Food intolerances are far more common than most people realize — and often go undiagnosed for years. Unlike food allergies, which trigger immediate immune responses, food intolerances are slower, more subtle, and often manifest as a collection of chronic symptoms that can negatively affect your quality of life. If you frequently feel bloated, exhausted, or are dealing with unexplained skin issues, your body might be reacting poorly to something you’re eating — without you even knowing it.

In this comprehensive guide, we’ll walk you through everything you need to know about food intolerance: the difference between intolerance and allergy, how it affects your body, common signs of a food intolerane and causes, testing for food intolerance and the best ways to manage it.

What are signs of a Food Intolerance?

Food intolerance refers to difficulty in digesting certain foods or food components. It often involves the digestive system, rather than the immune system. When a person consumes a trigger food, symptoms can take hours — sometimes even days — to appear. These symptoms can be misleading and mistaken for other health issues, which is why food intolerances are often overlooked.

Unlike allergies, food intolerances rarely involve life-threatening reactions. Instead, they produce chronic discomfort that can gradually erode your well-being. The mechanisms behind food intolerances vary — from enzyme deficiencies to chemical sensitivities — and understanding these mechanisms is the first step in healing.

What Is a Food Intolerance?

A food intolerance is the body’s inability to properly digest or react to certain foods. This may be due to enzyme deficiencies (e.g., lactose intolerance), chemical sensitivities (e.g., caffeine or histamine), or immune-related non-allergic responses (e.g., IgG reactions). While not immediately life-threatening like allergies, intolerances can significantly impact quality of life.

According to a 2014 review in the journal Nutrients, food intolerances affect up to 20% of the population, though the true number is likely higher due to underreporting and misdiagnosis.

The Gut–Immune Axis: The Root of Intolerance

The gut lining serves as both a digestive and immune barrier. When the gut becomes compromised — often referred to as “leaky gut syndrome” — larger food particles can cross into the bloodstream. This triggers the immune system to mount a response, creating inflammation and low-grade systemic symptoms.

A 2020 article in Frontiers in Immunology highlights the link between increased intestinal permeability and inflammatory conditions, including skin and fatigue disorders.

When the immune system is repeatedly activated by offending foods, it can lead to chronic inflammation, immune dysregulation, and a cycle of symptoms that worsen over time.

Bloating: A Classic Sign of Food Intolerance

Bloating is one of the most common digestive symptoms and signs of a food intolerance. It often arises from:

Fermentation of undigested food by gut bacteria (common with lactose, fructose, or FODMAP intolerances)

Increased gas production in the small intestine

Inflammation of the gut lining leading to fluid retention and distension

In many cases, the foods triggering bloating are healthy on the surface — like onions, garlic, apples, or beans — but high in fermentable carbohydrates (FODMAPs) that sensitive individuals struggle to digest.

Studies have shown that individuals with non-celiac gluten sensitivity and IBS often report significant bloating relief when intolerant foods are eliminated.

Fatigue: The Hidden Drain of Intolerances

Chronic fatigue is one of the most overlooked signs of a food intolerance. Here’s why:

Immune activation: Persistent low-grade immune responses sap energy

Nutrient malabsorption: Damaged gut lining impairs absorption of iron, B12, magnesium — all vital for energy

Disrupted sleep: Intestinal discomfort or inflammation can interfere with restful sleep

Adrenal stress: Chronic inflammation puts pressure on the adrenal glands, leading to hormonal imbalances and exhaustion

A 2017 study in Nutrients demonstrated that elimination diets tailored to individual intolerances significantly improved energy levels and quality of life.

Skin Issues: The Gut–Skin Connection

The skin is often a mirror of internal health, particularly the gut. Food intolerances can lead to skin conditions through:

Systemic inflammation triggered by immune reactions

Increased histamine from poorly digested foods, contributing to hives or rashes

Gut dysbiosis and leaky gut, both linked to eczema, acne, and psoriasis

A 2019 article in Dermatology Practical & Conceptual explored how elimination diets improved symptoms of eczema and acne in individuals with identified intolerances.

Moreover, Candida overgrowth — often tied to food intolerance and sugar intake — has been linked to persistent skin flare-ups.

