Food Intolerance or IBS? How to get a reliable test and diagnosis


Do you have gut symptoms such as diarrhoea, constipation, urgency, pain, wind or bloating that you would do absolutely anything to resolve?

IBS sufferer on floor

No wonder there are so many companies offering food intolerance tests out there. The good news is that for most people there is no need to pay for tests in the UK to get a diagnosis. Let me explain the route to getting an official diagnosis of IBS, the difference between IBS and a food intolerance, how to identify your food intolerance, as well as which tests you can and cannot trust.

Getting a diagnosis of IBS

The symptoms of IBS can also be seen in other conditions that will require different medical treatment. This includes bowel cancer, inflammatory bowel disease (Crohn’s and ulcerative colitis) and coeliac disease. IBS sometimes occurs alongside these conditions, but it’s important these conditions are not missed as they can have serious implications on your health.

If you think you have IBS, make an appointment with your GP. Your GP will be following NICE clinical guidelines. These state that to be considered for investigations into an official diagnosis of IBS any of the following symptoms should have been present for at least 6 months:

Abdominal pain or discomfort
Change in bowel habit
If this is the case your GP will first want to rule out any underlying disease by performing the following tests:

– A blood test including:
Full Blood Count,
ESR and CRP (a measure of inflammation in your body),
antibody testing for coeliac disease (an allergy to the protein in wheat, rye and barley).

– Take a stool sample:
to test for infection;
calprotectin to assess inflammation in your gut.
Remember, it’s really important for an accurate coeliac disease test that you have eaten gluten in your diet daily at two meals for 6 weeks before the test is taken. For example, this could be a Weetabix at breakfast and a slice of bread at lunch. If gluten is not eaten daily before the blood test this could lead to a false negative result.
Finally, if these tests come back clear, a diagnosis of IBS can be made if you have abdominal pain or discomfort that is relieved by passing a bowel movement or associated with a change in your poo – either how often you pass a bowel movement, or how hard or soft your poo it.

This also needs to be alongside two of the following symptoms:
– straining, urgency, incomplete evacuation
– bloating,
– mucus in your poo
– symptoms worse after eating.

Once you have this diagnosis you can ask your GP to refer you on to a specialist dietitian to help with diet and lifestyle modification for relief of your symptoms. Dietetic provision and waiting lists on the NHS vary, and some areas provide group support, whilst others offer 1:1 consultations. I run private Zoom clinics with access to appointments within 2 weeks, to find out more visit my website, or to find a local private dietitian visit the British Dietetic Association freelance directory .

So, what happens if you have gut symptoms that are definitely linked to food, but IBS and any underlying diseases have been ruled out? Sounds like you have a food intolerance or food allergy.

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

A food allergy involves the immune system and a food intolerance does not. Immediate food allergy (IgE mediated) occurs when the immune system reacts to protein in foods and causes histamine release. This can lead to itching and swelling, vomiting and diarrhoea, or breathing problems. Delayed food allergy (non IgE mediated) causes symptoms up to several days later including vomiting, diarrhoea and constipation, and/or eczema.

For children the most common food allergies are:

– Cow’s milk

– hens’ eggs

– peanuts

– tree nuts

– wheat

– soya

– fish

– shellfish

– sesame

In adults the most common food allergy is to raw fruits and vegetables (pollen food syndrome), fish, shellfish, legumes and seeds. If you think you have a food allergy you will need a specialist to diagnose this and prescribe treatment. This is available on the NHS in the first instance via your GP, and details can be found here.

Diagnosing Food Intolerance

If you suspect a specific food is causing your gut symptoms, and other conditions have been ruled out as explained above, it’s important to approach this issue in a systematic way. There are many different groups of food that can cause gut upset. If you cut all of them out at once, you will be left with an extremely restrictive diet lacking in key nutrients – this can then cause nutrient deficiencies and poor health, and still little insight into which food or food group was causing your gut upset.

