How do I get Enough Fiber on a Low FODMAP Diet?
As a dietitian, I frequently get questions about the low FODMAP diet. Can this be a beneficial diet? Absolutely! Is it beneficial for everyone long-term? Absolutely not. One reason being is that it is a diet low in fiber, specifically fermentable fiber.
If you're not familiar, FODMAPs are Fermentable Oligosaccharides, Disaccharides (lactose), Monosaccharides (fructose), and Polyols (sugar alcohols). These fibers act as "food" and are fermented by the bacteria in your large intestine. This fermentation process produces our natural gas and other beneficial compounds such as short-chain fatty acids (SCFAs) like butyrate.
Why Start a Low FODMAP Diet?
Most people start this diet because they've been dealing with ongoing digestive symptoms, like bloating, gas, stomach cramps, constipation, or diarrhea, that don't have a clear cause. It is also commonly recommended for people who have small intestinal bacterial overgrowth (SIBO) to help reduce the fermentation happening in the small intestine. This is one of the reasons why SIBO can be so uncomfortable, due to the gas produced in the small intestine rather than the large intestine which it should be.
It's worth noting this diet isn't meant for weight loss or general "healthy eating." It's a short-term diagnostic tool, best done with guidance from a doctor or dietitian, especially since it involves cutting out major food groups and beneficial nutrients for a period of time.
THE CAVEAT: If you have been on a low FODMAP diet for longer than necessary, you can essentially be cutting out the major food sources for your beneficial bacteria. These beneficial bacteria help with nutrient recycling, immune system function, neurotransmitter production, and so much more. So, it may seem like you are relieving symptoms, but this can also make your GI function (and symptoms) worse than in the long run. I have seen too often people who are on this diet for too long struggle with more food sensitivities and issues than they began within the first place.
How Long Should You Be on It?
The low FODMAP diet has three phases, and it's not meant to be followed forever:
Elimination phase (2 to 6 weeks): You remove high FODMAP foods to see if your symptoms improve. Most people know within a few weeks whether it's working.
Reintroduction phase (several weeks to a few months): You add FODMAP groups back one at a time to identify which ones actually trigger your symptoms and in what amounts.
Personalization phase (ongoing): You build a long-term diet based on what you learned, avoiding only the specific foods that bother you, rather than staying strictly low FODMAP indefinitely.
The problem that I see far too often, is that people stay in the elimination phase for too long. This can (and will) reduce the diversity of good bacteria in your gut and make it harder to get enough fiber and other nutrients.
Foods to Choose
If you want to keep your fiber intake up, here are some of the main foods to focus on. These options that are typically well-tolerated in appropriate portions. Serving sizes matter for FODMAP tolerance, so it's worth checking the Monash FODMAP app for the most current, specific portions.
Fruits
Kiwi, 1 medium = 3-4g
Orange, 1 medium = 3g
Strawberries, 1 cup = 3-4g
Raspberries, 1 cup = 8g
Banana, 1 firm/unripe medium = 3g
Vegetables
Carrot, 1 medium = 2g
Eggplant, ¾ cup = 2g
Zucchini, 1 medium = 2g
Spinach (raw), 1 cup = 1-2g
Potato with skin, 150g = 3g
Green beans, ¾ cup = 2g
Grains and Starches
Oats, ½ cup dry = 4g
Quinoa (cooked), 1 cup = 5g
Brown rice (cooked), 1 cup = 4g
Sourdough spelt bread, 1 slice = 2–3g
Nuts and Seeds
Chia seeds, 1 tablespoon = 4g
Ground flaxseed, 1 tablespoon = 3g
Walnuts, small handful (~30g) = 2g
Pumpkin seeds, small handful (~15g) = 2g
A Sample Low FODMAP Day
Here's what a full day could look like, spreading foods out across meals to avoid FODMAP "stacking." Fiber estimates are approximate.
Breakfast | Overnight oats: ½ cup dry oats + 1 tablespoon chia seeds + 1 tablespoon ground flaxseed + 5 medium strawberries, made with almond milk) | Fiber: ~12g |
Mid-Morning Snack | 1 firm/unripe medium banana + a small handful (30g) of walnuts | Fiber: ~5g |
Lunch | Quinoa bowl: 1 cup cooked quinoa + 1 cup raw spinach + 1 shredded medium carrot + grilled chicken | Fiber: ~8g |
Afternoon Snack | 1 medium kiwi + a small handful (15g) of pumpkin seeds | Fiber: ~4g |
Dinner | ½ medium baked potato (skin on) + ¾ cup green beans + ¾ cup roasted eggplant + baked salmon or chicken Fiber: about 5.75g | Fiber: ~6g |
Estimated daily total: ~ 34g of fiber
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