Understanding FODMAPs and Onions
FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols. These are types of carbohydrates that can be poorly absorbed in the small intestine. For individuals with Irritable Bowel Syndrome (IBS), these carbohydrates can ferment in the large intestine, leading to symptoms such as bloating, gas, abdominal pain, and altered bowel habits.
Onions, along with garlic and other members of the Allium family, are a common source of FODMAPs, specifically fructans. Fructans are chains of fructose molecules that the human body cannot break down. Therefore, when consumed, they pass undigested into the large intestine where gut bacteria rapidly ferment them, producing gas.
The Effect of Cooking on Onion's FODMAP Content
A common misconception is that cooking high-FODMAP foods like onions will destroy or remove their fructan content. While some types of processing can alter FODMAP levels, cooking typically does not. The critical factor is whether the fructans have a medium to leach into during cooking.
Water-based cooking vs. Oil-based cooking
Fructans are water-soluble but not oil-soluble. This fundamental chemical property dictates how you can and cannot use onions for flavor on a low FODMAP diet.
- Water-based cooking: When you cook onions in a liquid-based dish, such as a soup, stew, or sauce, the fructans will leach out of the onion and into the water. Even if you remove the onion pieces afterward, the dish will remain high in FODMAPs. This is why the strategy of cooking with whole onions and then removing them is ineffective for those with fructan sensitivity. The flavor comes with the FODMAPs.
- Oil-based cooking: In contrast, when you cook onions in oil, the fructans do not dissolve. The onion's flavor compounds infuse the oil, leaving the fructans trapped within the onion solids. The key is to remove the onion pieces entirely before adding any other ingredients to the oil. This method allows you to capture the savory flavor of onions without the problematic carbohydrates.
Safe Portions of White Onion for Low FODMAP
According to the Monash University FODMAP Diet App, which is the leading authority on FODMAP testing, raw white onion does have a small, low-FODMAP serving size.
- Raw White Onion: A low-FODMAP serving is approximately $10g$. For most people, this is a very small amount, such as one thin ring, and should be treated with caution, especially during the strict elimination phase of the diet. As the portion size increases, so does the fructan content.
Because cooking does not change the fructan content of the solid onion itself, the safe raw portion size can serve as a rough guide. However, due to the water-soluble nature of fructans, consuming any cooked onion pieces in a mixed, liquid-based dish carries a high risk of triggering symptoms. For this reason, many dietitians recommend avoiding cooked onion pieces entirely during the elimination phase of the low FODMAP diet.
Alternatives to White Onion for Flavor
For those who miss the depth of flavor that onions provide, several low-FODMAP alternatives can be used:
- The green tops of spring onions (scallions): The green, leafy part of a spring onion is low in FODMAPs, while the white bulb is not. The green tops can be chopped and added raw or cooked to dishes for a mild, onion-like flavor.
- Chives: Fresh or dried chives offer a delicate, oniony taste and have no detectable FODMAPs, according to Monash University testing.
- Leek greens: Similar to spring onions, the dark green, leafy portion of leeks is low in FODMAPs.
- Onion-infused oil: As previously mentioned, this is a safe and effective way to get authentic onion flavor. Commercially produced, certified low-FODMAP infused oils are available, or you can make your own by heating oil with onion chunks and then removing the solids before consumption.
- Asafoetida powder: Also known as 'hing,' a tiny pinch of this pungent spice mimics the taste of onion and garlic once cooked. Some brands contain wheat, so check the label for gluten-free options.
Comparison: Onions on a Low FODMAP Diet
| Feature | High-FODMAP Onion Use | Low-FODMAP Onion Use | Safe Low-FODMAP Alternatives |
|---|---|---|---|
| Type | White, yellow, red, shallots | Small portion of raw white onion ($10g$) | Green parts of spring onions, chives, leek greens |
| Cooking Method | Cooking in water-based dishes (soups, stews, sauces) | Infusing oil with onion flavor (removing solids) | Any cooking method |
| FODMAP Content | Fructans leach into the dish, making it high FODMAP | Fructans stay trapped in the solid onion, keeping the dish low FODMAP | Negligible or low FODMAP content |
| Risk of Symptoms | High risk, especially during elimination | Very low risk if oil infusion method is correctly followed | Minimal to no risk |
Practical Tips for Cooking with Onions on a Low FODMAP Diet
- Use Infused Oil Correctly: When making your own onion-infused oil, heat the oil with large pieces of onion for a few minutes. Ensure the oil does not get too hot, and remove all onion solids before continuing with your recipe. It is safest to store this oil in the refrigerator and use it within a few days.
- Go Green with Alliums: Rely on the green parts of spring onions and leeks for a fresh onion flavor. These are excellent garnishes or can be added to dishes at any stage of cooking.
- Choose Dried Alternatives: Dried chives are a pantry staple that provide a mild onion flavor. Consider using asafoetida powder, a potent spice that can replace onion and garlic flavor in cooked dishes. Start with a very small amount.
- Consult a Dietitian: The low FODMAP diet should be followed under the guidance of a registered dietitian. They can help you navigate the stages, identify personal triggers, and ensure your diet is nutritionally complete.
- Utilize the Monash App: The official Monash University FODMAP Diet App provides the most up-to-date information on tested foods and safe serving sizes, essential for navigating the diet effectively.
Conclusion
To answer the question, is cooked white onion low FODMAP? The verdict is largely no, but with important exceptions. Cooking a white onion in a water-based liquid will not make it low FODMAP, as the fructans will spread throughout the dish. However, a very small serving of raw white onion (around $10g$) is considered low FODMAP by Monash University. The most reliable and flavorful way to incorporate onion taste into your cooking on a low FODMAP diet is by making or purchasing onion-infused oil and using low-FODMAP alternatives like the green parts of spring onions or chives. By understanding the properties of FODMAPs and how they react to different cooking methods, you can confidently add flavor to your food without triggering digestive symptoms.
