Is Onion Low FODMAP? Understanding Onions and FODMAP Sensitivity
Onions are one of the highest FODMAP foods due to their fructan content. Learn which parts of onions are safest, alternatives, and how to cook without onions.
No, onion is not low FODMAP. Onions are among the highest FODMAP foods and are frequently cited as the number one dietary trigger for people with IBS. All common varieties of onion — white, brown, red, and yellow — contain high levels of fructans, a type of fermentable carbohydrate that can cause bloating, gas, cramping, and diarrhea in sensitive individuals. Even small amounts of cooked or raw onion can provoke symptoms.
Like garlic, onion is a staple ingredient in most cuisines, which makes avoiding it a significant challenge. However, understanding which parts of the onion family are safe, and knowing the right substitutes, makes low-FODMAP cooking entirely manageable.
Why Are Onions High in FODMAPs?
Onions are loaded with fructans, which are part of the oligosaccharides group in the FODMAP classification. Fructans are chains of fructose molecules that the human body cannot digest. They pass through the small intestine intact and arrive in the large intestine, where gut bacteria ferment them rapidly, producing gas.
For people with IBS, this fermentation process triggers a cascade of symptoms. The gas causes intestinal distension, which activates the hypersensitive nerve endings in the gut that are characteristic of IBS. This is why even a small amount of onion in a dish can lead to hours of discomfort.
FODMAP Breakdown: Onion Varieties
| Onion Type | Serving Size | FODMAP Level | FODMAP Type |
|---|---|---|---|
| White onion | 1/4 cup (30g) | High | Fructans (Alliums) |
| Brown/yellow onion | 1/4 cup (30g) | High | Fructans (Alliums) |
| Red onion | 1/4 cup (30g) | High | Fructans (Alliums) |
| Shallots | 1 shallot (15g) | High | Fructans (Alliums) |
| Spring onion (white part) | 1 bulb (8g) | Moderate-High | Fructans (Alliums) |
| Spring onion (green part) | 1 cup (20g) | Low | Minimal fructans |
| Chives | 1 tablespoon | Low | Minimal fructans |
The key insight from this table is that the green parts of spring onions and chives stand out as safe options. The fructans in allium vegetables are concentrated in the bulb — the white, fleshy base that grows underground. The further you move from the bulb toward the green leaves, the lower the fructan content.
Why Is Onion the Number One IBS Trigger?
Research and clinical experience consistently identify onion as one of the most common FODMAP triggers. A few factors explain why.
Onion is everywhere. It forms the flavor base of most savory cooking. French mirepoix, Italian soffritto, Indian tadka, and countless other foundational recipes start with onion. It is present in soups, stews, sauces, gravies, curries, casseroles, stir-fries, salads, and most restaurant dishes.
Onion has a very low trigger threshold. According to Monash University FODMAP testing, onion registers as high FODMAP even at modest serving sizes. There is no established “safe” portion of regular onion during the elimination phase.
Onion is difficult to detect. It dissolves into sauces and soups, making it virtually invisible. It appears in ingredient lists under various names, including “dehydrated onion,” “onion powder,” “onion extract,” and simply “seasoning” or “natural flavors.”
What Are the Best Onion Alternatives?
Replacing onion in cooking is one of the most important skills for anyone following a low-FODMAP diet. Here are the most effective substitutes.
Spring Onion Greens (Scallion Greens)
The green tops of spring onions are your best friend. They provide a fresh, mild onion flavor and are low FODMAP at servings up to 1 cup. Slice them thinly and use them:
- As a garnish on soups, stir-fries, and rice dishes
- Sauteed lightly as a base flavor in place of onion
- Mixed into salads, omelets, and dips
- Added to sauces and dressings
Important: Cut cleanly where the color changes from white to green. Discard the white bulb portion.
Chives
Chives offer a subtle onion flavor and are safe at 1 tablespoon. They work well as a finishing herb on potatoes, eggs, soups, and cream-based dishes.
Asafoetida (Hing)
This pungent Indian spice mimics the savory depth of onion and garlic combined. Use it sparingly — a tiny pinch (1/16 to 1/8 teaspoon) goes a long way. It is particularly effective in curries, dals, and rice dishes. Make sure to buy asafoetida that is not blended with wheat flour if you are also avoiding gluten.
Onion-Infused Oil
Similar to garlic-infused oil, you can gently cook onion pieces in oil, then strain them out. The fat-soluble flavor compounds transfer to the oil while the water-soluble fructans remain in the onion. This technique gives you the savory base note that onion provides without the FODMAP content.
Other Flavor Builders
- Celery: Adds aromatic depth (low FODMAP at 1/4 stalk)
- Fennel bulb: Low FODMAP at 1/2 cup; provides a sweet, savory flavor
- Leek leaves (green part only): The dark green tops of leeks are low FODMAP
- Lemongrass: Adds complexity to Asian-style dishes
Does Cooking Onion Reduce FODMAPs?
