Consumption of Olive Oil and Risk of Total and Cause-specific Mortality Among U.S. Adults

The experimental setup in the study "Consumption of Olive Oil and Risk of Total and Cause-specific Mortality Among U.S. Adults" involves several key components:

Study Population

  1. Cohorts:

    • Nurses' Health Study (NHS): Initiated in 1976, including 121,701 women aged 30 to 55.

    • Health Professionals Follow-up Study (HPFS): Began in 1986, with 51,525 men aged 40 to 75.

    • Baseline: 1990, when olive oil consumption was first included in the food frequency questionnaires (FFQ).

  2. Exclusions:

    • Participants with a history of cardiovascular disease (CVD) or cancer at baseline.

    • Those with missing data for olive oil consumption or implausible energy intakes.

    • After exclusions, 60,582 women and 31,801 men were included in the analysis.

Dietary Assessment

  1. Method: A validated semi-quantitative food frequency questionnaire (FFQ) with over 130 items, administered every 4 years.

  2. Olive Oil Intake: Calculated from three items in the FFQ:

    • Olive oil use for salad dressings.

    • Olive oil added to food or bread.

    • Olive oil used for baking and frying at home.

    • One tablespoon was considered equivalent to 13.5 grams of olive oil.

  3. Other Fats: Intake of other vegetable oils, margarine, butter, and dairy fat was also assessed based on reported brands and types of fats used for cooking and added at the table.

Ascertainment of Death

  1. Sources: State vital statistics records, National Death Index, reports from next of kin or postal authorities.

  2. Cause of Death: Determined by physician review of medical records, medical reports, autopsy reports, or death certificates.

  3. Classification: Deaths were grouped into five major categories: cardiovascular disease (CVD), cancer, neurodegenerative disease, respiratory disease, and other causes.

Assessment of Covariates

  1. Lifestyle Factors: Updated every 2 years through mailed questionnaires, including body weight, smoking status, physical activity, medication use, and chronic disease diagnoses.

  2. Diet Quality: Modified Alternate Healthy Eating Index (AHEI) score, excluding polyunsaturated fatty acids and alcohol components, was used to assess overall diet quality.

Statistical Analysis

  1. Models: Age-stratified Cox proportional hazards models were used to assess the associations between olive oil consumption and total and cause-specific mortality.

  2. Person-Time: Calculated from baseline until the end of follow-up (June 30, 2018, for NHS and January 31, 2018, for HPFS) or death, whichever occurred first.

  3. Dietary Variables: Updated upon a report of cancer or CVD to avoid confounding by changes in diet after the development of these conditions.

  4. Olive Oil Consumption: Categorized by frequency and linear trends evaluated using the Wald test.

  5. Multivariable Models:

    • Model 1: Adjusted for age and calendar time.

    • Model 2: Further adjusted for ethnicity, Southern European/Mediterranean ancestry, marital status, living alone, smoking status, alcohol intake, physical activity, family history of diabetes, myocardial infarction or cancer, personal history of hypertension or hypercholesterolemia, multivitamin use, aspirin use, menopausal status and hormone use in women, total energy intake, and body mass index.

    • Model 3: Additionally adjusted for intake of red meat, fruits and vegetables, nuts, soda, whole grains, and trans fat.

  6. Substitution Analyses: Estimated the risk of total and cause-specific mortality when replacing 10 grams per day of other fats (margarine, butter, mayonnaise, dairy fat) with olive oil.

Sensitivity Analyses

  1. Socioeconomic Status: Adjusted for census-tract median family income, median home value, and percentage with college degree.

  2. Continuous Diet Update: Diet was continuously updated until the end of follow-up.

  3. AHEI Adjustment: Models adjusted for modified AHEI (excluding polyunsaturated fatty acids and alcohol).

  4. Cumulative Average: Used the cumulative average of olive oil intake instead of the average of the two most recent FFQs.

  5. Mutual Adjustment: Models mutually adjusted for other types of fat.

  6. Personal History of Diabetes: Further adjusted for personal history of diabetes.

  7. Exclusion of BMI: Conducted analyses excluding BMI as it could be in the causal pathway.

  8. Dementia-Related Death: Separate analysis for dementia-related death.

  9. Competing Risk Regression: Applied a competing risk regression model for cause-specific mortality.

Results

  1. Follow-Up: 28 years, with 36,856 deaths documented.

  2. Olive Oil Consumption: Mean consumption increased from 1.6 grams per day in 1990 to about 4 grams per day in 2010.

  3. Hazard Ratios (HRs): Participants in the highest category of olive oil consumption had lower risks of total and cause-specific mortality compared to those in the lowest category.

  4. Substitution Analyses: Replacing 10 grams per day of margarine, butter, mayonnaise, and dairy fat with olive oil was associated with lower risks of total and cause-specific mortality.

Strengths and Limitations

  1. Strengths:

    • Large sample size.

    • Long-term follow-up.

    • Detailed and repeated measurements of diet and lifestyle.

    • Numerous sensitivity analyses.

  2. Limitations:

    • Potential residual confounding despite adjustments.

    • Predominantly non-Hispanic white population, limiting generalizability.

    • Self-reported dietary assessment with potential measurement errors.

    • Inability to distinguish between different varieties of olive oil.

Conclusions

The study found that higher olive oil consumption was associated with lower risks of total and cause-specific mortality. Replacing other types of fat with olive oil was also associated with lower mortality risks. The results support current dietary recommendations to increase the intake of olive oil and other unsaturated vegetable oils.

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