Major nutrients such as carbohydrates and vitamin C are generally similar from one color and variety of onion to another. Color and pungency, however, indicate the presence of beneficial anti-oxidants that do vary in amount and type with the onion variety. If you use both red and yellow onions, you can take advantage of the best qualities of each.
The best locally grown onions arrive at your supermarket in August, after the fully matured bulbs reach their peak nutritional content and go dormant. Bulb onions survive storage for three to six months, according to the Alabama Cooperative Extension. Yellow storage onions keep their nutritional qualities longer than mild red bulb onions because of higher levels of protective chemical compounds. These phytochemicals preserve the major nutrients in the onion bulb and prevent bulb rot. Phytochemicals also provide important health benefits.
Differences in the nutritional content of onions arise as the bulbs mature. Yellow storage onions develop tough-skinned pungent bulbs with high levels of anti-oxidant phytochemicals. Phytochemicals such as phenolics and flavonoids protect onions from fungal diseases, viruses and bacteria. Onions with greater pungency have higher levels of these anti-oxidants. Of the 10 most common onion varieties sold in the United States, Western Yellow onions rank highest in flavonoids, according to an article published in 2004 in the Journal of Agricultural and Food Chemistry. Northern Red onions rank fourth. Western Yellow onions rank second in overall anti-oxidant activity, while Northern Red onions again rank fourth.
Red vegetables including red onions contain phytochemicals with unique anti-oxidant properties. Chemical compounds such as lycopene and anthocyanin give onions a red or reddish purple color. These anti-oxidants help prevent cholesterol from sticking to the walls of your arteries. Other health benefits include better memory and fewer urinary tract infections. Further, Western Yellow and Northern Red onions contain high amounts of anti-oxidants that slow the growth of cancer cells in the liver and the colon, according to the 2007 issue of the "Journal of Agricultural and Food Chemistry."
Supermarkets typically carry a limited selection of the many varieties of onions that are grown around the world. If you grow your own produce, try some varieties that offer better nutritional value than the commercial offerings. Pungency and color mark the qualities needed for high anti-oxidant levels in onions, so choose varieties known for long storage lifetimes rather than mild sweet flavor. Plant darker types of red onions for higher anthocyanin content. Short-day-length onions grow best in the southern United States, but long-day-length onions grow better in northern climates. Ask your extension agent for advice on the best choices for your region.