General information with sources. Never a substitute for your doctor.

MethodologyAbout the authorESEN
SaciedadSearch

Vegetables and starchy roots

Onion

Onions are 89% water, so 100 g (3.5 oz) brings only 40 kcal and 1.7 g of fiber.

USDA description: Onion, raw

  • Low energy density
  • Hydrating
  • May cause bloating

Estimated satietyHow it is calculated

Starting point+1
Very low energy density+2
A lot of fiber per calorie+2
High water content+1
Sum6
Capped at 55

Our own estimate from the values per 100 g: useful for comparing foods with each other, not a measurement in people. Holt index (1995): this food was not measured.

Prepared by
Last updated
Next review
Sources
7 primary
Data
USDA FDC 170000

No clinical review. No advertising from drug companies and no affiliate links.How we work

Nutrition facts

Calories
20 kcal per serving
40 kcal per 100 g (3.5 oz)
Protein
0.6 g per serving
1.1 g per 100 g (3.5 oz)
Carbohydrates
4.7 g per serving
9.3 g per 100 g (3.5 oz)
of which sugars
2.1 g per serving
4.2 g per 100 g (3.5 oz)
Fiber
0.9 g per serving
1.7 g per 100 g (3.5 oz)
Fat
0.1 g per serving
0.1 g per 100 g (3.5 oz)
of which saturated
0 g per serving
0 g per 100 g (3.5 oz)
Water
44.6 g per serving
89.1 g per 100 g (3.5 oz)

Source: USDA FoodData Central: Onions, raw (FDC ID 170000), accessed September 23, 2026.

A typical 50 g (2 oz, about 1/4 cup) serving provides about 20 kcal, 0.6 g of protein, and 0.9 g of fiber.

The typical serving (50 g (2 oz, about 1/4 cup)) is a common amount used for the math, not a recommendation. Ounces and cups are rounded US equivalents; the values come from the database entry cited in the sources.

What it is and how to eat it

Onions (Allium cepa) are sold dry, in white, yellow and red varieties, and fresh with green stalks, as cebolleta in Spain, cebollín in Mexico and Chile, cebolla de verdeo in Argentina and cebolla larga in Colombia. They are almost always used in small amounts to flavor sofritos, stews, sauces and salads, raw or cooked. This page covers raw onion, with 40 kcal, 1.7 g of fiber and 89.1 g of water per 100 g according to USDA.

A 50 g (1.8 oz) serving provides 20 kcal, 0.9 g of fiber and 0.6 g of protein, and this site's single-serving recipes use less, such as the 15 g in the scrambled eggs with beans and tomato or the 20 g in the small arepa with perico scrambled eggs. The US National Institutes of Health list onions among the high-FODMAP foods, hard-to-digest carbohydrates, and gas and bloating are common side effects of semaglutide (Wegovy) in its 68-week trials, according to its label. If you notice it does not sit well, try using less, and remember that the 2025 nutrition advisory on GLP-1 therapy suggests starting meals with the protein, not the side dish.

What the evidence says

The NIDDK, part of the US National Institutes of Health, lists onions, along with garlic, among the high-FODMAP vegetables that are limited on the low FODMAP diet used for irritable bowel syndrome. By contrast, its list of foods commonly linked to reflux symptoms names citrus fruits, tomatoes, chocolate, coffee, mint, alcohol, high-fat foods and spicy foods, but not onions, although the same institute notes that some people with reflux find that certain foods or drinks make their symptoms worse. With 7.4 mg of vitamin C per 100 g it falls short of the 12 mg the EU requires for a "source of vitamin C" claim (Regulations (EC) No 1924/2006 and (EU) No 1169/2011), and I have not found trials showing that onion itself helps with weight loss.

The sources for this section are listed under "Sources" at the end of the page.