Common Causes of Food Intolerance

Here are some of the most well-known — and lesser-known — causes of food intolerance:

• Lactose Intolerance – One of the most common forms, this occurs when the body lacks the enzyme lactase, needed to digest lactose found in dairy. Symptoms include bloating, cramping, and diarrhea.
• Histamine Intolerance – Caused by an accumulation of histamine in the body due to reduced breakdown, often resulting in headaches, hives, and nasal congestion after eating histamine-rich foods like aged cheese, wine, and smoked meats.
• Non-Celiac Gluten Sensitivity – Different from celiac disease, this intolerance to gluten can lead to fatigue, joint pain, bloating, and mood disturbances without an autoimmune component.
• Fructose Malabsorption – A condition where the body is unable to absorb fructose efficiently, leading to gas, bloating, and diarrhea.
• Food Additive Sensitivities – Many people react to artificial additives like MSG, sulfites, or food dyes. Symptoms may include wheezing, rashes, or behavioral changes.
• Salicylate Sensitivity – Salicylates are naturally occurring chemicals in fruits, vegetables, and certain medications. Intolerance can cause asthma-like symptoms, nasal polyps, and skin reactions.

Signs You Might Have a Food Intolerance

Identifying a food intolerance can be challenging because symptoms are often vague, delayed, or attributed to other causes. However, certain symptoms tend to appear consistently:

1. Bloating and Gas

A classic sign of poor digestion. If your abdomen feels distended or you’re passing gas regularly, especially after eating, a food intolerance could be interfering with your gut function.

2. Fatigue and Brain Fog

Feeling sluggish after meals or constantly drained even after a good night’s sleep? Inflammation caused by food intolerance can impair energy metabolism and concentration.

3. Skin Reactions

Breakouts, rashes, and eczema often stem from gut-related imbalances. If you’ve tried every topical remedy without success, it might be time to look at what you’re eating.

4. Headaches or Migraines

Certain food chemicals, like histamine or nitrates, are known migraine triggers. Keeping a food diary can help identify patterns between diet and headaches.

5. Digestive Symptoms

Nausea, cramps, loose stools, or constipation may all signal trouble digesting specific foods. These are common in conditions like lactose intolerance or fructose malabsorption.

6. Mood Changes

Mood swings, anxiety, and irritability can be connected to gut inflammation and poor digestion. A disrupted gut microbiome affects neurotransmitter production — especially serotonin.

7. Acid Reflux or Heartburn

Reflux may not always be due to excess acid — it can be a sign that your body isn’t processing certain foods properly.

8. Respiratory Issues

Chronic sinus congestion, post-nasal drip, or even asthma-like symptoms may be related to a food intolerance, particularly to dairy or additives like sulfites.

9. Joint Pain or Muscle Aches

Low-grade inflammation caused by food sensitivities can contribute to joint stiffness, especially in the morning.

10. Sleep Disturbances

Night sweats, difficulty falling asleep, or waking up frequently could stem from hormonal disruption triggered by food reactions.

How to Identify a Food Intolerance

Step 1: Keep a Food and Symptom Diary

Start by tracking everything you eat and how you feel afterwards. Look for patterns. Symptoms may not appear until hours later, so detail is key.

Step 2: Try an Elimination Diet

Remove common triggers — such as gluten, dairy, eggs, soy, corn, and additives — for 2–4 weeks. Gradually reintroduce one food at a time and observe any reactions. This can be quite a difficult process so feel free to book a consultation with an expert.

Step 3: Get Functional Testing

• Book a food intolerance test in London
• Buy a food intolerance test kit
• Buy a Lactose or Fructose Breath Tests–
• Consider Histamine/Mast Cell Testing
• Consider a Gut Microbiome Analysis

These tests can provide deeper insights and complement your symptom tracking.

Step 4: Consult a Practitioner

A functional medicine practitioner can help guide you through testing, interpret results, and support your gut healing process.

Why Blood Testing Is More Reliable Than Hair Testing

When it comes to identifying food intolerances, not all testing methods are created equal. Two of the most common are IgG blood testing and hair analysis (bioresonance testing). While both are popular in the wellness space, blood testing is considered far more evidence-based and clinically reliable.

IgG blood testing measures levels of immunoglobulin G (IgG) antibodies that your body produces in response to specific foods. Elevated levels may indicate a delayed immune response, commonly associated with food intolerance. According to a review published in Nutrients (2017), IgG testing can help identify reactive foods that contribute to IBS, fatigue, and skin disorders — providing a clinical foundation for elimination diets.

In contrast, hair testing evaluates the energetic response of your body to foods, but lacks scientific validation. A 2015 review in the journal Clinical & Experimental Allergy concluded that there is “no evidence that bioresonance testing is a reliable method for diagnosing food intolerances.”

