# Does Olive Oil Harm Blood Vessels After a Meal? A Patient's Guide to the 2000 Mediterranean Diet Study

A groundbreaking study published in the Journal of the American College of Cardiology in 2000 investigated how different components of the Mediterranean diet affect blood vessel function after meals. Researchers at the University of Maryland fed 10 healthy volunteers five different high-fat meals and measured how well their arteries dilated. They discovered that olive oil—despite being a cornerstone of the heart-healthy Mediterranean diet—actually temporarily impaired blood vessel function by 31%, while omega-3-rich canola oil and salmon did not. However, adding antioxidant vitamins or vinegar and salad to the olive oil largely prevented this harmful effect, pointing to antioxidants and omega-3 fats as the true protective components of this famous diet.

# Does Olive Oil Harm Blood Vessels After a Meal? A Patient's Guide to the 2000 Mediterranean Diet Study

## Table of Contents

- Key Points
- Background: Why Study the Mediterranean Diet?
- Study Methods: How the Research Was Conducted
- Key Findings: Detailed Results With All Numbers
- Discussion: What the Results Mean
- What Other Research Has Shown
- Study Limitations: What This Study Couldn't Prove
- Conclusions and Recommendations for Patients
- Frequently Asked Questions
- Source Information

## Key Points

- In 10 healthy volunteers, a meal of extra-virgin olive oil and bread reduced flow-mediated vasodilation by 31% three hours after eating.
- Canola oil and salmon meals, rich in omega-3 fatty acids, did not significantly impair blood vessel function in the same 10 volunteers.
- Adding vitamins C and E reduced the olive oil-induced impairment by 71%; adding vinegar and salad reduced it by 65%.
- All five test meals raised serum triglycerides by 27% to 45%, and higher triglyceride rises correlated with greater declines in blood vessel function.
- This short-term study of single meals cannot show long-term heart attack or stroke outcomes, and its findings warrant further investigation.

## Background: Why Study the Mediterranean Diet?

The Mediterranean diet—consisting largely of olive oil, pasta, fruits, vegetables, fish, and wine—has long been associated with a surprisingly low rate of cardiovascular events (heart attacks and strokes). This observation seemed unexpected because people following this diet have serum cholesterol levels similar to those of people in countries with much higher rates of heart disease.

Observational studies have found connections between lower rates of coronary disease and higher intake of fish, fruits, and vegetables. The famous **Lyon Diet Heart Study** reported a reduction in cardiovascular events in heart disease patients who were randomly assigned to a Mediterranean diet. But here's the twist: that study substituted **alpha-linolenic acid (omega-3 fatty acid) enriched canola oil** for the traditionally consumed olive oil, which is rich in omega-9 fatty acids.

Importantly, this modified Mediterranean diet did not change serum lipoproteins (blood fats), suggesting that the benefit came from some mechanism other than cholesterol reduction. Other prospective diet trials found that heart disease patients advised to eat more fruits and vegetables, eat more fish, or take omega-3 fish oil supplements had fewer cardiovascular events.

High-saturated-fat diets are potentially atherogenic (artery-clogging) because they raise serum cholesterol, increase blood coagulation (clotting), and impair endothelial function. The endothelium is the thin layer of cells lining blood vessels, and **endothelial dysfunction is a known risk factor for atherosclerosis** (hardening of the arteries).

A high–saturated-fat diet impairs endothelial function, possibly because of oxidative stress (damage to cells by free radicals). Intravenously administered fat emulsions also produce endothelial dysfunction. Because the Mediterranean and Lyon Diet Heart Study diets are not associated with low cholesterol levels, the researchers hypothesized that olive, fish, and canola oils have other vasoprotective (blood-vessel-protecting) properties—specifically, that they do not impair endothelial function after meals.

## Study Methods: How the Research Was Conducted

### Study Participants

The study included **10 healthy hospital volunteers (5 men, 5 women) ages 28 to 56 years** with serum cholesterol and triglyceride levels below 200 mg/dl. No subjects had coronary risk factors other than age and gender. All subjects ate unrestricted (ad libitum) diets, and none was taking supplemental vitamins or medications—except one male subject who had been on an HMG-CoA reductase inhibitor (statin) for two years.

