EAL

DLM: Fiber (2007)

Citation:

Study Design:

Class:

- Click here for explanation of classification scheme.

Quality Rating:

Research Purpose:

This study examined the hypothesis that intake of dietary fiber is inversely related to the risk of cardiovascular disease (CVD) and myocardial infarction (MI) in a large prospective cohort of women.

Also, to evaluate the effects of different types of fiber on risk of CVD and MI.

Inclusion Criteria:

39,310 female health professionals who participated in the Women’s Health Study (WHS) over six years (1993-2000) and provided diet information via a semi-quantitative food frequency questionnaire (SFFQ).

Exclusion Criteria:

No heart disease, stroke or cancer at baseline.

More than 70 blanks on participants SFFQ.

Energy intake <2,514kJ (600 kcal).

Energy intake >14,665kJ (3,500 kcal).

Description of Study Protocol:

A 131-item SFFQ was administered to all participants at the beginning of the study.

Each participant accumulated follow-up time beginning at baseline and ending in the month of diagnosis of a CVD or censoring (death from causes other than CVD, or March 2000).

Data Collection Summary:

A 131-item SFFQ was administered to all participants at the beginning of the study. Baseline data were collected, events were recorded during the 6 year follow up and at the end of the follow up period

For each food, a portion size was specified, and participants asked how often, on average, during the previous years they had consumed that amount.

Nine responses were possible: ranging from “never” to “Six or more times per day.”

Dietary fiber was computed by multiplying the frequency of consumption of each food item by the fiber content of the specific portions.

Dietary fiber content was obtained from the Harvard Food Composition Database.

Five groups were identified in quintiles of intake: 1) Lowest 12.5, 2) 15.7, 3) 18.2, 4) 21.1, and 5) the highest 26.3

Primary end point was CVD, which included MI, stroke, percutaneous transluminal coronary angioplasty, CABG, and fatal CVD.

Description of Actual Data Sample:

Beginning: 38,480 Female health care workers

Summary of Results:

At baseline: Higher intake of fiber was found to be associated with certain category of women, these include:

1. Older

2. 2. Less likely to be current smokers

3. More likely to exercise

4. Use postmenopausal hormones

5. Use multivitamin supplements or vitamin C

6. A history of high cholesterol

7. Lower BMI[S1]

History of DM [S2] , HTN, and Parental MI did not differ significantly between groups.

Fiber intake between the highest and lowest quintiles (median: 26.3g/day vs. 12.5 g/day [S3] )

Women in the highest quintile of fiber:

Lower total and saturated fat

Lower dietary glycmic index

Higher intake of protein, CHO, and folate

570 incident cases of CVD were confirmed.

A significantly smaller number of cases occurred in the highest quintile of intake than in the lowest quintile of intake (99 cases vs. 140 cases).

RR for the highest quintile was 0.65 p=0.001 after age and randomized treatment effects adjustments

Dietary fiber intake in all quintiles (excluding the lowest quintile with a RR of 1 and 12.5 g/day fiber intake) resulted in a reduced CVD risk

Relative Risk [S4] for CVD risk in the quintiles after age and randomized treatment effects adjustments are as follows:[S5]

2^nd quintile 0.71

3^rd quintile 0.72

4^th quintile 0.64

5^th quintile 0.65

MI events followed a similar trend at but with a higher inverse relationship p=0.0005

2^nd quintile 0.54

3^rd quintile 0.54

4^th quintile 0.43

5^th quintile 0.46

Adjustments for other known risk CVD factors such as smoking (the strongest confounding factor), and dietary variables, folate, fat, protein and total energy intake, attenuated the effects(effects were no more statistically significant - p values were >0.05 but the trends remain the same.

Similar and statistical significant trends were found for fruit fiber, soluble fiber and insoluble fiber. The RR however higher than in total dietary fiber.

There were no statistical significant effects for cereal fiber and vegetable fiber in the analysis.

[S1]These two points were included in the article (p.50)

[S2] What does DM mean? Will everyone know?

[S3] There is an ambiguity in the article on this point. The text at the bottom of p.50 indicates that the median intake of dietary fiber for the lowest quintile is 18.2 g/day. However, in Table 1 on p.51 the median intake for the lowest quintile is 12.5 g/day. Which is correct? The authors state that there was “approximately 1.5-fold difference in total dietary fiber intake between the highest and lowest quintiles of the study population”. Clearly, this would be a gross miscalculation if the 12.5 were the correct number. The difference between 18.2 and 26.3 is closer to 1.5—though still not exact. However, Table 3 (p.53) corroborates the numbers in Table 1—although Table 3 reports g/day of fiber in terms of means rather than median (as in Table 1). In short, we have to make a best guess here as the article is unclear. However, as a best guess, I’d tend toward the 12.5 median for the lowest quintile (as the same number is reported twice) and flag this ambiguity in the authors comments. This is the kind of thing you can count on the expert panel to catch!

[S4] Because Relative Risk scores are meaningful only with respect to the comparison among groups, you will need to report the risk scores of the various quintiles for the numbers to be meaningful.

[S5]Give the reader a p-value as you do in the following paragraph.

Author Conclusion:

In six-year prospective cohort of female health professionals >45 yrs, a modest but not statistically significant inverse association between intake of dietary fiber intake and risk of CVD.

Results may indicate a true benefit from high intake of dietary fiber, they may also be explained by other heart-healthy lifestyle and dietary factors associated with a greater intake of dietary fiber.

After adjusting for multiple CVD risk factors and dietary variables, most inverse trends remained, although they were no longer statistically significant.

After excluding participants who were overweight or whoever smoked, generally revealed a stronger inverse association between dietary fiber intake and CVD risk. However, these had wider 95% CIs.

Funding Source:

Government:

NIH

Reviewer Comments:

This study gave the impression that fiber is an important part of a healthy diet but other dietary and lifestyle factors may play a greater role in CVD.

The population only filled out the FFQ at the beginning of the 6yr study. Is it reasonable to believe that some of the people would change their eating habits over 6 years?

The article reported a median daily intake of fiber for the lowest quintile as variously 12.5 g/day and 18.2 g/day. It was not entirely clear from the article which measure was correct. However, the measurement in Tables 1 and 3 for this group corresponded: 12.5 g/day. Unfortunately, Table 1 reported this value as a median, while Table 3 reported this value as a mean.

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?	N/A
	1.2.	Was (were) the outcome(s) [dependent variable(s)] clearly indicated?	N/A
	1.3.	Were the target population and setting specified?	N/A
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?	N/A
	2.2.	Were criteria applied equally to all study groups?	N/A
	2.3.	Were health, demographics, and other characteristics of subjects described?	N/A
	2.4.	Were the subjects/patients a representative sample of the relevant population?	N/A
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.)	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?	N/A
	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?		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?	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?		Yes
	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?	Yes
	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?	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?	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?		N/A
	7.1.	Were primary and secondary endpoints described and relevant to the question?	N/A
	7.2.	Were nutrition measures appropriate to question and outcomes of concern?	N/A
	7.3.	Was the period of follow-up long enough for important outcome(s) to occur?	N/A
	7.4.	Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures?	N/A
	7.5.	Was the measurement of effect at an appropriate level of precision?	N/A
	7.6.	Were other factors accounted for (measured) that could affect outcomes?	N/A
	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?	N/A
	10.2.	Was the study free from apparent conflict of interest?	N/A