Benefits of an Onion a Day for Weight Loss

The old adage about an apple a day may apply to the onion too. Both pieces of produce are rich in fiber, nutrients and an antioxidant called quercetin, all of which may contribute to weight loss. Onions also are easy to add to a variety of dishes, bringing the benefit of low-calorie flavoring.

Onions are a good source of the antioxidant quercetin. Quercetin may help increase energy expenditure, or calorie burn, in your body while also reducing inflammation, says Laura K. Stewart, lead author for a 2008 study published in the journal "Metabolism." However, more rigorous research is needed before firm conclusions can be drawn. One problem with many quercetin studies is imprecise information about the amount of compound actually provided, which leads to uncertainty about the effective dose of quercetin needed to gain results, Stewart notes. In addition, your body may adapt to the antioxidant's early effect on energy expenditure, thus reducing its benefit over time, Stewart says. This antioxidant also is found in apples, tea, citrus fruits and red wine, according to University of Maryland Medical Center.

Cooking with onions is a good way to add flavor to your foods without adding lots of extra calories, as fats and oils tend to do, recommends Fred A. Stutman, author of "100 Weight-Loss Tips That Really Work." Combining onion with garlic is an especially tasty way to achieve this. Increasing the volume of a dish with low-calorie foods, such as by adding onions, also helps you to feel fuller on fewer calories, according to the Mayo Clinic. A medium onion has 44 calories, according to the U.S. Department of Agriculture.

Adding onions to a dish can help you gain more fiber, according to the Reader's Digest book, "Fight Back With Food." Fiber is the part of a plant food that you cannot digest. Eating fiber helps you stay full longer, which helps control your weight. It also helps control your blood sugar levels, which may help reduce your risk for type 2 diabetes, according to the Mayo Clinic. A medium onion has 1.9 g fiber, according to the USDA. The quercetin in onions also may help increase your "good" high-density lipoprotein cholesterol levels, which boosts overall health, says Irish nutritionist Anne Collins.