HTN: Minerals (2007)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:
To evaluate the relation between calcium intake from dairy products and the intake of vitamin D on systolic and diastolic blood pressure.
Inclusion Criteria:
All men and women over 24 years of age, who were living in the municipality of Tromso, Norway, were invited to participate in a health survey carried out by the National Health Screening Service.
Exclusion Criteria:
  • Subjects taking drugs for hypertension or heart disease
  • Those taking calcium tablets
  • Subjects reporting cardiovascular disease
  • Pregnant women.
Description of Study Protocol:
  • Recruitment: Health survey conducted in 1994-1995
  • Design: Cross-sectional study
  • Intervention: Subjects completed a self-administered health survey and had blood pressure measured.

Statistical Analysis

  • To enable main effects and interactions to be identified, data were evaluated initially with a three-factor ANOVA, with sex, calcium intake and vitamin D intake as fixed factors, SBP and DBP as dependent variables and age, BMI, alcohol and coffee consumption, physical activity and cigarettes smoked as independent variables
  • Associations were evaluated using Pearson correlation coefficients
  • Differences between men and women were evaluated with ANOVA
  • Linear trends evaluated with linear regression
  • Comparisons of calcium intake between subjects with and without HTN were done with multiple linear regression, with calcium intake as a dependent variable; SBP or DBP as fixed factor and age, BMI, alcohol and coffee consumption, physical activity, cigarettes and vitamin D as independent variables
  • All test were done two-sided and the Bonferroni correction was used as a post-hoc test.
Data Collection Summary:
  • Timing of measurements: Subjects participated in a self-administered health survey and had blood pressure measured
  • Dependent variables: Blood pressure measured with automatic device
  • Independent variables: Calcium and vitamin D intakes calculated from a self-administered FFQ, but only regarding dairy products.

Control Variables

  • Age
  • BMI
  • Alcohol consumption
  • Coffee consumption
  • Physical activity
  • Cigarette smoking
  • Vitamin D intake.
Description of Actual Data Sample:
  • Initial N: Over 27,000 subjects originally examined in the health survey
  • Attrition (final N): 7,543 men, 8,053 women included in analyses
  • Age: 25 to 69 years
  • Ethnicity: Not mentioned
  • Location: Tromso, Norway.
Summary of Results:

 

  Adjusted SBP Adjusted DBP
Men (zero to 199mg Ca per day) 131.9 76.1

Men (200 to 399)

131.7

75.9

Men (400 to 599) 131.8 75.8
Men (600 to 799) 131.0 75.3
Men (800 to 999) 130.6 75.1
Men (over 1,000) 130.3 74.9
P for linear trend <0.001 <0.001
Women (zero to 199) 123.3  71.6
Women (200 to 399) 122.6 71.3
Women (400 to 599) 123.4 71.5
Women (600 to 799) 122.4 71.1
Women (800 to 999) 121.8 70.9
Women (over 1,000) 120.6 70.5

P for linear trend

<0.01

<0.05

Adjusted for age, BMI, alcohol and coffee consumption, physical activity, smoking and vitamin D intake.

Other Findings

  • Women: Intake of calcium was 444.4±236.0mg per day, vitamin D was 5.8±4.6ug per day
  • Men: Intake of calcium was 563.8±305.0mg per day, vitamin D was 6.8±4.7ug per day
  • After correction for age, BMI, alcohol and coffee consumption, physical activity, cigarette smoking and vitamin D intake, there was a significant linear decrease in systolic and diastolic blood pressure with increasing dairy calcium in both sexes (P<0.05)
  • However, the difference in blood pressure between subjects with the highest and the lowest calcium intake was less than one to three mm Hg
  • Similarly, with increasing blood pressure there was a significant (P<0.001) linear decrease in age-adjusted calcium intake from dairy sources; the difference between the highest and the lowest blood pressure groups was 3% to 10%
  • After adjustment for other variables, vitamin D intake had no significant effect on blood pressure.
Author Conclusion:
In conclusion, we found a weak negative association between calcium intake from dairy sources and blood pressure. This could be important because small reductions in blood pressure might have a significant effect on primary prevention of cardiovascular diseases.
Funding Source:
Government: Research Council of Norway
Reviewer Comments:
  • Large sample size
  • Looked at calcium from dairy products only since diet questionnaire did not cover other sources
  • Self-administered FFQ.
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? 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
  3.1. 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? N/A
  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? 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.) Yes
  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? 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? Yes
  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? 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? Yes
  6.6. Were extra or unplanned treatments described? Yes
  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? ???
  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? ???
  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? No
  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? 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