No. This is one of the most persistent myths in FODMAP management. Fructans are thermally stable carbohydrates. Boiling, frying, roasting, grilling, or caramelizing onion does not break down or remove fructans in any meaningful way.
Caramelized onions taste sweeter because the natural sugars undergo the Maillard reaction, but the fructan content remains the same. Similarly, French onion soup, despite long cooking, is very high in FODMAPs.
The only cooking technique that separates fructans from flavor is oil infusion, where the onion is cooked in fat and then physically removed before eating.
How to Spot Hidden Onion in Foods
Onion hides in an enormous number of prepared and processed foods. Here is where to look:
- Sauces and condiments: Tomato sauce, barbecue sauce, salsa, chutney, gravy, and most salad dressings
- Soups and stocks: Almost all commercial stocks and bouillon cubes contain onion
- Spice blends: Taco seasoning, Italian seasoning, Cajun seasoning, curry powder, and many others
- Processed meats: Sausages, burgers, meatballs, and deli meats often include onion powder
- Snack foods: Many chips, crackers, and flavored nuts contain onion powder
- Restaurant food: Assume onion is present unless you specifically ask
Tools like FODMAPSnap can help you scan meals and identify hidden onion and other FODMAP triggers, making it easier to stay on track whether you are cooking at home or eating out.
Reintroducing Onion After Elimination
When you reach the reintroduction phase, onion falls under the fructans — alliums challenge. A typical protocol:
- Day 1: 1/8 of a medium onion (about 10g), cooked
- Day 2: 1/4 of a medium onion (about 20g), cooked
- Day 3: 1/2 of a medium onion (about 40g), cooked
Wait 24-48 hours between each challenge day and monitor symptoms carefully. Many people discover they can tolerate very small amounts of onion but react at moderate portions. Knowing your personal threshold is valuable — it means you may not need to avoid onion completely in the long-term personalization phase.
Tracking your reintroduction challenges with FODMAPSnap can help you build a clear picture of your fructan tolerance and make confident food choices going forward.
Onion Sensitivity and SIBO
Onion is particularly problematic for people with small intestinal bacterial overgrowth (SIBO). The fructans in onion can feed the overgrown bacteria in the small intestine, amplifying symptoms like bloating, pain, and excessive gas. If you have SIBO, your healthcare provider may recommend a more extended onion-free period.
Key Takeaways
- All common onions (white, brown, red, yellow) are high FODMAP
- The green parts of spring onions are low FODMAP and the best direct substitute
- Cooking onion does not reduce its fructan content
- Onion-infused oil captures flavor without fructans
- Onion powder and dried onion are high FODMAP and should be avoided
- Shallots are high FODMAP, similar to regular onion
- Chives and asafoetida are excellent alternative seasonings
Related Reading
- Is Garlic Low FODMAP? — The other major allium trigger
- Is Rice Low FODMAP? — A safe base for onion-free meals
- Bananas and FODMAPs — Understanding ripeness and FODMAP levels
This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional or registered dietitian before making changes to your diet. The low-FODMAP diet should ideally be followed under the guidance of a FODMAP-trained dietitian to ensure nutritional adequacy and proper reintroduction. Individual tolerance varies significantly.
Track Your Personal FODMAP Triggers
Everyone's gut is different. FODMAPSnap uses AI to analyze your meals for FODMAP content and learns your unique sensitivities over time — so you can eat with confidence.
Frequently Asked Questions
Are spring onion greens low FODMAP?
Yes, the green parts of spring onions (scallions) are low FODMAP at up to 1 cup (20g). Only the white bulb portion contains concentrated fructans. Cut the spring onion where the color transitions from white to green, and use only the green tops. This is one of the best ways to add a mild onion-like flavor to low-FODMAP dishes.
Is cooked onion lower in FODMAPs than raw onion?
No, cooking onion does not reduce its FODMAP content. Fructans are heat-stable carbohydrates that remain intact regardless of cooking method. Whether onion is raw, sauteed, caramelized, or boiled, the fructan content stays essentially the same. The only exception is onion cooked in oil and then removed — the oil captures flavor but not fructans.
Can I use onion powder on a low-FODMAP diet?
No, onion powder is high FODMAP and should be avoided. It is made from dehydrated onion and contains concentrated fructans. Even small amounts can trigger symptoms. Check spice blend labels carefully, as onion powder is one of the most common hidden ingredients in seasonings, sauces, and processed foods.
What about shallots — are they low FODMAP?
Shallots are high FODMAP. They are closely related to onions and contain similar levels of fructans. Even small amounts of shallots can trigger symptoms in people sensitive to fructans. Use the same alternatives you would for onion, such as the green parts of spring onions, chives, or asafoetida.