All participants purposely refrained from exercise on the study days and fasted for 12 hours overnight before being studied. All gave written informed consent, and the protocol was approved by the Institutional Review Board of the University of Maryland.

### The Five Test Meals

Each subject ate all five meals in varied order, starting between 8:00 and 10:00 AM, with at least a one-week interval between meals. All meals contained **50 g of fat and 900 kcal**. The meals were:

1.
**Extra-virgin olive oil (50 g)** with nonpreservative whole-grain bread (120 g)
1.
**Canola oil (50 g)** with nonpreservative whole-grain bread (120 g) — contained approximately 5 g of omega-3 fatty acids
1.
**Canned red salmon (420 g)** with crackers (30 g) — contained approximately 6 g of omega-3 fatty acids
1.
**Extra-virgin olive oil (50 g)**, whole-grain bread (120 g), plus **vitamin C (1 g) and vitamin E (d,l, alpha-tocopherol, 800 IU)**

1.
**Extra-virgin olive oil (50 g)**, whole-grain bread (120 g), plus **balsamic vinegar (100 ml) and salad** [romaine lettuce (1.5 cup), carrot (1 medium-sized), tomato (1 medium-sized)]

### Measurements

Fasting blood was drawn for total, low-density lipoprotein (LDL), and high-density lipoprotein (HDL) cholesterol, triglycerides, and glucose. Heart rate, blood pressure, and **brachial artery flow-mediated vasodilation (FMD)** were assessed before the meal and again 3 hours after completion of the meal.

FMD is a noninvasive measure of endothelial function. Using 11.0-MHz linear array ultrasound, the brachial artery (the main artery in the upper arm) was imaged at baseline, then again 1 minute after a 5-minute upper arm blood pressure cuff occlusion. The FMD was calculated as the percent diameter change of the post-occlusion measurement relative to the average of two baseline measurements.

The study had **80% power to detect a 3% reduction in FMD** and **90% power to detect a 3.6% reduction**. Group values were expressed as mean ± standard deviation (SD). Statistical analysis used analysis of variance (ANOVA) for repeated measures, the two-tailed paired t test, Duncan's Multiple Range Test, and the Tukey test. A p value less than 0.05 was considered statistically significant.

## Key Findings: Detailed Results With All Numbers

### Blood Tests: Triglycerides Rose After Every Meal

All five meals significantly raised serum triglycerides (blood fats), with increases ranging from **27% to 45%** (p < 0.05). However, no significant changes were observed in serum total cholesterol, LDL ("bad") cholesterol, HDL ("good") cholesterol, or glucose levels after any meal.

Here are the triglyceride changes for each meal (measured in mg/dl):

- Olive oil and bread: increased from 78 ± 41 to 107 ± 74
- Canola oil and bread: increased from 63 ± 28 to 88 ± 49
- Salmon and crackers: increased from 66 ± 47 to 84 ± 58
- Olive oil with vitamins C and E: increased from 71 ± 36 to 99 ± 69
- Olive oil with salad and vinegar: increased from 82 ± 44 to 119 ± 50

### The Key Result: Olive Oil Impaired Blood Vessel Function

The **olive oil and bread meal reduced flow-mediated vasodilation (FMD) by 31%**, from 14.3 ± 4.2% to 9.9 ± 4.5% (p = 0.008). This means that after eating olive oil, the blood vessels' ability to dilate (widen) in response to increased blood flow was significantly reduced.

This change in FMD following the olive oil meal was significantly different from that following the canola oil and salmon meals (by ANOVA, p = 0.02). The olive oil meal reduced FMD statistically more (p < 0.05) than the other four meals by Duncan's Multiple Range Test, and more (p < 0.05) than the other meals—excepting the olive oil, vinegar, and salad meal—by the Tukey test.

A **significant inverse correlation was observed between postprandial changes in serum triglycerides and FMD** (r = -0.47, p < 0.05). In plain language, the more triglycerides rose after a meal, the more blood vessel function declined.

