Onion benefits have proven more than just anecdotal. Different varieties provide many health advantages like antioxidants, protection against cancer, and immune system support. Including onions in your diet is a nutritious way to improve your overall health.

Onions (Allium cepa) are considered to be a highly nutritious vegetable. They contain phytochemicals, compounds plants produce to ward off harmful bacteria, viruses, and fungi. These phytochemicals have the following properties and provide health benefits:

1. May Help Fight Bacteria

Onions may have the potential to fight bacteria like:

• Escherichia coli (E. coli)
• Staphylococcus aureus (S. aureus)
• Bacillus cereus

Research suggests that quercetin, one of the flavonoids found in onions, can inhibit the growth of many types of bacteria.

2. May Have Anticancer Effects

Though research is variable, some studies show that eating onions can decrease certain cancers, especially those affecting the digestive tract.

Onions contain quercetin, which research supports may inhibit the growth of Helicobacter pylori (H. pylori), a type of bacteria associated with stomach (gastric) cancer and a rare type of non-Hodgkin lymphoma.

3. May Reduce Inflammation

Onions are packed with healthy components that help fight inflammation. They are a rich source of flavonoids (which combat free radicals in your body), like quercetin.

There are several studies in humans investigating the correlation between quercetin levels and immunomodulatory effects, and a 2016 review confirmed the benefits of increasing your quercetin intake.

4. Has Antioxidant Effects

Along with other related vegetables in the Allium plant genus (like garlic, leeks, and shallots), onions contain:

• Flavonoids
• Glutathione
• Selenium compounds
• Vitamins E and C

All of these components contribute to the antioxidant properties of onions.

5. Helps Control Blood Sugar

The extract of onion bulb Allium cepa has been shown to significantly lower high blood glucose (sugar) in animal studies.

When given alongside the antidiabetic drug metformin, onion extract lowered fasting blood sugar levels by 35% to 50%, depending on the dose. This is a promising finding, especially considering the affordability and safety of onions.

6. Supports Bone Health

One of onions’ most powerful compounds is theorized to give a boost to bone health. A peptide found in onions called GPCS has been shown in studies to prevent the breakdown of bone, decrease bone loss, and lower fracture risk.

7. Promotes Digestive Health

Fiber is crucial for a healthy gut biota. Onions are among the best probiotic foods (along with leeks, chicory roots, celery, apples, and beans) to protect your gut lining.

Onions along with other vegetables and fruits, contain inulin—a prebiotic compound that encourages the healthy growth of gut bacteria.

While more research is needed, onions have been shown to help build a healthy gut. Instead of eating them as a condiment, those with gut issues may benefit from including onions in their diet more regularly.

8. Provides Many Nutrients

Onions are a highly nutritious food that can contribute to a healthy diet. Here is the nutrition information for 1/2 cup raw onion:

• Total calories: 30
• Total fat: 0 grams (g)
• Cholesterol: 0 milligrams (mg)
• Carbohydrates: 7 g
• Dietary fiber: 1 g
• Total sugars: 5 g
• Protein: 1 g
• Sodium: 0 g
• Iron: 0.2 mg or 1% Daily Value (DV)
• Potassium: 126 mg or 4% DV
• Vitamin C: 5 mg or 9% DV
• Vitamin B6: 0.1 mg or 5% DV
• Calcium: 16 mg or 2% DV
• Folic Acid: 15.2 mg or 4% DV
• Selenium: 0.5 mg or 1% DV
• Zinc: 0.2 mg or 1% DV

9. May Help Support Weight Management

Being overweight is associated with metabolic and inflammatory changes related to eating high amounts of saturated fat. Including onions in your diet can significantly reduce body weight, body fat, and blood triglyceride levels due to the beneficial effects of the anti-inflammatory flavonoid quercetin.

In a 2023 systematic review and meta‐analysis, these effects were particularly noticeable when onion skin (often discarded during food processing) was consumed.

What’s Healthier, Raw or Cooked Onions?

Though onions are beneficial whether eaten raw or cooked, cooking onions reduces the number of thiosulfinates, compounds that give onions their antimicrobial, antifungal, and antibiotic properties. However, research shows that when onions are crushed before cooking, they retain their health benefits.

Of the various cooking methods, boiling and frying onions seem to cause the most significant loss in nutritious value. Other preparation methods that decrease health benefits include sautéing, steaming, and microwaving.

In contrast, baking onions is shown to increase their flavonoids in onions. Consuming dried, powdered onions can also offer nutritious value to food, especially if the powder is freeze-dried.

Summary

Onions are a nutritious vegetable that has antifungal, antibacterial, anti-inflammatory, and many other healthy properties. They provide health benefits regardless of how they are prepared, though onions lose some of their nutritional value when cooked.

Part of what makes onions so healthy is that they contain flavonoids, glutathione, selenium compounds, vitamin E, and vitamin C. When selecting onions, look for those that are without blemishes or discoloration, feel firm, and have dry, papery skins.