Recipes with this food

See all recipes

Satiety

The 1 to 5 score is our own estimate: it starts at 1 point, adds points for low energy density, for protein and fiber per 100 kcal, and for the water in solid foods, and subtracts points for drinks with calories. It is useful for comparing foods with each other; it is not a measurement in people.

What to keep in mind on a GLP-1 medication

Low energy density
It has 100 kcal or less per 100 g: plenty of volume for few calories, which helps you feel full. On a GLP-1 medication, which already reduces your appetite, make sure these foods do not push protein foods off your plate.
Hydrating
It has 85 g of water or more per 100 g, so it contributes to hydration. That helps because with a smaller appetite people also tend to eat and drink less, and not drinking enough can make constipation worse.
May cause bloating
Beans and other legumes, cruciferous vegetables, onions, and carbonated drinks cause gas in some people. Because GLP-1 medications slow down stomach emptying, that bloated feeling can be more noticeable. This is an editorial judgment: many people tolerate these foods well in moderate servings.

Frequently asked questions about onions

How many grams of protein are in a 50 g (2 oz) serving of onions?

100 g (3.5 oz) of onions has 1.1 g of protein. A typical 50 g (2 oz) serving has about 0.6 g of protein. That is 2.8 g of protein per 100 kcal.

How many kcal are in a 50 g (2 oz) serving of onions?

100 g (3.5 oz) of onions has 40 kcal. A typical 50 g (2 oz) serving has about 20 kcal. That is a very low energy density.

How much fiber is in a 50 g (2 oz) serving of onions?

100 g (3.5 oz) of onions has 1.7 g of fiber. A typical 50 g (2 oz) serving has about 0.9 g of fiber.

Do onions keep you full?

Yes, very filling for the calories they provide: our estimated satiety score is 5 out of 5. It is based on: very low energy density; a lot of fiber per calorie; high water content. It is an estimate based on its composition, not a measurement in people.

How do onions fit into eating on Ozempic, Wegovy, Mounjaro, or Zepbound?

Based on our tags, this is what matters about onions on a GLP-1 medication. “Low energy density”: plenty of volume for few calories; make sure it does not push protein foods off your plate. “Hydrating”: a lot of water, which contributes to hydration when you eat and drink less. “May cause bloating”: it can cause gas or bloating in some people, and on a GLP-1 medication that feeling can be more noticeable. These are general guidelines, not medical advice: everyone tolerates foods differently.

What is a typical serving of onions?

We use 50 g (2 oz) as a typical serving, about 1/4 cup. That amount provides about 20 kcal, 0.6 g of protein, and 0.9 g of fiber. It is a reference for calculations, not a recommendation of how much to eat.

See all foods

Sources

Sources for the data on this page, with the access date.

  1. Official databaseU.S. Department of Agriculture, Agricultural Research Service.USDA FoodData Central: Onions, raw (FDC ID 170000).accessed .
  2. InstitutionNational Institute of Diabetes and Digestive and Kidney Diseases (NIH).Eating, Diet, & Nutrition for Irritable Bowel Syndrome (low FODMAP diet).accessed .
  3. Drug labelEuropean Medicines Agency.Wegovy (semaglutide): EU Summary of Product Characteristics, section 4.8 (undesirable effects), current version.accessed .
  4. Clinical guidelineObesity (Wiley).Mozaffarian D, et al. Nutritional priorities to support GLP-1 therapy for obesity: A joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. Obesity. 2025;33(8):1475-1503. doi:10.1002/oby.24336.accessed .
  5. InstitutionNational Institute of Diabetes and Digestive and Kidney Diseases (NIH).Eating, Diet, & Nutrition for GER & GERD.accessed .
  6. RegulatorOfficial Journal of the European Union (EUR-Lex).Regulation (EC) No 1924/2006 on nutrition and health claims made on foods (Annex).accessed .
  7. RegulatorOfficial Journal of the European Union (EUR-Lex).Regulation (EU) No 1169/2011 on the provision of food information to consumers (Annex XIII, nutrient reference values).accessed .