Key advantages of IgG blood testing include:

Quantifiable antibody levels

Reproducible lab results

Better alignment with published clinical research

For these reasons, many functional medicine practitioners — including us at Nutrition and Vitality — prioritise IgG blood testing as part of a structured, evidence-based approach to gut healing.

If you’re experiencing persistent bloating, fatigue, or skin issues, uncovering the root cause starts with proper testing. At Nutrition and Vitality, we offer comprehensive food intolerance testing using IgG antibody analysis, which can help identify delayed immune responses to specific foods.

How to Manage Food Intolerances

• Avoid Offending Foods – The most obvious, but often hardest, step. Once you’ve identified your intolerances, remove those foods from your diet.
• Read Labels Carefully – Many packaged foods contain hidden additives or allergens under different names.
• Use Enzyme Support – Supplements like lactase (for lactose) or diamine oxidase (for histamine) can improve tolerance when avoiding food isn’t practical.
• Strengthen Gut Health – Incorporate prebiotics, probiotics, L-glutamine, and anti-inflammatory herbs like turmeric or marshmallow root to repair the gut lining.
• Support Detoxification – Ensure your liver and elimination pathways are supported with B vitamins, leafy greens, water, and fiber to process food compounds efficiently.
• Rotate Foods – Avoid eating the same foods daily to reduce the risk of developing new sensitivities.
• Manage Stress – Chronic stress weakens digestion and immunity. Mindfulness, yoga, or even a short daily walk can improve
  
  
  
• Once intolerances are identified, the healing process includes:
• Removing trigger foods for at least 4–6 weeks
• Supporting gut repair with nutrients like L-glutamine, zinc carnosine, and omega-3s
• Balancing gut flora using probiotics, prebiotics, and in some cases, anti-microbials
• Restoring digestive function with enzymes, HCl support, or bitters
• Reintroducing foods strategically to test tolerance and build resilience

Frequently Asked Questions

What’s the difference between a food allergy and a food intolerance?

A food allergy involves the immune system and can be life-threatening. Food intolerances affect the digestive system and typically cause delayed, less severe symptoms. Allergies can trigger anaphylaxis; intolerances generally do not.

Can food intolerance cause weight gain?

Yes. Inflammation, bloating, and hormonal disruption from intolerances can lead to fluid retention and weight gain.

Can I develop a food intolerance later in life?

Absolutely. Gut damage, stress, infections, or dietary patterns can lead to new intolerances developing at any age.

Are food additives a common cause of intolerance?

Yes. Many people react to artificial preservatives, flavor enhancers, and colorings like MSG, sulfites, and tartrazine. Reactions may include rashes, wheezing, or digestive upset.

Can intolerances go away over time?

In many cases, yes — especially if you remove the food, heal your gut, and reduce systemic inflammation. Some people regain tolerance over months of focused support.

Final Thoughts

The signs of a food intolerance can be sneaky and chronic — showing up as digestive discomfort, persistent fatigue, or frustrating skin problems. These symptoms are not just a dietary nuisance — they are a signal that something deeper is off in the gut–immune system.

The good news is, with the right testing, a personalised elimination plan, and gut-healing support, these symptoms can be reversed. As a functional medicine practitioner, identifying and addressing food intolerances can often be the missing key that unlocks profound transformation for your patients.

Author – Victoria Tyler BSc Hons MBANT

Victoria Tyler owns and runs two busy clinics : Nutrition and Vitality and the IBS and Gut Disorder Clinic.

Nutrition and Vitality, along with the IBS and Gut Disorder Clinic, were founded with the goal of helping patients alleviate IBS symptoms by uncovering and addressing the root causes of their digestive issues.

As a Registered Nutritional Therapist, Victoria holds a BSc (Hons) in Nutritional Therapy and has trained with the Institute of Functional Medicine. She is also accredited by BANT and CNHC.

Before transitioning into health, Victoria earned a degree in Economics and an MBA, working with corporations including Canon and Vodafone. However, her own health challenges led her to pursue a career in Nutritional Therapy.

With a passion for learning, Victoria is committed to staying at the forefront of Functional Medicine. She helps patients manage IBS and other digestive disorders, including SIBO, Candida, and IBD, by identifying and addressing their root causes.

Victoria strongly believes that every symptom has an underlying cause, and there is always a solution to every health condition. To learn more, or to see patient reviews, visit Victoria’s profile on Google.

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