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Onion benefits come down to a few solid facts and a lot of hype. In USDA data, a medium raw onion has about 44 calories, 1.9 grams of fiber and some vitamin C and vitamin B6. Onions are also among the richest everyday sources of flavonoids such as quercetin, and red onions add anthocyanin pigments. They contain fructans, a fermentable fiber that feeds gut bacteria but can cause gas in people sensitive to FODMAPs. Below are the USDA onion nutrition facts, how red vs white onion and other types compare, what the research does and does not show, and why onions are on the list of foods to keep away from pets.
Peptide Labs does not sell onions or onion products. This article is general nutrition information, not medical advice.
Key Takeaways
- A medium raw onion (110 g) has about 44 calories, 1.9 grams of fiber (7% DV) and 9% of the Daily Value for vitamin C in USDA data.
- Onions are a major dietary source of flavonols, mostly quercetin compounds; a 2007 review found red onions at the higher end of the range.
- USDA SR Legacy has no separate red or white onion entry; the color difference is mainly pigment and flavor.
- Onions are rich in fructans, a FODMAP that can trigger gas and bloating in people with IBS.
- The ASPCA warns that onions can damage red blood cells in cats and dogs.
Onion nutrition: USDA values per onion and per cup
The table uses the USDA FoodData Central SR Legacy entry for raw onions (FDC 170000) at USDA’s portion weights for a medium onion (110 grams) and a cup chopped (160 grams). The percent Daily Value (%DV) uses the FDA Daily Values printed on Nutrition Facts labels.
| Nutrient (raw onion) | 1 medium (110 g) | % DV | 1 cup chopped (160 g) | % DV |
|---|---|---|---|---|
| Calories | 44 | Not applicable | 64 | Not applicable |
| Carbohydrate | 10.3 g | 4% | 14.9 g | 5% |
| Fiber | 1.9 g | 7% | 2.7 g | 10% |
| Total sugars | 4.7 g | No DV | 6.8 g | No DV |
| Protein | 1.2 g | 2% | 1.8 g | 4% |
| Vitamin C | 8.1 mg | 9% | 11.8 mg | 13% |
| Vitamin B6 | 0.13 mg | 8% | 0.19 mg | 11% |
| Folate | 21 mcg DFE | 5% | 30 mcg DFE | 8% |
| Manganese | 0.14 mg | 6% | 0.21 mg | 9% |
| Potassium | 161 mg | 3% | 234 mg | 5% |
Onion calories are low because onions are about 89% water. Their sweetness comes from natural sugars (glucose, fructose and sucrose) that become more noticeable when onions are cooked slowly and browned. Onions contain essentially no vitamin A or vitamin K, which sets them apart from green Allium vegetables such as scallions and the leeks in our sibling guide. For context on fiber across foods, see our guide to fiber-rich foods; onions contribute modestly, mostly because people eat them often.
Raw vs cooked onions and other onion types
Per 100 grams, the main USDA onion entries look like this:
| Per 100 g (USDA FDC ID) | Calories | Fiber (% DV) | Total sugars | Vitamin C (% DV) | Potassium (% DV) |
|---|---|---|---|---|---|
| Onions, raw (170000) | 40 | 1.7 g (6%) | 4.24 g | 7.4 mg (8%) | 146 mg (3%) |
| Onions, boiled and drained (170001) | 44 | 1.4 g (5%) | 4.73 g | 5.2 mg (6%) | 166 mg (4%) |
| Sweet onions, raw (170008) | 32 | 0.9 g (3%) | 5.02 g | 4.8 mg (5%) | 119 mg (3%) |
| Scallions, tops and bulb (170005) | 32 | 2.6 g (9%) | 2.33 g | 18.8 mg (21%) | 276 mg (6%) |
| Shallots, raw (170499) | 72 | 3.2 g (11%) | 7.87 g | 8 mg (9%) | 334 mg (7%) |
Cooking concentrates onions slightly as they lose water, while some vitamin C is lost to heat and cooking water. Sweet onions taste milder, yet USDA lists them at 5.02 grams of sugars per 100 grams versus 4.24 grams for general raw onions, not a big gap. Scallions bring more vitamin C, and their green tops are rich in vitamin K, a point covered in our guide to vitamin K foods for anyone on warfarin who keeps intake consistent.
