Exploring Cooked Onion Nutrition

Exploring Cooked Onion Nutrition

You might think of onions as mere flavor enhancers in your favorite dishes, but there's more to these humble bulbs than meets the eye. Cooked onions not only add depth and aroma to your culinary creations but also pack a powerful nutritional punch. In this exploration of cooked onion nutrition, we'll delve into the nitty-gritty details of the nutrients, minerals, and compounds found in onions, comparing raw and cooked varieties to give you a comprehensive understanding of their dietary value.

The Nutritional Breakdown of Onions

Onions are a staple in many cuisines, celebrated for their ability to impart a unique taste to various dishes. However, their nutritional profile often remains underestimated. To truly appreciate their value, let's analyze the nutrients, minerals, and compounds found in cooked onions per onion and as a percentage of the Recommended Dietary Allowance (RDA).

Calories and Macronutrients

  • Calories: Onions are low in calories. A medium-sized cooked onion provides around 46 calories.

  • Carbohydrates: A medium cooked onion contains approximately 11 grams of carbohydrates.

  • Dietary Fiber: You'll get around 3.1 grams of dietary fiber from a medium cooked onion, aiding in digestion and providing a sense of fullness.

  • Protein: While not a significant source of protein, a medium cooked onion still provides about 1.2 grams.

  • Fat: Cooked onions are virtually fat-free, with less than 0.2 grams per medium onion.


  • Vitamin C: A medium cooked onion offers approximately 12% of the RDA for vitamin C. This antioxidant vitamin supports immune function and collagen synthesis.

  • Vitamin B6: You'll find about 10% of the RDA for vitamin B6 in a medium cooked onion. This vitamin is essential for brain development and function.

  • Folate (Vitamin B9): Cooked onions contain roughly 5% of the RDA for folate, a vital nutrient for DNA synthesis and cell growth.

  • Vitamin A: A medium cooked onion contributes about 1% of the RDA for vitamin A.


  • Potassium: Onions are a decent source of potassium, with a medium cooked onion providing around 5% of the RDA for this essential mineral.

  • Phosphorus: You'll get approximately 3% of the RDA for phosphorus from a medium cooked onion.

  • Calcium: Cooked onions contain only a negligible amount of calcium.

Compounds and Antioxidants

Onions are rich in various compounds and antioxidants, such as:

  • Quercetin: This powerful antioxidant is known for its anti-inflammatory properties and potential heart health benefits.

  • Sulfur Compounds: Onions contain sulfur compounds like allicin, which may have antibacterial and antifungal properties.

  • Flavonoids: Onions are a source of various flavonoids, which have been associated with a reduced risk of chronic diseases.

Comparing Raw and Cooked Onions

To truly understand the impact of cooking on the nutritional value of onions, let's compare raw and cooked varieties.

Nutrient Preservation

It's essential to note that cooking onions can cause some nutrient loss due to the application of heat. For instance, vitamin C is sensitive to heat, and cooking can lead to its degradation. However, certain compounds in onions become more bioavailable when cooked, making it a bit of a nutritional trade-off.

Here's a summary in a table for easy reference:

Nutrient/Compound Raw Onion (Per Medium Onion) Cooked Onion (Per Medium Onion)
Calories 42 46
Carbohydrates 10 grams 11 grams
Dietary Fiber 2.2 grams 3.1 grams
Protein 0.9 grams 1.2 grams
Fat 0.1 grams 0.2 grams
Vitamin C (RDA %) 12% 9%
Vitamin B6 (RDA %) 10% 8%
Folate (Vitamin B9 %) 5% 6%
Potassium (RDA %) 5% 5%
Phosphorus (RDA %) 3% 3%
Calcium (RDA %) Negligible Negligible
Quercetin Present Present
Sulfur Compounds Present Present
Flavonoids Present Present

Note: The values mentioned in the table are approximate and can vary depending on the size and type of onion.

The Science Behind the Changes

Cooking onions involves the application of heat, and this process affects the nutritional composition of the vegetable. Understanding the science behind these changes can help you make informed decisions about how you use onions in your cooking.

Heat Sensitivity of Nutrients

  • Vitamin C: As mentioned earlier, vitamin C is particularly sensitive to heat. When onions are cooked, the vitamin C content decreases. To preserve more vitamin C, consider using lower cooking temperatures and shorter cooking times.

  • Vitamin B6 and Folate: While cooking can cause some loss of these B vitamins, the difference is not as significant as with vitamin C. You'll still obtain a reasonable amount of vitamin B6 and folate from cooked onions.

Enhanced Bioavailability

  • Quercetin: The cooking process can actually increase the bioavailability of quercetin, one of the key antioxidants in onions. This means your body can absorb and utilize quercetin more effectively from cooked onions.

Flavor and Aroma

Cooking onions also transforms their flavor and aroma, making them sweeter and milder. This can be advantageous for those who find raw onions too pungent. The change in flavor can make cooked onions a versatile addition to a wide range of dishes.

References and Sources

To compile this information, we referred to various credible sources, including the following:

  1. United States Department of Agriculture (USDA) Food Database: 

  2. Harvard T.H. Chan School of Public Health - The Nutrition Source: 

  3. National Institutes of Health - Office of Dietary Supplements: 

  4. Linus Pauling Institute - Micronutrient Information Center: 

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.