What is the Nutritional Value of an Onion?

December 1, 2025 •

4 min read

According to Healthline, a 100-gram serving of raw onion contains only about 40 calories, making this common kitchen staple a nutrient-dense and low-calorie food. Knowing what is the nutritional value of an onion is key to appreciating its health benefits beyond its distinctive flavor.

Quick Summary

Onions are low-calorie, nutrient-rich vegetables packed with vitamins, minerals, prebiotic fiber, and antioxidants like quercetin and anthocyanins. They support heart health, gut flora, and bone density, while also offering notable anti-inflammatory and antibacterial benefits.

Key Points

Low-Calorie & Nutrient-Dense: A 100g serving of raw onion contains only about 40 calories while providing essential vitamins and minerals.
Rich in Antioxidants: Onions are loaded with powerful antioxidants, particularly quercetin and anthocyanins, which combat oxidative stress and inflammation.
Excellent Source of Prebiotics: The fructan fiber in onions acts as a prebiotic, nourishing beneficial gut bacteria to support a healthy digestive system.
Supports Heart and Bone Health: The compounds in onions help lower cholesterol, reduce blood pressure, and are linked to increased bone density, especially in older adults.
Boasts Anti-Inflammatory & Antibacterial Properties: Quercetin acts as a natural anti-inflammatory, while sulfur compounds provide antimicrobial benefits that fight harmful bacteria.
Flavor with Functional Benefits: Adding onions to your meals enhances flavor while delivering a wide array of health benefits that support overall well-being.

Onions are a cornerstone of cuisines around the world, celebrated for their unique pungent and savory flavor. While often used as a simple seasoning, they are a vegetable packed with a surprising array of vitamins, minerals, and potent plant compounds that contribute significantly to a healthy diet. This comprehensive guide will explore the specific nutritional breakdown of the common onion, highlighting its key health-promoting properties.

Core Nutritional Profile per 100g

Raw onions are an excellent source of nutrients without adding significant calories or fat to a meal. Below is a typical breakdown for a 100-gram (3.5 oz) serving of raw onion:

Calories: Approximately 40 kcal
Water: About 89%
Carbohydrates: Roughly 9.3 grams, primarily from natural sugars and fiber.
Sugar: Approximately 4.2 grams
Dietary Fiber: About 1.7 grams, including prebiotic fructans.
Protein: Around 1.1 grams
Fat: Negligible, at about 0.1 grams

Essential Vitamins and Minerals

Beyond their basic macronutrient profile, onions offer a good supply of several vital micronutrients that are crucial for bodily functions.

Vitamin C: A powerful antioxidant that supports immune function, aids in collagen production for healthy skin and hair, and assists in iron absorption. Red onions, in particular, may offer higher levels.
Vitamin B6 (Pyridoxine): This vitamin is involved in metabolism and the formation of red blood cells.
Folate (B9): Important for cell growth and metabolism, making it especially crucial during pregnancy.
Potassium: An essential mineral that plays a key role in cellular function, fluid balance, nerve transmission, and maintaining heart health.
Manganese: Onions also provide manganese, which supports bone health and metabolism.

Powerful Plant Compounds and Antioxidants

Most of the health benefits of onions are attributed to their wealth of potent plant compounds, known as phytochemicals. Onions are particularly rich in flavonoids, being one of the best dietary sources. These include Quercetin, a potent antioxidant and anti-inflammatory compound especially abundant in the outer layers of red and yellow onions, linked to improved heart health and lower blood pressure. Red onions contain Anthocyanins, the pigments that give them their color, which are also powerful antioxidants associated with a reduced risk of heart disease. Additionally, the characteristic flavor and odor of onions come from sulfur compounds like allyl sulfides and thiosulfinates, which are being studied for potential anti-cancer, antimicrobial, and blood-thinning properties.

