Independent Variables

  • Dietary Intake: measured using the CARDIA Diet History which asks about usual dietary practices and obtains a quantitative food frequency of past 28 days. Dairy products in particular were identified as any items reported during diet history interview that were 100% dairy (e.g., milk) or included dairy as 1 of the main ingredients (e.g, dips made with sour cream). Weekly frequency of consumption for each food (times/week) was used to estimate relative consumption/week for each food for each individual. Also considered were intake of other food groups that may have confound associations between dairy intake and IRS (fruits, non-starchy and starchy vegetables, fruit juices, soft drinks and sugar-sweetened beverages, whole and refined grains, mean, and fish). 
  • Clinic Measurements: (a) height-measured with a vertical ruler to nearest 0.5 cm; (b) weight-measured to nearest 0.2 kg with calibrated balance beam scale; (c) BMI-computed as weight in kg devided by ht in meters squared; (d) waist and hip measurements-these were measured using a tape in duplicate to the nearest 0.5 cm around minimal abdominal girth and maximal protrusion of hips at level of symphysis pupblic, respectively; (e) waist-hip ratio-computed from average of waist and hip measurements; (f) blood pressure-measured at each exam on right arm using Hawksley random 0 sphygmomanometer; (g) fasting insulin and glucose-drawn using vacuum tubes with no preservative; (h) fasting triglyceride levels- were estimated using enzympatic procedures; (i) fasting HDL-C levels-measured according to method of Warnick et al.; (j) fasting LDL cholesterol-measured by the Northwest Lipid Research Clinic Laboratory (along with other lipids)     
  • IRR Measurements: (a) abnormal glucose homeostasis-defined as a fasting plasma insulin concentration of at least 20 uU/mL, fasting glucose of at least 100 mg/DL, or use of medication to control blood glucose; (b) obesity-defined as a BMI of at least 30 or a WHR or at least 0.85 for women or 0.90 for men; (c) elevated blood pressure-defined as blood pressure of at least 130/85 mm Hg or use of antihypertensive medications; (d) dyslipidemia-defined as low HDL-C (<35 mg/dL) or high trigylceride (>200 mg/dL) levels; (e) IRS-defined as presence of 2 or more of the 4 components: abnormal glucose homeostasis, obesity, elevated blood pressure, and dyslipidemia.     

Dependent Variable

  • 10-year cumulative incidence of IRS

Control Variables

  • kcal intake
  • alcohol
  • fiber (g per 1000 kcal/day)
  • caffeine (mg/day)
  • % of kcal from CHO, pro, total fat, saturated and unsaturated fatty acids
  • micronutrients from supplements and foods including calcium, sodium, potassium, and vitamin D

 

Description of Actual Data Sample:

Initial N: 5115

Final N (after exclusions): 3157  

Age: ages 18-30 at baseline

Ethnicity: black (n=1409) and white (n=1748)  

Location: Study was conducted at 4 study centers: Birmingham, AL; Minneapolis, MN; Chicago, IL; and Oakland, CA.   

 

Summary of Results:

Dietary Intake and Specific Dairy Food Groups by Race

  1. Dairy intake was higher in white than in blacks (dairy product intake/week of 16.6 and 13.2 for normal weight and overweight black participants, respectively vs. 23.2 and 23.3 in normal weight and overweight white participants, respectively) (P<0.001), with difference consistent across dairy subgroups (e.g., milk and milk drinks, cheese and sour cream, butter and cream, yogurt, dessert). 
  2. Overweight participants consumed dairy products at a lower frequency than normal weight participants (P<0.001), with differences larger for blacks than for whites.

Demographic, Lifestyle, and Dietary Correlates of Dairy Intake

Higher dairy consumers were much less likely to be black and somewhat more likely to be women. Dairy intake was positively associated with whole grain, fruit, vegetable, and saturated fat intake and inversely correlated with sugar-sweetened soft drink intake.

Ten-Year Cumulative Incidence Rates for IRS and Each IRS Component

  1. Incidence rates for all components were higher for indivduals who were overweight at baseline, with blacks having higher rates of each IRS component except dyslipidemia. Incidence of IRS (developing 2 or more components over 10 years) was nearly 4-fold higher in overweight blacks and nearly 5-fold higher in overweight whites compared with their normal-weight counterparts.
  2. There was a consistent decrease in incidence for each of the 4 IRS components with increasing categories of dietary intake for overweight participants only (associations were much weaker and less consistent in normal-weight participants).
  3. With IRS as dependent variable, there was an interaction between dairy intake and baseline overweight status (P=0.03), with no association observed between dairy intake and IRS in those who were normal weight at baseline. In overweight participants of either race, incidence of IRS decreased by >50% from lowest to highest categories of dairy intake, behaving in a dose-response manner for blacks.    

ORs for IRS According to Categories of Total Dairy Intake

  1. There was a substantial decrease in the odds of IRS over the 10-year period with increasing category of dairy intake ( reduction in odds=71% for highest category of dairy intake relative to lowest category) (P-value for linear trends across all quartiles<0.001). In those not overweight or obese at baseline, the OR of IRS for those in highest category of dairy intake was 0.72 (P for trend =0.22).
  2. There was little evidence of confounding by other lifestyle and dietary factors.
  3. The association between dairy intake and IRS incidence was very similar for both races and sexes. Odds were typically lower, and usually considerably decreased, with increasing intake of all types of dairy products.   
Author Conclusion:
  1. Dairy intake was inversely related to the incidence of all IRS components among participants who were overweight (BMI>25) at baseline but not among participants who were of normal weight at baseline (BMI<25).
  2. Adjusted odds of developing IRS were 72% lower (odds ratio. 0.28) among overweight participants in the highest (>35 times/week) vs. lowest (<10 times/week) category od dairy intake.
  3. Each daily occasion of dairy consumption was associated with a 21% lower odds of IRS (odds ratio-0.79); these associations were similar for blacks and whites and for men and women. 
  4. Other dietary factors did not explain the association between dairy intake and IRS incidence.
  5. Dietary patterns involving increased dairy intake have a strong inverse association with IRS in overweight young adults and may reduce risk of type 2 diabetes and CVD.     
Funding Source:
Government: NHLBI, NIDDK
Industry:
General Mills
Food Company:
Not-for-profit
0
Foundation associated with industry:
Reviewer Comments:
  1. Study was observational so researchers can't rule out residual confounding or conclude that increased dairy intake reduced IRS in a causal manner.
  2. Dietary intake data was self-reported.  
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.) 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 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? No
  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? ???
  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? ???
  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.) ???
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? ???
  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? Yes
  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? 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? 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? 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)? 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? 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