The reduction in FMD following the olive oil and bread meal was associated with a 2% decrease in post-occlusion arterial diameter (p = 0.09) and a 1.8% increase in baseline arterial diameter (p = 0.14).

### What Happened With the Other Four Meals?

**No significant changes in FMD were observed after any of the other four meals.** The FMD values before and after each meal were:

- Canola oil and bread: 13.0 ± 3.4% before, 11.6 ± 4.4% after (not significant)
- Salmon and crackers: 13.1 ± 5.2% before, 12.8 ± 5.1% after (not significant)
- Olive oil + vitamins C and E: 13.3 ± 6.8% before, 12.1 ± 5.7% after (not significant)
- Olive oil + salad and vinegar: 13.5 ± 3.5% before, 12.1 ± 3.5% after (not significant)

No significant changes were observed in blood pressure or heart rate after any of the five meals.

### The Protective Power of Antioxidants

Perhaps the most striking finding was that **vitamins C and E reduced the olive oil–induced impairment in FMD by 71%**—the impairment was largely prevented when these antioxidant vitamins were added to the meal. Similarly, **balsamic vinegar and salad reduced the impairment by 65%**.

## Discussion: What the Results Mean

Contrary to part of the researchers' original hypothesis, this study found that **omega-9 (oleic acid)-rich olive oil impairs endothelial function after a meal**. The change in FMD correlated with the change in serum triglycerides, so the effect probably results from triglyceride-containing lipoproteins (fat-carrying particles in the blood).

The mechanism appears to be **oxidative stress**, because the decrease in FMD was reduced by 71% when antioxidant vitamins C and E were given alongside the meal. This finding supports the idea that the transient impairment in blood vessel function after fatty meals is caused by free radical damage that can be neutralized by antioxidants.

### Why Would Olive Oil Impair Endothelial Function?

Olive oil is highly monounsaturated (72% monounsaturated fat), and it does have beneficial effects on serum lipoproteins. The major unsaturated fatty acids in olive oil are oleic acid (18:1n-9) and linoleic acid (18:2n-6). However, a recent report showed that a high-oleic and linoleic acid (sunflower oil) meal impairs FMD compared with a low-fat meal—supporting the current study's observations.

Laboratory studies have shown that albumin-bound oleic and linolenic acids reduce vascular ring endothelium-dependent vasodilation, and remnant particles (partially digested fat particles) obtained from postprandial (after-meal) human blood impair rabbit aorta endothelium-dependent vasodilation in a dose-dependent fashion. Olive oil also has a 17% saturated fat content, and in a clinical study, olive oil activated coagulation factor VII (a blood-clotting protein) to the same extent as butter.

The authors concluded: **"Olive oil does not have a clearly beneficial effect on vascular function."**

### A Note on Canola Oil

Canola oil, which contains approximately 5 g of omega-3 fatty acids per 50 g serving, did **not** significantly reduce FMD in this study. The researchers suggested that canola oil may share some of the unique vasoprotective (blood-vessel-protecting) properties of other omega-3-rich oils, such as fish oil.

## What Other Research Has Shown

### High-Fat Meals and Endothelial Function

The research team had previously reported that a high–saturated-fat meal transiently impairs endothelial function, probably as a result of increased circulating levels of free fatty acids and/or remnant particles. Similar decreases in FMD have been reported with high–saturated-fat meals in other studies. This impairment can be reversed by prior administration of an **angiotensin-converting enzyme (ACE) inhibitor** (a blood pressure medication).

Purple grape juice, which is rich in polyphenol antioxidants, improved FMD in one of two reported studies. Free radical production, measured by electron paramagnetic resonance spectroscopy, was found to increase in subjects fed a high-fat meal, supporting the oxidative stress theory.

### The Mediterranean Diet Evidence Base

The **Seven Countries Study** found rates of coronary heart disease to be much lower on the Greek island of Crete than would be expected for the mean serum cholesterol level. The **Lyon Diet Heart Study** reported a reduction in cardiovascular events in heart disease patients given a Mediterranean diet modified by substituting alpha-linolenic acid (omega-3 fatty acid)-enriched canola oil for traditionally consumed olive oil, compared with a prudent French diet.

