Food intolerances and sensitivities are often confusing. Some people seem to tolerate every food without issue, while others experience bloating, headaches, or fatigue after certain meals. And for many, these reactions can appear suddenly after years of eating the same foods without a problem.
So why do food sensitivities develop? Let’s explore the science behind them.
The role of genetics
In some cases, your genes play a key role.
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Enzyme deficiencies – For example, adult lactose intolerance is linked to a genetic reduction in the enzyme lactase. Without enough lactase, the body struggles to digest lactose, leading to cramps, bloating, or diarrhoea.
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Inherited tendencies – Some families may be more prone to certain intolerances or sensitivities, though environment and lifestyle also play a part.
Genetics may set the foundation, but they’re rarely the only factor.
Environmental and lifestyle factors
Your surroundings and habits can also shape how your body reacts to food.
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Antibiotic use – These can disrupt the balance of your gut microbiome, sometimes leading to new intolerances or sensitivities.
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Dietary patterns – Highly processed foods, artificial additives, or low-fibre diets may overwhelm the digestive system and increase the risk of intolerances or sensitivities.
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Pollution and chemicals – Ongoing exposure to environmental toxins can add to the body’s “load,” making it less tolerant to certain foods.
Gut health changes
The gut is at the heart of many food reactions. When it’s working smoothly, it digests food and absorbs nutrients efficiently. But when compromised, intolerances and sensitivities may appear.
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Leaky gut or increased intestinal permeability – A weakened gut lining may allow food particles to interact with the immune system, creating intolerances or sensitivities.
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Microbiome imbalances – A lack of diverse gut bacteria can reduce the body’s resilience, making it more reactive to certain foods.
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After illness or infection – Temporary intolerances often develop after stomach bugs or gastroenteritis, but can improve once the gut heals.
The cumulative effect
Sometimes intolerances or sensitivities don’t stem from a single cause, but from accumulation.
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Eating multiple moderate-reactivity foods in one day may create symptoms, even if each food alone wouldn’t.
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Regular exposure to additives or highly processed ingredients can gradually reduce tolerance.
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Over time, stress and poor sleep may weaken digestion, adding to the problem.
When do intolerances and sensitivities develop?
Reactions can appear at any age.
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In childhood – Some children are born with intolerances, such as lactose intolerance or sensitivities to soy or certain fruits. Many grow out of these as their digestive system matures.
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In adulthood – Intolerances often develop later in life, sometimes after illness, stress, hormonal changes, or gradual enzyme decline. For example, it’s common for lactose intolerance to first appear in the teenage years or adulthood.
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In older age – Digestive efficiency often declines naturally with age, making older adults more prone to intolerances.
Links to stress, hormones and digestion
The gut doesn’t work in isolation. Stress and hormones can directly affect digestion and tolerance.
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Stress – Chronic stress alters gut motility and reduces enzyme production, making it harder to process foods.
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Hormonal shifts – Changes during puberty, pregnancy, or menopause can influence gut function and sensitivities.
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Nervous system connection – The gut-brain axis means mood and digestion are closely linked—stress, anxiety, or low mood can all increase the likelihood of food reactions.
Bottom Line
Food intolerances and sensitivities can develop for many reasons — from genetics and gut health to stress, environment, or even natural changes as we age. While they’re not usually dangerous (unlike allergies), they can have a real impact on how you feel day to day.
The good news? Understanding your body’s reactions is the first step to feeling better. By supporting gut health, managing stress, and identifying trigger foods, many people find they can ease — or even prevent — uncomfortable symptoms.
If you often experience bloating, fatigue, or headaches after eating, our Body Intolerance Test can help you uncover what’s really going on. It analyses 1,500 items from the comfort of home, helping you pinpoint your personal triggers and take back control of your wellbeing.