Health Benefits Supported by Research

The diverse nutritional content of onions translates into a variety of potential health benefits:

Improved Heart Health: The quercetin and organosulfur compounds in onions help combat inflammation, lower cholesterol, and improve blood vessel function, all of which contribute to a reduced risk of heart disease.
Enhanced Gut Health: Onions contain fructans, a type of prebiotic fiber that feeds beneficial bacteria in the gut. A healthy gut microbiome is linked to improved digestion, enhanced immunity, and overall well-being.
Bone Density Support: Studies, particularly in postmenopausal women, have shown that regular onion consumption is linked to increased bone density and a reduced risk of hip fractures.
Blood Sugar Regulation: Some studies, including human and animal trials, suggest that raw onion consumption can help lower blood sugar levels. This may be especially helpful for individuals with diabetes.
Potential Anti-Cancer Effects: Numerous observational studies have linked higher consumption of allium vegetables, like onions, to a lower risk of certain cancers, such as stomach, colorectal, and breast cancer. The organosulfur compounds are believed to play a role.
Antibacterial Properties: The quercetin and sulfur compounds in onions exhibit powerful antibacterial properties that may inhibit the growth of harmful microbes like E. coli and Staphylococcus aureus.

Raw vs. Cooked: How Preparation Affects Nutrients

The method of cooking can influence an onion's nutritional value, as some compounds are sensitive to heat. The table below compares the general state of key nutrients in raw versus cooked onions.


Nutrient/Compound	Raw Onion	Cooked Onion (e.g., Sautéed)
Sulfur Compounds	Maximum potency, strong flavor.	Significantly reduced, less pungent.
Vitamin C	Higher due to heat sensitivity.	Lower, as vitamin C is partially destroyed by heat.
Flavonoids (Quercetin)	High levels, especially in red/yellow varieties.	Generally retained, with some cooking methods potentially increasing bioavailability.
Prebiotic Fiber (Fructans)	High levels, can cause digestive issues for sensitive individuals.	Intact, but texture softens, potentially aiding digestibility for some.
Antioxidant Activity	High due to active compounds.	Still present, though potentially reduced depending on method.

A Nutrient-Packed Addition to Your Diet

For a small calorie investment, the onion provides a significant nutritional payoff, from heart-healthy antioxidants to gut-supporting prebiotic fiber. Whether you enjoy them raw for maximum sulfur compound potency or cooked to soften the flavor, incorporating onions into your meals is an easy and delicious way to boost your health.

To retain the most nutrients, consume them raw in salads or salsas when possible. To minimize irritation when cutting, consider chilling the onion first or cutting near an open window. For more culinary ideas and nutritional details, check out resources like the National Onion Association, which champions the health benefits of this versatile vegetable. The onion's status as a superfood is well-deserved, proving that even the most common ingredients can be powerful allies for your health.

Conclusion

In summary, the onion is a nutrient-dense powerhouse rich in vitamins, minerals, and antioxidants. Its impressive profile of quercetin, anthocyanins, and prebiotic fibers supports everything from heart and bone health to gut function and immune response. A simple, flavorful addition to any meal, the onion offers significant nutritional value that makes it a must-have in a health-conscious diet.

Frequently Asked Questions

Red onions tend to have a higher antioxidant content, specifically anthocyanins, which give them their deep color. Yellow onions are also high in flavonoids like quercetin, often having more antioxidants than white onions.

Cooking can reduce the levels of certain heat-sensitive compounds, like some sulfur compounds and vitamin C. However, other antioxidants, such as flavonoids, are often retained or may even become more bioavailable through cooking.

Yes, onions are low in calories and fat while providing fiber, which can promote feelings of fullness and aid in digestion, potentially supporting weight management efforts.

When an onion is cut, its cells release sulfur compounds. These compounds react with enzymes to produce a gas called propanethial S-oxide, which irritates the eyes and causes tears.

Yes, onions are a decent source of fiber, providing both soluble and insoluble types. The soluble fiber, specifically fructans, acts as a prebiotic to feed healthy gut bacteria.

Onions are considered a high-FODMAP food due to their fructan content, which can cause digestive issues like bloating and gas in sensitive individuals with Irritable Bowel Syndrome (IBS). Cooked onions may be tolerated better than raw ones.

While there is no official daily recommendation, consuming a moderate amount, such as ½ to 1 medium onion per day, is a reasonable way to reap the antioxidant and anti-inflammatory benefits.