Understanding Why Am I So Tired on a Low FODMAP Diet?

November 11, 2025 •

4 min read

According to a 2019 study published in Gastroenterology, the low FODMAP diet is associated with a significant reduction in fatigue for many with Irritable Bowel Syndrome (IBS) over the long term, yet many people initially find themselves asking, 'Why am I so tired on a low FODMAP diet?'. The answer lies in several potential nutritional and lifestyle factors that can be managed effectively.

Quick Summary

Initial fatigue on a low FODMAP diet often results from inadequate calorie intake, nutrient deficiencies, or eliminating caffeine. Addressing energy gaps, ensuring sufficient nutrient intake from safe sources, and managing the overall lifestyle can resolve persistent tiredness.

Key Points

Check Calorie Intake: Ensure you are consuming enough overall energy by replacing high-FODMAP foods with appropriate low-FODMAP alternatives like rice, potatoes, and proteins.
Evaluate Caffeine Consumption: Be aware that eliminating or reducing caffeine can cause temporary withdrawal symptoms, including fatigue and headaches, as your body adjusts.
Mind Nutritional Gaps: Be vigilant for potential deficiencies in key nutrients like iron, B vitamins, and calcium, and seek alternative sources or supplementation if necessary.
Support Your Gut Health: Recognize that temporary changes in your gut microbiome can occur during the elimination phase, which might affect energy and mood initially.
Rule Out Other Causes: Consider non-dietary factors such as poor sleep, chronic stress, or other health issues that could contribute to your fatigue.
Work with a Professional: The most effective way to navigate the diet and prevent nutritional imbalances is under the guidance of a registered dietitian.
Focus on the Phased Approach: The elimination phase is not meant to be permanent. The goal is to reintroduce foods to tolerance, allowing for a more varied and less restrictive diet.

Common Culprits Behind Low-FODMAP Fatigue

The low FODMAP diet is a highly effective tool for managing symptoms associated with Irritable Bowel Syndrome (IBS) by eliminating fermentable carbohydrates. However, the restrictive nature of the initial elimination phase can lead to unintended side effects, including fatigue. Understanding the root cause is the first step toward regaining your energy. Some common reasons for feeling tired include:

Inadequate Calorie Intake

When transitioning to a low FODMAP diet, people often cut out many staple foods like bread, certain fruits, and legumes without replacing the lost calories. This unintentional reduction in overall energy intake can lead to lethargy and fatigue as the body receives less fuel than it requires to function optimally. Proper meal planning and ensuring adequate portions of low-FODMAP carbohydrates, proteins, and fats are essential to counteract this effect.

Caffeine Withdrawal

For many, high-FODMAP-containing foods and beverages are not the only things eliminated during the diet. Many individuals also eliminate or significantly reduce their intake of caffeinated products, such as coffee, tea, and soda, as they can sometimes be gut irritants. For regular caffeine consumers, this sudden reduction can trigger withdrawal symptoms like headaches, fatigue, and brain fog as the body adjusts. The fatigue is often temporary but can be a major contributor to feeling run down during the early stages of the diet.

Nutritional Deficiencies

The removal of several key food groups on a low FODMAP diet can increase the risk of micronutrient deficiencies if not managed carefully. High-FODMAP foods like wheat, certain dairy products, and legumes are rich in important nutrients. Missing these can impact energy levels. Key nutrients to monitor include:

Iron: Many grains and legumes are high-FODMAP and are also important sources of iron. Low iron levels can cause fatigue and lethargy.
B Vitamins: Fortified grains and certain dairy products are sources of B vitamins, crucial for energy production. Removing them can cause a dip in your intake.
Calcium: Eliminating dairy is a common step, which can reduce calcium intake, affecting muscle function and energy.
Fiber: Many high-FODMAP foods are fibrous. Reduced fiber intake can lead to sluggish digestion and constipation, which is physically draining.

Gut Microbiome Adjustments

FODMAPs serve as fuel for beneficial gut bacteria. When these are removed, the gut microbiome undergoes changes. While this is the goal for reducing IBS symptoms, the initial disruption to the bacterial balance can have temporary effects on mood and energy, as the gut-brain axis is highly sensitive to changes. For most, this balances out and improves overall well-being, but initial adjustment periods can be rough.

The Importance of a Balanced Approach

To combat fatigue, a low FODMAP diet should not be about elimination alone, but about strategic substitution. It's a temporary tool designed for diagnosis, not a permanent restrictive lifestyle. Working with a dietitian trained in FODMAPs is highly recommended to ensure you don't inadvertently create nutritional imbalances.

