Prospective Cohort Study

B - Click here for explanation of classification scheme.

POSITIVE: See Quality Criteria Checklist below.

To examine associations between habitual fruit and vegetable intake and saturated fat intake, separately and in combination, and subsequent coronary heart disease and total mortality among men in the Baltimore Longitudinal Study of Aging (BLSA). 

Men in the Baltimore Longitudinal Study of Aging (BLSA).

• Had completed fewer than 4 days of a 7-day diet record for greater than 1 biennial visit
• Born after 1929 or age greater than 80 years at the first dietary record
• History of angina pectoris or myocardial infarction at baseline

Recruitment

The BLSA began in 1958 and was designed to study normal human aging and precursors to disease and death.  The current analysis includes a subset of 501 BLSA participants that met the inclusion criteria.

Design:  Prospective Cohort Study

Men recruited for this prospective cohort completed a biennial visit which included a 7 day diet record.  BLSA participants were trained to record their food intake by dietitians during their examination visit and ambiguous or incomplete records were clarified by telephone interview.  Vital status was followed for all participants and cause of death was determined by death certificate, hospital and physician records and autopsy data as available.

Blinding used (if applicable):  No blinding was used.

Intervention (if applicable):  No intervention was initiated.

Statistical Analysis

• Cox proportional hazards analysis was used to estimate risk of total and CHD mortality associated with dietary factors. 
• Unpaired t tests and X² analysis compared characteristics in survivors with men who died of any cause and with those who died of CHD. 

Timing of Measurements

Dietary data were collected by 7-day diet record during 4 time periods: 1961-1965; 1968-1975; 1984-1991 and 1993-present time.  Participants completed diet records at biennial visits.  In this specific analysis, BLSA participants completed records for 4-49 days (mean = 19 days) at 1-7 visits over a mean follow-up of 18 years.  Dietary records collected <2 years before death or subsequent development of clinical CHD were excluded due to the possibility diet may be affected by major illness. 

Dependent Variables

• All cause mortality
• CHD mortality
• Diagnosis of clinical CHD

Independent Variables

• Intake of fruits and vegetables by serving
• Intake of saturated fat by gram
• Intake of micronutrients
• Intake of fiber

Control Variables

• Age at first visit
• BMI
• Energy intake
• Physical activity score
• Smoking status
• Alcohol use
• Vitamin supplement use

 

Initial N: 501 men

Attrition (final N):  All subjects included in the final analysis

Age: Born before 1929 and <80 years of age at the time of first diet record

Ethnicity: Not reported

Other relevant demographics: Demographics not reported in this paper but the authors did note that participants in the BLSA were not a random sample and are predominantly white men of relatively high socioeconomic status

Anthropometrics Significant differences were observed at baseline between survivors and CHD deaths in age at baseline (participants who died of CHD were slightly older at baseline) and smoking status.

Location: Baltimore, Maryland, United States

 

Key Findings:

• After adjustment for covariates, each serving of fruits and vegetables was associated with a 6% reduction in risk for total mortality (P<0.05) and a 21% reduction in CHD death (P<0.01)
• When examines separately, intake of fruit was associated inversely with total mortality (P<0.05) and intake of vegetables was inversely associated with CHD mortality (P<0.01) with a risk reduction of 40% per serving
• Each gram of saturated fat intake was associated with a 7% increased risk of CHD death (P<0.001)
• The inclusion of both saturated fat and fruit and vegetable intake into a model attenuated most of the associations with mortality; however intake of total fruits and vegetables and vegetables alone remained significantly protective against CHD mortality (P<0.05 and P<0.01, respectively).  Saturated fat also remained significantly associated with CHD mortality after adjustment (P<0.05)
• A 50% reduction in CHD mortality per 100 mg of magnesium intake was found after adjustment for covariates and saturated fat (p<0.001)
• β-carotene showed a 10% reduction in risk for CHD mortality per mg (P<0.05)
• Dietary fiber showed a 6% reduction in risk for CHD mortality per gram (P<0.05)
• When examined together, participants consuming >5 servings of fruits and vegetables per day and <12% of energy intake came from saturated fat were 31% less likely to die of any cause (P<0.05) and 76% less likely to die from CHD (P<0.001)

 

In conclusion, the results of this study support earlier observations that dietary intakes low in saturated fat or high in fruits and vegetables each offer protection against CHD mortality.  In addition, however, our data suggest that the combination of both high fruits and vegetables with relatively low saturated fat intake offers greater protection against both total and CHD mortality than either practice alone.

Limitations noted by the authors:

• As in any long term study, one of the limitations is changes made to the methodology over the years
• Because men entered the study over a 34-year period, the effects of secular trends in dietary habits were difficult to separate from those of diet per se
• The current analysis was limited to men because women were not recruited into the BLSA until 1978 

While the authors noted this limitation in their discussion of findings, it is important to note that this cohort was made up of only men and little demographic information was provided in this particular paper (although it may have been published elsewhere).  Therefore, it is difficult to extrapolate the findings of this cohort study to the population as a whole.

It is unclear in reading the paper whether or not the investigators were blinded to outcome when conducting biennial visits with the participants, including collecting a diet history.