### Fish, Fish Oil, and Heart Health

Fish consumption is inversely associated with the incidence of cardiovascular events. Fish oils decrease serum very-low-density lipoproteins (VLDL), triglycerides, platelet aggregation (clotting), and inflammation, and they increase membrane stability and endothelial function. The clinical benefit of fish and fish oil has been confirmed in prospective trials:

- The **Diet and Reinfarction Trial (DART)** found a **29% reduction in mortality** in post–myocardial infarction (heart attack) patients advised to eat more fish.
- The **Indian Experiment of Infarct Survival** found significant reductions in cardiac death, nonfatal heart attacks, and cardiovascular events in subjects given either omega-3 fatty acid supplementation (2 g/day) or omega-3 fatty-acid-containing mustard oil.
- The **GISSI study** (Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto miocardico) reported a **15% reduction in mortality in 11,324 post–heart attack patients** given omega-3 fatty acid supplementation (1 g/day).

### Fruits and Vegetables

The consumption of fruits and vegetables has been associated with lower incidences of cardiovascular events. Postulated mechanisms include increased antioxidant intake, increased fiber intake (with resulting decreased serum LDL), increased potassium intake, and decreased blood pressure. An Indian study found that adding fruits, vegetables, nuts, and grain products to a fat-reduced diet lowered cardiovascular events, including mortality, in post–heart attack patients.

## Study Limitations: What This Study Couldn't Prove

The authors were careful to acknowledge several important limitations:

- Although considerable data support an atherogenic role for endothelial dysfunction, the assumption that endothelial function predicts progression of atherosclerosis and/or cardiovascular event risk is supported only by preliminary data.
- The study documented postprandial changes after a large morning meal. Whether similar effects would be observed at other times of day is unknown, especially in view of circadian variations in hepatic and digestive metabolism.
- The study involved a small subject group (10 people), although significant changes were still observed after the olive oil and bread meal. Several other studies employing the brachial artery FMD technique have been of similar size.
- The researchers did not measure responses to nitroglycerin (a medication that directly relaxes blood vessels), because they and other investigators have reported that these are unaffected by a fatty meal. This means they could not fully distinguish between endothelium-dependent and endothelium-independent effects.
- The study does not establish that the decrease in FMD is entirely endothelium-dependent, because the majority—but not all—of the decline in FMD was due to a decrease in post-occlusion arterial diameter. The data do not exclude a direct vasodilator effect (endothelium-independent) of the olive oil by itself, as there was a trend toward increased baseline diameter. Two prior studies observed a direct vasodilator effect of a high-fat meal, and one study observed a direct vasodilator effect after intravenous lipid emulsion—but most other prior studies have not.

## Conclusions and Recommendations for Patients

The researchers concluded that, in terms of their effects on postprandial (after-meal) endothelial function, **the beneficial components of the Mediterranean and Lyon Diet Heart Study diets appear to be the antioxidant-rich foods—vegetables, fruits, and their derivatives such as vinegar, and omega-3-rich fish and canola oils—not olive oil.**

This conclusion has several practical implications for people interested in heart-healthy eating:

-
**Antioxidants matter.** Adding vitamin C and vitamin E, or simply eating a salad with vinegar alongside a fatty meal, substantially reduced the temporary decline in blood vessel function caused by olive oil (by 71% and 65%, respectively).
-
**Omega-3 fats are protective.** Canola oil and salmon, both rich in omega-3 fatty acids, did not impair endothelial function after a meal, unlike olive oil.
-
**Olive oil is not a free pass.** While olive oil has beneficial effects on cholesterol levels, this study suggests it may temporarily impair blood vessel function after consumption—a finding that warrants further investigation.
-
**Fruits and vegetables provide protection.** Dietary intake of fruits, vegetables, and their products appears to provide some protection against the direct impairment in endothelial function produced by high-fat foods, including olive oil.