The 3 ‘R’s

To identify a food intolerance, follow the ‘three R method’:

1. Record. It’s important before restricting any specific foods to document when your symptoms occur, alongside what food and drink you are eating for at least a week. This helps you identify the potential culprit. Good old-fashioned pen and paper are great for this, or you might want to try an app such a Bowelle (Apple), mySymptoms or caraCare to help pick out the associations which can be multifaceted. Try and do this at a time when you can keep other lifestyle factors such as exercise and stress under control if possible, and keep a note of anything out of the ordinary (lack of sleep, illness etc).
2. Restrict. Once you have identified a potential culprit it’s time to cut that food out of your diet for a period of 2-4 weeks. Give yourself a week to prepare and clear out the cupboards, stocking up with nutritionally equivalent food. A registered dietitian can help you keep your diet balanced during this stage, to avoid any nutrient deficiencies and resultant negative effect on health and wellness. If there are multiple culprits (e.g. lactose, wheat, histamine) exclude one group at a time. Multiple exclusion diets are best discussed with your dietitian.
3. Reintroduce. Gradually reintroducing the restricted food is essential. If you have received no benefits this is relatively quick and the food group can be reintroduced before you move onto the next restriction phase. If you found relief on the restriction phase it’s still important to reintroduce. If you don’t your sensitivity could worsen, and you’ll be left on a diet which is tricky to follow due to lack of choice and less healthy due to lack of variety. Food intolerances are not black and white but you will have a personal threshold of how much of that food you can tolerate. Day 1 of reintroduction involves eating a third of your usual portion of the test food, day two is 2/3 and day 3 is a whole portion. If you do get symptoms wait for 3 days symptom – free before moving on to the next test food.

Common food intolerances are lactose, wheat, histamine (and other food chemical intolerances), fructose and caffeine. To find out more about managing gut symptoms that are not caused by disease have a read of Dr Megan Rossi’s Eat Yourself Healthy book, which is a comprehensive scientifically based guide to gut health. Remember that any in depth exclusion diet, particularly multiple exclusion diets should be carried out with the support of a registered dietitian.

Can’t I just take a Food Intolerance test?

The absolute gold standard for diagnosing a food intolerance is the ‘3 R method’ described above. There are a few tests that can help with diagnosis if you cannot tolerate the discomfort and inconvenience of reintroducing foods you know are a trigger for your gut symptoms:

Lactose intolerance:

1. Hydrogen breath test

This involves testing the concentration of hydrogen in your breath, and how this is affected by drinking a lactose solution. Lactose intolerance can cause the bacteria in the colon (large intestine) to produce more hydrogen than normal. This test is sometimes available on the NHS, but can give false positives and false negatives, so it’s not 100% accurate.

2. Lactose Tolerance test

This involves drinking lactose solution and then having your blood tested to see how much blood sugar (glucose) it contains. If you are lactose intolerant your blood sugar will rise only a small amount if at all, because your body is unable to digest lactose.

3. Nutrigenomic DNA testing
We can see if your body has the ability to produce the enzyme for digesting lactose by testing your DNA. The test can be done at home using a cheek swab, and sent to labs for analysis. I regularly arrange DNA testing for my clients in my private dietetic clinic, for more information and case studies, have a read of my in depth article on nutritional genomics.

Coeliac disease, and wheat intolerance: Nutrigenomic DNA testing can show if your body is predisposed to developing coeliac disease or not. If you are not predisposed to coeliac disease your risk of developing coeliac disease is very low at 1 in 2000, and a coeliac screen is therefore less necessary. If you are predisposed to coeliac disease your risk of developing the condition rises to 1 in 35, meaning a coeliac screen would be advisable for a definitive diagnosis. In those with high risk genetics who then go on to test negative for coeliac disease there is a high rate of non-coeliac gluten sensitivity where there is symptom relief from reducing gluten in the diet.
This test is also part of the DNA screen I offer through my clinics. If you think you might benefit, book in for a discovery call and we can discuss the best way forward.

Which food intolerance tests should I avoid?

There is no scientific basis for hair analysis, IgG or kinesiology testing in diagnosing a food intolerance. Please save your money! IgG is an indicator of repeated exposure to a food, so often the lists of foods to avoid from these tests are full of foods you eat regularly. Remember diagnosing a food intolerance or IBS is not easy, and symptoms could be related to underlying disease or food allergy. This means it’s really important to involve healthcare professionals such as registered dietitians. Dietitians are bound by law to give the most up to date science-based advice and treatment there is within their own specialist area.

I hope you enjoyed this article. Our next blog in October will be on “How to increase variety and enjoyment on a low FODMAP diet”. In the mean-time please share your thoughts and comments on “Food Intolerance or IBS? Getting a reliable test and diagnosis”.

Article written by Helen Phadnis, Registered Dietitian and Nutrition Coach at The Bespoke Nutrition Coach

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