There have been many intervention studies adding garlic or onion to the diet and evaluating cardiovascular and metabolic indicators.1 Studies have used a variety of forms of onions and garlic, including garlic powder, garlic extract, onion peel extract, or the raw vegetables themselves. The evidence suggests that phytochemicals from these flavorful vegetables support cardiovascular health in several different ways:
Here’s some news that might bring tears (of joy) to your eyes: Onions, garlic and other Allium vegetables aren’t just delicious, nutritious and – let’s face it – highly odiferous. They are also the source of phytochemicals that have powerful benefits for your heart health.
Within about one minute after garlic is chopped or crushed, the enzyme alliinase produces an organosulfur compound called allicin; then allicin is converted to a number of other organosulfur compounds with beneficial antioxidant, anti-inflammatory, and anti-cancer effects.2 Other organosulfur compounds are thought to be absorbed intact, and then converted by the body to bioactive compounds.3, 4 Onions (and other Allium vegetables – chives, shallots, leeks, scallions, etc.) also contain some of these organosulfur compounds. The evidence suggests waiting 10 minutes to cook after chopping for the full effect of alliinase.5
Cooking tip: Looking for an easy way to crush garlic? Try using a hand-held grater.
Onions are particularly rich in the flavonoid phytochemical quercetin, which is the most abundant flavonoid in onions. Red onions also contain anthocyanins. The outer layers of the onion contain more flavonoids, as does the portion nearer the root end. Flavonoids are known for their effects on cell signaling leading to anti-inflammatory and antioxidant effects.6,7
Cooking tip: To get the most “bang” for your caloric “buck” when eating red onions, be sure not to peel off too much of the outer layers, and cut it as close to the root end as possible. That’s because the outer layers of the onion contain more flavonoids, as does the portion nearer the root.
Allium vegetable consumption and health: An umbrella review of meta-analyses of multiple health outcomes
Garlic and onions: their cancer prevention properties
Garlic (Allium sativum L.): A Brief Review of Its Antigenotoxic Effects
Biological Properties and Bioactive Components of Allium cepa L.: Focus on Potential Benefits in the Treatment of Obesity and Related Comorbidities
Onions: A Source of Flavonoids
The organosulfur compound allicin and another garlic phytochemical called ajoene are known to inhibit the enzyme responsible for cholesterol synthesis in vitro, suggesting that garlic could influence cholesterol levels.8 Multiple meta-analyses of randomized controlled trials on the effect of garlic supplements on lipid levels have been published. Overall, total and LDL cholesterol were reduced in both healthy participants and patients with elevated cholesterol.1
Only a few trials have been conducted on onions or onion extract. In women with normal cholesterol levels, two weeks of supplementation with onion peel extract decreased total and LDL-cholesterol compared to placebo.9 Similarly, raw red onion twice a day for 8 weeks reduced cholesterol in overweight or obese women with polycystic ovary syndrome.10
Video: How to Lower Blood Pressure and Cholesterol (free for members)
Ajoene, a garlic compound, inhibits protein prenylation and arterial smooth muscle cell proliferation
Effect of onion peel extract supplementation on the lipid profile and antioxidative status of healthy young women: a randomized, placebo-controlled, double-blind, crossover trial
Effects of raw red onion consumption on metabolic features in overweight or obese women with polycystic ovary syndrome: a randomized controlled clinical trial
Garlic phytochemicals could affect blood pressure in two ways:
Meta-analyses have found that supplemental garlic reduced systolic and diastolic blood pressure in participants with hypertension.1
Quercetin and other flavonoids are thought to reduce blood pressure in multiple ways:
In a few studies, supplemental onion improved vascular function or blood pressure. Onion peel extract supplementation – twice a day for 12 weeks – in overweight and obese participants improved vascular function, compared to placebo.14 In another study of onion skin extract, in overweight or obese participants with pre-hypertension or stage 1 hypertension, systolic blood pressure was reduced, compared to placebo.15
Effect of garlic on blood pressure: a meta-analysis
Habitual intake of flavonoid subclasses and incident hypertension in adults
Actions of Quercetin, a Polyphenol, on Blood Pressure
Effect of onion peel extract on endothelial function and endothelial progenitor cells in overweight and obese individuals
Effects of a quercetin-rich onion skin extract on 24 h ambulatory blood pressure and endothelial function in overweight-to-obese patients with (pre-)hypertension: a randomised double-blinded placebo-controlled cross-over trial
In 40 patients with metabolic syndrome, raw crushed garlic twice a day for 4 weeks reduced systolic and diastolic blood pressure, triglycerides, and fasting blood glucose.16 In women with breast cancer, eating fresh yellow onion daily for 8 weeks decreased fasting blood glucose compared to a placebo group during doxorubicin chemotherapy, a treatment which may reduce sensitivity to insulin. The onion group also showed improvement in a measure of insulin sensitivity.17
In a meta-analysis of studies on patients with diabetes, garlic supplementation reduced fasting blood glucose and HbA1c, and the effect on HbA1c was greater with a longer duration of supplementation.18
Effect of Raw Crushed Garlic (Allium sativum L.) on Components of Metabolic Syndrome
Consumption of Fresh Yellow Onion Ameliorates Hyperglycemia and Insulin Resistance in Breast Cancer Patients During Doxorubicin-Based Chemotherapy: A Randomized Controlled Clinical Trial
The effect of garlic on lipid profile and glucose parameters in diabetic patients: A systematic review and meta-analysis
Organosulfur compounds from garlic and onions inhibit platelet aggregation in vitro. Trials on garlic supplementation overall have found a modest but significant antiplatelet effect,19 which could help prevent heart attacks by slowing blood clotting.
A comparison of different Allium vegetables that tested antiplatelet activity on blood from healthy volunteers found that garlic and shallot had the strongest antiplatelet effects, and chives and onion had milder effects.20
Garlic shows promise for improving some cardiovascular risk factors
Relationships Between Bioactive Compound Content and the Antiplatelet and Antioxidant Activities of Six Allium Vegetable Species
Read: The End of Heart Disease
Joel Fuhrman, M.D. is a board-certified family physician, seven-time New York Times bestselling author and internationally recognized expert on nutrition and natural healing, who specializes in preventing and reversing disease through nutritional methods. Dr. Fuhrman coined the term “Nutritarian” to describe his longevity-promoting, nutrient dense, plant-rich eating style.
For over 30 years, Dr. Fuhrman has shown that it is possible to achieve sustainable weight loss and reverse heart disease, diabetes and many other illnesses using smart nutrition. In his medical practice, and through his books and PBS television specials, he continues to bring this life-saving message to hundreds of thousands of people around the world.
*There is no guarantee of specific results. Results can vary. All material provided on the DrFuhrman.com website is provided for informational or educational purposes only. Consult a physician regarding the applicability of any opinions or recommendations with respect to your symptoms or medical condition.