To maintain energy, focus on these low-FODMAP substitutions:

Instead of wheat-based bread, choose spelt sourdough or gluten-free bread options.
For calcium, swap regular milk for lactose-free milk or fortified plant-based milk made from soy protein.
Increase low-FODMAP protein sources, like plain meat, fish, eggs, firm tofu, and smaller portions of nuts and seeds.
Ensure adequate low-FODMAP carbohydrate intake from sources like potatoes, rice, quinoa, and oats.
Boost healthy fats with olive oil, small amounts of nuts and seeds, and small servings of avocado.
Fortify meals by adding extra low-FODMAP energy and protein. For example, add lactose-free yogurt to fruit salad or grated cheese to soup.

Comparing Fatigue Causes: FODMAP vs. Non-FODMAP Factors

Sometimes, fatigue on a low FODMAP diet isn't caused by the diet itself but by other underlying issues. It's crucial to differentiate between diet-related fatigue and other factors. A detailed comparison can help clarify the situation and guide next steps.


Cause of Fatigue	Diet-Related (Low FODMAP)	Other Potential Factors
Energy Intake	Unintentional calorie reduction due to fear of eating or lack of suitable alternatives.	Chronic stress, poor sleep hygiene, over-exercising, or other restrictive diets.
Caffeine	Withdrawal symptoms from reducing or eliminating coffee and tea.	Unchanged caffeine habits can still disrupt sleep and energy levels.
Nutrients	Deficiency in iron, B vitamins, calcium, or fiber due to cutting out fortified grains and dairy.	Malabsorption due to underlying, undiagnosed conditions like celiac disease or Small Intestinal Bacterial Overgrowth (SIBO).
Gut Health	Temporary disruption of the gut microbiome during the initial elimination phase.	Chronic inflammation or imbalance unrelated to FODMAPs.
Psychological	Anxiety around dietary restrictions or cooking new meals.	General life stress, anxiety, or depression that can exist independently of diet.

Conclusion

While experiencing fatigue on a low FODMAP diet can be frustrating, it's often a temporary side effect that can be managed by making mindful adjustments. The key is to ensure you are consuming enough calories and nutrients from safe, low-FODMAP sources to prevent energy dips. Addressing potential caffeine withdrawal and recognizing the temporary impact on your gut microbiome is also crucial. If persistent fatigue continues, it is important to consider that the cause may lie outside of your dietary plan. Consulting with a healthcare professional or a registered dietitian is the most effective way to navigate these challenges, ensure nutritional adequacy, and create a sustainable, personalized diet plan that keeps your energy high while managing your digestive symptoms. For many, the long-term benefit of reduced IBS symptoms outweighs the initial adjustment period, leading to improved quality of life and reduced fatigue overall.

For more detailed guidance on a low FODMAP diet, resources like the Monash University FODMAP blog can provide additional information and recipes.

Frequently Asked Questions

Yes, it is common to experience some fatigue or tiredness during the initial elimination phase. This can be due to a decrease in overall calorie intake, the body's adjustment to changes in the gut microbiome, or caffeine withdrawal.

Yes, it is possible. The diet's restrictive nature, especially if not well-planned, can lead to lower intake of nutrients like iron, B vitamins, and calcium. These play key roles in energy production and can cause fatigue if deficient.

To maintain energy, include low-FODMAP carbohydrates like rice, quinoa, and potatoes, along with healthy fats such as olive oil and seeds. Ensure you are also getting enough protein from sources like eggs, meat, and firm tofu.

If you were a regular caffeine consumer, suddenly eliminating it can cause withdrawal symptoms, including temporary fatigue and headaches. You can switch to low-FODMAP caffeine sources, such as limited coffee or black tea, or wean off it gradually.

If your fatigue persists even with adequate calorie and nutrient intake from low-FODMAP sources, it might be due to other factors like stress, poor sleep, or an underlying medical condition. Consulting a doctor or dietitian is recommended to rule out other causes.

For most people, initial fatigue is temporary and subsides as the body adjusts, often within a few weeks. If you address potential issues like low calorie intake and nutrient gaps, you should start feeling better as your gastrointestinal symptoms improve.

Yes, working with a dietitian experienced in the low FODMAP diet is highly recommended. They can help you create a balanced meal plan, ensure proper nutrient intake, and guide you through the reintroduction process to prevent long-term nutritional issues and fatigue.