Updated December 16, 2014.
Onions are prized in cooking all over the world for the flavors they impart to almost any dish. Did you know that the reason they irritate the eyes is due to a mild sulfuric acid which is produced and released when chopping them? Some remedies to try (in declining order of effectiveness, but all have been shown to be of some help): wearing goggles, chilling the onion before slicing, breathing through your mouth when slicing, and lighting candles around the cutting board.

Carbohydrate and Fiber Counts for Onions

• ½ cup chopped raw onion: 6 grams effective (net) carbohydrate plus 1 gram fiber and 32 calories

• 1 medium onion (2.5" in diameter; about 4 oz.): 8 grams effective (net) carbohydrate plus 2 grams fiber and 44 calories
• 1 slice raw onion, 1/8 inch thick: 1 gram effective (net) carbohydrate and 6 calories

Looking for a lower-carb substitute for onions? Try green onions (scallions, spring onions).

Glycemic Index for Onions

As with most non-starchy vegetables, there is no scientific study of the glycemic index of onions However, some of the carbohydrate in onions is in the form of oligosaccharides, which would tend to lower the glycemic effect.
More Information about the Glycemic Index

Estimated Glycemic Load of Onions

• ½ cup raw chopped onions: 2
• 1 medium onion (2.5" in diameter; about 4 oz.): 4
• 1 slice raw onion, 1/8 inch thick: 0
  

More Information about the Glycemic Load

Health Benefits of Onions

Onions are a good source vitamin C and a very good source of chromium. They are also high in quercetin, and as mentioned above, oligosaccharides.
Onions have been thought to have many different types of health benefits.

There are hints in animal and some human studies that they are heart-healthy and may have anti-diabetic properties, as well as promoting anti-inflammatory and anti-bacterial activity in the body. The evidence is stronger that onions have an anti-cancer effect, particularly in regards to gastrointestinal cancers, including colon and stomach cancers.

Low-Carb Recipes with Onions

I would venture to say that most of my low-carb main dishes have at least a little onion in them. Here's a sampling of some of my recipes which include onion:

• Sugar-Free Cranberry Chutney
• Green Bean Casserole with Onion Topping
• Italian Squash Casserole
• Creamy Spicy Pumpkin Soup
• Denver Casserole
• Crockpot Ropa Vieja
• Crockpot Hungarian Goulash
• Chicken or Beef Fajitas
• Easy Thai Coconut Curry Chicken
• Cincinnati Chili
• Low-Carb Pasta with Chicken and Roasted Red Pepper Sauce
• Turkey or Chicken Tetrazinni

More Information About Onions at Calorie Count Plus.
More Carb Profiles:

• Fruits
  
  
• Vegetables
  
  
• Nuts and Seeds
  
  
• Grains and Legumes
  
  
• Milk, Cream, Soy Milk, etc.

<sub>Sources:

Galeone C, Pelucchi C, Levi F, Negri E, Franceschi S, Talamini R, Giacosa A, La Vecchia C. Onion and garlic use and human cancer. Am J Clin Nutr. 2006 Nov;84(5):1027-32.

Leroux, MarcusFoster-Powell, Kaye, Holt, Susanna and Brand-Miller, Janette. "International table of glycemic index and glycemic load values: 2002." American Journal of Clinical Nutrition. Vol. 76, No. 1, 5-56, (2002).

USDA National Nutrient Database for Standard Reference, Release 20.</sub>