Red vs white onion: what is actually different
The USDA SR Legacy database has a general raw onion entry and a sweet onion entry but no separate red or white onion entries, so there is no USDA basis for saying one color has more calories, fiber or vitamins. The differences that are documented are in pigments and flavor compounds. A 2007 review of onion flavonoids in the Journal of Agricultural and Food Chemistry reported that flavonols, mostly quercetin compounds, are the main pigments in onions, that red onions span a higher range than yellow onions, and that red onions also contain anthocyanins, mainly cyanidin compounds, which give them their color. The review gave no figures for white onions.
| Onion type | Color pigments | Flavonols (2007 review, per kg fresh weight) | Typical flavor and use |
|---|---|---|---|
| Red | Flavonols plus anthocyanins (cyanidin compounds) | 415 to 1,917 mg | Mild to sharp; often eaten raw in salads and salsas |
| Yellow | Flavonols | 270 to 1,187 mg | Pungent raw, sweet when cooked; all-purpose |
| White | No red pigment | Not reported in the review | Crisp and sharp; common in Mexican dishes |
| Sweet (for example Vidalia) | Flavonols | Not reported separately | Mild; eaten raw or grilled; USDA entry 170008 |
Two practical notes. The review noted that the dry outer skins contain additional quercetin compounds, but those papery skins are usually discarded rather than eaten. And a higher flavonoid count in a lab analysis is not the same as a proven health effect in people. If you like the taste of one onion over another, that is a fine reason to choose it.
Onion benefits: what the research shows
- Flavonoids: onions are among the richest dietary sources of flavonols, especially quercetin compounds. Quercetin is studied mostly as an isolated supplement; our quercetin guide covers that evidence and its limits, and our guide to antioxidant-rich foods explains why lab antioxidant scores do not translate directly into health effects.
- Fructans and gut bacteria: onions supply fructans, fermentable fibers that gut bacteria use for food, which is why onions appear in our guide to prebiotic foods.
- Fiber, vitamin C and B6: modest per serving, but onions are eaten so often that they add up across a week.
- Flavor without much salt: onions add depth to food, which can help when cooking with less sodium.
Claims that onions lower blood sugar, cholesterol or cancer risk come mostly from lab, animal and observational studies, which can find associations but cannot show that onions cause an effect. Onions are a healthy everyday vegetable; they are not a remedy for any condition.
Why onions make you cry
When an onion is cut, its cells break and sulfur-containing compounds meet enzymes. A 2002 paper in Nature identified a specific enzyme, lachrymatory-factor synthase, that produces the volatile compound that stings the eyes. Chilling onions before cutting, a sharp knife and good ventilation are common kitchen tricks for fewer tears. The same family of sulfur compounds is responsible for onion breath.
Onions, fructans, pets and other cautions
- FODMAPs: Monash University lists onion among vegetables particularly rich in fructans, and NIDDK names onions among high-FODMAP vegetables for people with IBS. Onion also hides in marinades, sauces, spice blends and processed meats. Our low FODMAP diet guide and gas and bloating guide explain the three-step approach, which Monash says should be done with a dietitian.
- Pets: the ASPCA Animal Poison Control Center says onion, garlic and chives can irritate the digestive system and damage red blood cells, possibly leading to anemia; cats are more susceptible, and dogs are also at risk depending on the amount eaten. If a pet eats onion, call your veterinarian or the ASPCA Animal Poison Control Center at (888) 426-4435. Raisins are another common kitchen hazard; see our raisins guide.
- Allergy: onion is not one of the nine major food allergens that US labels must declare, so people who react need to read ingredient lists and ask about restaurant dishes. See an allergist for suspected reactions, and get urgent care for swelling or breathing trouble.
- Smaller meals: if you eat less on a GLP-1 medicine, our GLP-1 diet guide covers how vegetables fit in.
Frequently Asked Questions
What are the main onion benefits?
Onions are low in calories, supply some fiber, vitamin C and B6, and are a major source of flavonoids such as quercetin. Health claims beyond that are mostly from lab and observational studies.
How many calories are in an onion?
About 44 calories in a medium raw onion and about 64 in a cup chopped, in USDA data.
Is a red or white onion healthier?
USDA has no separate red and white entries. Red onions contain anthocyanin pigments and, in a 2007 review, a higher range of flavonols than yellow onions. Neither has proven health advantages.
Are cooked onions still nutritious?
Yes. Cooked onions keep their fiber and minerals, though some vitamin C is lost to heat and cooking water.
Why do onions cause gas?
Onions are rich in fructans, which gut bacteria ferment and which are a common trigger for people with IBS.
Are onions toxic to dogs?
Yes. The ASPCA warns that onions can damage red blood cells in dogs and cats, with cats more susceptible. Call your veterinarian if a pet eats onion.
Questions about a label? Contact us or read our Buyer’s Guide. Food comes first; follow the label and ask a healthcare provider before starting a supplement, especially if you take medicines, are pregnant or have a food allergy. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Last reviewed October 2026.
Related guides: Leeks: What They Are, Leeks Nutrition and How to Cook Leeks, Prebiotic Foods: A Practical List and What the Research Shows, Quercetin: Benefits, Side Effects, Interactions and Bromelain Combos, Shallots: What They Are, USDA Nutrition and Shallot vs Onion, Types of Onions: Colors, Day Length and Allium Vegetables.