Are Onions Healthier Raw or Cooked?

Onions are bulb-shaped vegetables that grow in the ground. Even though eating onions raw will let you reap all their health benefits, there can also be several downsides.

Onions (scientifically known as Allium cepa) are bulb-shaped vegetables that grow in the ground. They're closely related to other root vegetables in the Allium family, including shallots, leeks, scallions, chives, and garlic.

Onions are known for their pungent smell and distinct flavor and come in several varieties and sizes. They can be eaten cooked or raw and are a staple food worldwide. In addition to their flavoring abilities, onions are a rich source of vitamins, minerals, and fiber and offer several health benefits. 

But these properties can sometimes change depending on whether you cook your onions or eat them raw. How you cook them can change the properties as well. Knowing these differences can help you make the best choices for how to include these nutrient powerhouses in your diet.

All onions fall into three broad categories: short-day, medium-day, and long-day. These refer to the number of hours of sunlight required for the onion bulb to form, typically falling between 11 and 15 hours of sunlight.

More commonly, onions are identified and categorized by their color, with yellow, red, and white being the three most popular types of onions consumed in the U.S. 

Yellow onions. Yellow or "all-purpose" onions are the most common, accounting for over 85% of the onions grown in the U.S. They're often juicier than other types of onions, and their flavor ranges from mild to strong depending on the variety. In addition to being eaten raw, yellow onions can be cooked, sautéed, grilled, baked, caramelized, or roasted. When cooked, yellow onions often release a sweet flavor and smell, making these a popular option to cook with.  

Red onions. Less than 10% of the U.S. onion crop is red. Red onions can range from crisp, sweet, and very mild to spicy, sharp, and extremely pungent. They can be grilled or roasted and are commonly eaten raw in salads or as a garnish.

White onions. White onions amount to about 5% of U.S. onion production. They're rarely sweet or mild and usually have a medium to strong pungent taste. They can be eaten raw, grilled, sautéed, or cooked, and are most commonly used in white sauces. 

The main nutritional properties of a medium-sized (100-gram) raw onion include:

• Calcium — 23 milligrams
• Calories — 40
• Carbohydrates — 9.3 grams 
• Fiber — 1.7 grams
• Magnesium — 10 milligrams
• Potassium — 146 milligrams
• Protein — 1.1 grams
• Sugar — 4.2 grams
• Vitamin C — 7.4 milligrams
• Water — 89%

In addition to being low in calories and providing a good source of minerals and vitamins, onions have other health benefits. They contain high amounts of antioxidants and sulfur compounds. 

Sulfur-containing compounds are abundant in onions and can help reduce your risk of cancer. Other health benefits of eating onions include the following:

Bone health. The antioxidants in onions can improve bone density, decrease bone loss, and help prevent various bone-related problems — in particular, osteoporosis.

Heart health.  Quercetin, also known as flavonoid, is known to lower cholesterol and blood pressure, which reduces the risk of nonfatal heart attacks and strokes.

Digestive health. Onions contain prebiotics, a type of fiber that promotes the growth of healthy bacteria in your stomach. This bacteria is responsible for creating short-chain fatty acids, which strengthen your gut health, reduce inflammation, and promote digestive health.  p>

Blood sugar regulation. Eating raw onions can help reduce your blood sugar levels, which is a key factor in controlling type 1 and 2 diabetes. 

Reduced cancer risk. The sulfur compounds and quercetin found in onions have been linked to the reduction of various cancers — including stomach, prostate, lung, and ovarian cancers.

There are many ways to prepare onions, and the method and duration of cooking can impact an onion’s properties. Although cooking onions won’t destroy all their nutritional value, it can reduce some of the benefits that onions offer.

For example, raw onions have a high concentration of pyruvate, a type of acid that promotes anti-platelet activity, which is what helps prevent blood clots. As you heat onions, the concentration of pyruvate is lowered, decreasing the strength of the onion’s anti-platelet agents. 

Steaming. Steaming onions can have some of the most dramatic effects on anti-platelet activity. Just 3 to 6 minutes of steady steaming can destroy pyruvate concentrations entirely. Anything more than 10 minutes might begin to have negative effects on the onion's nutritional value.

Boiling. Boiling onions for up to 3 minutes hasn’t shown any dramatic loss of key compounds, but this begins to change as onions continue to boil. 

Cooking in general. As onions are cooked, they lose some of their sulfur compounds and quercetin, two of their most beneficial elements.

Even though eating onions raw will let you reap all their health benefits, there can also be several downsides.

Heartburn. Eating raw onions may cause or worsen pain and irritation in the stomach. This irritation is commonly known as heartburn, which is a symptom of acid reflux.

Eye irritation. Cutting, handling, and eating raw onions can lead to watering and irritation of the eyes.

Bad breath. Halitosis, more commonly known as bad breath, is another drawback to eating raw onions.

Stomach pains. Onions can aggravate irritable bowel syndrome symptoms, including bloating, passing gas, and stomach pains. This is especially true when you eat onions raw.

Allergies. Some people may be allergic to onions and experience common symptoms like itchiness, runny nose, red eyes, nasal congestion, or a rash when eating onions.

Onions have several health benefits. Including these nutrient-packed, compound-rich vegetables in your diet is generally a good idea. 

How you choose to prepare and eat onions depends on individual preference, taste, and circumstances. As a general rule, lightly cooking onions for 1 or 2 minutes is unlikely to cause a notable change in their properties. If you’re looking for maximum health benefits, onions should only be mildly cooked, or eaten raw.