It's important to remember that this was a short-term study measuring immediate effects after a single meal—not a long-term trial of heart attack or stroke outcomes. The body of evidence supporting the overall health benefits of a Mediterranean-style diet remains strong. However, this study provides valuable insight into *which components* of the Mediterranean diet confer the most vascular protection, and suggests that emphasizing omega-3-rich fish and canola oil, along with antioxidant-rich vegetables and fruits, may be more beneficial than relying primarily on olive oil alone.

## Frequently Asked Questions

### What did this study find about olive oil and blood vessel function after a meal?

In 10 healthy volunteers, a meal of extra-virgin olive oil and whole-grain bread reduced flow-mediated vasodilation, a measure of blood vessel function, by 31% three hours after eating. This temporary impairment was significantly different from the effects of canola oil and salmon meals, which did not impair blood vessel function.

### What are flow-mediated vasodilation (FMD) and endothelial function?

FMD is a noninvasive ultrasound measure of how well the brachial artery in the upper arm widens in response to increased blood flow. It reflects endothelial function, the health of the thin layer of cells lining blood vessels. Endothelial dysfunction is a known risk factor for atherosclerosis, or hardening of the arteries.

### Did adding vitamins or salad prevent the olive oil effect?

Yes. Adding vitamins C and E to the olive oil meal reduced the impairment in FMD by 71%, and adding balsamic vinegar and salad reduced it by 65%. This suggests that antioxidants in these additions largely prevented the temporary blood vessel impairment caused by the olive oil meal.

### What were the limitations of this study?

The study included only 10 healthy volunteers and measured effects after a single large morning meal, so it could not show long-term heart attack or stroke outcomes. It also could not fully distinguish endothelium-dependent from endothelium-independent effects. The authors noted that the link between endothelial function and cardiovascular events is supported only by preliminary data.

### What do the triglyceride changes mean for patients?

All five meals raised serum triglycerides by 27% to 45%, but only the olive oil meal impaired FMD. The study found an inverse correlation between triglyceride changes and FMD, meaning the more triglycerides rose after a meal, the more blood vessel function declined. This suggests the effect may involve triglyceride-containing lipoproteins.

### Should I stop eating olive oil based on this study?

This was a short-term study of immediate effects after one meal, not a long-term trial of heart attack or stroke outcomes. The body of evidence supporting the overall health benefits of a Mediterranean-style diet remains strong. The findings suggest emphasizing omega-3-rich fish and canola oil, along with antioxidant-rich vegetables and fruits, may be more beneficial than relying primarily on olive oil alone.

### Should I get a second opinion before changing my heart-healthy diet based on the 2000 olive oil study?

A second opinion is reasonable before making dietary decisions based on a single-meal study of 10 healthy volunteers. That research measured brachial artery flow-mediated vasodilation three hours after each meal; olive oil and bread reduced it by 31%, while canola oil and salmon did not, and adding vitamins C and E or salad with vinegar reduced the impairment by 71% and 65%. It did not measure heart attacks or strokes over time. A clinician can help interpret these short-term findings alongside your own risk factors. Diagnostic Detectives Network provides independent expert second opinions.

## Source Information

**Original Article Title:** vogel olive oil study 2000

**Authors:** Robert A. Vogel, MD, FACC; Mary C. Corretti, MD, FACC; Gary D. Plotnick, MD, FACC (Division of Cardiology, Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland)

**Journal:** Journal of the American College of Cardiology, Vol. 36, No. 5, 2000, pages 1455–1460. © 2000 by the American College of Cardiology. Published by Elsevier Science Inc.

**Funding:** Supported by funds from the Center for Vascular Biology and Hypertension, University of Maryland School of Medicine, Baltimore, Maryland.

This patient-friendly article is based on peer-reviewed research originally published in a medical journal. It is intended for educational purposes and should not replace personalized medical advice from a healthcare professional.


---
Publisher: Diagnostic Detectives Network (https://diagnosticdetectives.com) — independent multi-expert medical second opinions, worldwide, private-pay.
Author byline: Anton Titov, MD, PhD. Contact: https://diagnosticdetectives.com/pages/contact
Canonical page: https://diagnosticdetectives.com/products/does-olive-oil-harm-blood-vessels-after-a-meal-a-patients-guide-to-the-2000-mediterranean-diet-study