Onions are a modest but consistent source of several vitamins, most notably vitamin C and vitamin B6. A half cup of chopped raw onion (about 80 grams) provides roughly 5 mg of vitamin C (9% of your daily value) and 0.1 mg of vitamin B6 (5% of your daily value). They won’t rival citrus fruits or leafy greens, but because onions show up in so many meals, their nutritional contribution adds up over time.
Vitamins in Bulb Onions
Yellow, white, and red onions share a similar vitamin profile. The standout is vitamin C, with a medium onion (110 grams) delivering about 9 mg. Vitamin C supports immune function and helps your body absorb iron from plant-based foods. Vitamin B6, which plays a role in brain development and keeping your nervous system running smoothly, is present in smaller but meaningful amounts.
Onions also contain folate (vitamin B9), which is important for cell division and is especially relevant during pregnancy. The amounts are small per serving, but onions are rarely eaten in isolation. Tossed into a stir-fry, soup, or salad, they contribute to the cumulative vitamin intake of the whole meal.
Green Onions Are a Different Story
If you’re looking for more vitamin variety, green onions (scallions) punch well above their weight compared to bulb onions. A 100-gram serving of green onions delivers about 997 IU of vitamin A and 207 mcg of vitamin K. Bulb onions contain almost none of either. Vitamin A supports vision and skin health, while vitamin K is essential for blood clotting and bone metabolism.
This difference comes from the green tops, which function like leafy greens. If you’re tossing green onions into a dish, using both the white and green parts gives you the best nutritional return.
Quercetin and Other Beneficial Compounds
Beyond standard vitamins, onions are one of the richest dietary sources of quercetin, a plant compound with antioxidant properties. Red and yellow onions contain the most, ranging from about 54 to 286 mg per kilogram of fresh weight in red varieties and 117 to 202 mg per kilogram in yellow ones. White onions generally have lower levels, though there are exceptions.
Red onions also contain anthocyanins, the pigments responsible for their color. Research shows that onions with higher levels of both anthocyanins and quercetin tend to have stronger overall antioxidant activity. The anthocyanin content in red onions ranges from 39 to 240 mg per kilogram of fresh weight.
Onions are also rich in sulfur-containing compounds, which are responsible for that familiar eye-stinging effect when you cut them. These include compounds like cysteine sulfoxides and thiosulfinates, which have demonstrated antibacterial properties in lab studies. One specific compound, onionin A, has shown potential to inhibit tumor cell growth in early research, though this work is still at the laboratory stage.
How Cooking Affects Vitamin Content
Heat breaks down vitamin C, and onions are no exception. In one analysis measuring vitamin C retention across cooking methods, raw onions contained about 4.67 mg per 100 grams. Steaming preserved the most, leaving about 3.52 mg. Boiling dropped levels to 2.38 mg, and frying was the most destructive, reducing vitamin C to just 1.76 mg per 100 grams.
That’s a loss of roughly 25% from steaming, nearly 50% from boiling, and over 60% from frying. If you’re specifically trying to maximize the vitamin C you get from onions, eating them raw or lightly steamed is the way to go. For soups and stews where onions are boiled, some of the vitamin C leaches into the broth, so you’ll recapture a portion if you consume the liquid.
Quercetin is more heat-stable than vitamin C, so cooking doesn’t wipe it out as dramatically. Sautéing onions briefly is a reasonable middle ground between flavor development and nutrient retention.
Outer Layers Have More Nutrients
The outermost edible layers of an onion are more nutrient-dense than the inner core. Research has found roughly a three-fold difference in flavonol concentration between the outer fleshy scales and the inner portions. The dry papery skin itself is even richer in total flavonoids, though it’s not typically eaten.
The practical takeaway: peel off the dry skin and any damaged layers, but don’t strip away more than you need to. That first fleshy layer just beneath the skin is where flavonoid concentration peaks. Over-peeling means tossing out the most nutritious part of the onion.
Choosing Onions for Nutrition
If your goal is to maximize vitamins, green onions offer the broadest range, with significant amounts of vitamins A, C, and K. For bulb onions, all colors provide similar levels of vitamin C and B6. Red onions edge ahead on antioxidant content thanks to their quercetin and anthocyanin levels, making them a good default choice for raw preparations like salads and salsas where those compounds stay intact.
Yellow onions, the kitchen workhorse, still carry solid quercetin levels and hold up well to cooking. White onions tend to have the mildest flavor and the lowest antioxidant content, though they remain a decent source of vitamin C and B6. Whichever variety you reach for, eating onions regularly, even in small amounts across multiple meals, contributes meaningfully to your overall nutrient intake.