Calories in Onions, yellow

What makes onions so powerful?

When you slice a yellow onion, you are triggering a sophisticated chemical defense mechanism. Crushing the cells releases the enzyme alliinase, which converts sulfoxides into sulfenic acid, rapidly rearranging into syn-propanethial-S-oxide—a volatile gas that irritates the eyes. But this same sulfur chemistry is what makes onions so beneficial. Yellow onions are among the richest dietary sources of quercetin, a flavonoid antioxidant that accumulates in the outer layers. Quercetin has been extensively studied for its ability to help lower blood pressure and reduce inflammation by modulating oxidative stress in the cardiovascular system. Unlike some nutrients, quercetin is actually quite heat-stable, meaning that slow-cooking onions does not destroy their primary antioxidant benefits. Historically, onions were a staple in ancient Egyptian, Greek, and Roman diets, often consumed by laborers building the pyramids for their perceived stamina-enhancing properties. Today, the yellow onion remains the workhorse of the culinary world, chosen for its ideal balance of pungency when raw and deep sweetness when caramelized, a transformation driven by the Maillard reaction breaking down its natural carbohydrates.