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
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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 | |