How to Write a (Useful) Food Intolerance Diary

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One of the best ways to try and find out if a food is causing symptoms like bloating or gas is to write a food symptom diary – but, if you accurately want to get a good picture of what might be causing your symptoms there’s a bit more to keeping a food intolerance diary than just noting down what you ate and when your symptoms occur – as our experts explain.

This article was reviewed by Jennifer May, Clinical Nutritionist. See more about her and our editorial accuracy process here

Notepad and pencil photographed on a yellow background

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The Big Surprise

Now at this point, you might be thinking ‘Surely it’s the thing I ate just before the symptoms happened that causes my bloating?’ Well, not necessarily.

While a reaction to lactose (a sugar in dairy products) can happen quite quickly after eating something containing it, other intolerances can take hours to manifest.

As accredited practising dietitian Molly Warner from Satisfy Nutrition explains, ‘bloating is caused when bacteria ferment food in the digestive tract. But, because the gastrointestinal tract is eight to nine metres long it can take between one to three days for the food to get through the large intestine (or colon) where those bacteria live and so, if bloating is your prime symptom, it’s actually unlikely to be the food you just ate – and it could be something you had a few meals ago.’

So why do symptoms occur shortly after eating something else?

‘Because when you eat the meal the stretch receptors in the stomach send signals to the rest of the intestine that effectively says ‘more food on the way, shuffle everything along’ – and so whatever is already in there gets moved along to the part where fermentation starts to occur. But if you’re getting gases building up through the afternoon or evening it’s not likely to be lunch that’s the cause but breakfast or last night’s dinner moving further down the digestive tract,’ says Molly.

Not knowing exactly which meal is hitting the bacteria when is one reason why it can be hard to tell exactly which food is causing your bloating.

Consider the Sweetcorn Challenge

One way to get a rough idea of how long things take to move through your system is to eat a meal containing sweetcorn (after not eating any for a while).

Your body can’t digest sweetcorn very easily and so when you see kernels coming out the other end, you know that’s roughly how long things take to go from one end to the other with you. This might help you pin things down a little.

You can also do it with muffins containing blue food dye – known as the blue poop challenge.

How Long Should You Keep Your Diary For?

There is no hard and fast rule around this,’ says accredited practising dietitian Bianca Woger at Big Sis Nutrition. ‘Ideally, it will be 5 days or more to get an idea of the types of foods you are having, when symptoms are presenting (how long after eating), the type of symptoms you are experiencing and importantly, what foods or compounds in those foods are commonly causing issues.

But the longer you fill out a food diary the better, so that patterns become clearer.’

What Should You Note Down?

Molly says you need to write down what you ate and when you ate it – and be very specific about it.

You don’t need to weigh every ingredient, but at least give a rough serving size- 2 slices of bread, 3 handfuls of quinoa as this can help show whether there’s an amount that might set off your symptoms.

And don’t ignore small things like whether or not you added soy sauce to your sushi, butter on your lunchtime sandwich or ketchup on your fries.

Woman sitting on her bed wearing a pink tee shirt and denim shorts while filling in a notepad - illustrating how to fill in a food diary

Also, if it’s a packaged food, note down the brand – if you can’t see any obvious main ingredients causing your issues it might be something much smaller in the food like a sweetener or sugar that’s used in certain brands.

And don’t forget drinks and sweets. ‘People tend to think bloating only comes from food, but some drinks can affect it – drinking fizzy water for example – as can things you easily forget about consuming like chewing gum or mints,’ says Molly.

There’s a whole heap of ingredients that can lead to bloating (we’ve pinpointed 10 in this post) and the more details you can give the more likely it is you’ll spot if a pattern involving any of them emerges.

Also note down what symptoms you’re experiencing, and the time they start and stop ‘and I also like people to take note of the severity – so, rate it from one to five or one to ten. This can also show patterns,’ says Molly.

Illustration showing the different type of stool represented in the Bristol Stool Chart

If you can bear it, it can also help to note your stool consistency using the Bristol Stool chart. Sometimes constipation can be the cause of bloating so it’ll help give you an idea of whether your stools are healthy.

Don’t Just Think About Food

Your digestive system is not only affected by the food you consume – your lifestyle can also impact upon it. ‘Sleep and stress both have an effect on digestive function and so it’s also a good idea to note down how well you’ve slept and whether it has been a stressful day,’ says Molly.

Lastly, note down what you did that day – i.e. went to the gym, had your monthly appraisal meeting, went for drinks with Jane.

As we explained in our piece on gym bloat, exercise can lead to your swallowing air which can trigger bloating – and what you might find is that your symptoms have nothing to do with food, but happen after your gym visit, after a difficult meeting with your boss or, after a night out with the girls.

‘Hormones can also affect bloating, so it might also be a good idea to note down where in your menstrual cycle you are if that’s applicable to you,’ says Molly.

Download a Printable Food Diary Template Here

If right now you’re thinking, ‘I’ll never remember that lot’ – we’ve got your back. We’ve made a simple printed food diary template that prompts you to remember everything you might need to include.

Simply sign up with the form below to get your free downloads.

When Should You Fill in Your Diary

Ideally just after you’ve eaten anything. ‘If you try and remember everything you’ve eaten at the end of the day it can be hard to recall everything – and it’s even worse at the end of a week,’ says Molly.

Page from our Printable Food Intolerance Diary Template showing the checklist of what you should include in a food intolerance diary

What Should You Look For?

So, once you have your diary filled in. How do you interpret it correctly?

Well basically you’re looking for patterns that might indicate a problem – so, 18 hours after you eat eggplant you develop gas (find out why that happens here) or if you have three servings of dairy in a day you get stomach cramps.

But it’s important not to make some common mistakes when you do this.

The Big Mistakes Not To Make With Your Food Symptom Diary

Even the most meticulously filled-in food diary can be interpreted incorrectly if you’re not careful. So, make sure you’re not falling into of the traps below…

Don’t Just Blame the Big Hitters

The most common foods people assume are triggering bloating and gas are gluten and dairy – and yes, they can cause problems in lots of people – but so can mushrooms, chickpeas, fizzy drinks, chia seeds, eggplant, beans – and many more items, for lots of different reasons.

In fact, in one interesting trial researchers gave people who thought they had a sensitivity to gluten some cereal bars – some contained gluten, some contained fructans (a type of sugar that can cause bloating) and a placebo bar. The volunteers in the study didn’t know what they were eating, but when the results were analysed, the fructan bars caused more problems than the gluten ones.

‘Another thing people do is assume that they have the same issue as their friends,’ says Molly. ‘If you have a friend who has solved their issues on a dairy-free diet you’ll immediately look to dairy as the culprit and might misinterpret your results.

That’s why showing your diary to a professional is such a good idea – they don’t have any preconceived ideas of what the issue might be.’

Loaves of bread on a shelf marked gluten free

Don’t Rule Out Favourite Foods

Just because you’ve eaten something your whole life and your bloating only started two months ago, that doesn’t mean it’s not that food.

It’s possible to develop an intolerance to a food you’ve eaten your whole life. Exactly why this happens isn’t known but reasons can include a disruption to the gut bacteria after an infection, a change in our ability to produce the enzyme needed to adequately digest the food involved.

Don’t Start Cutting Out Whole Food Groups

‘This is the main mistake people make when trying to find out what food is causing their problems and it can have a lot of negative effects on your diet,’ says Bianca. ‘A lot of gluten-containing foods, for example, are high in fibre. By changing fibre intake, you could also be affecting bowel habits, and impacting gut health. Dairy is also the major source of calcium in most of our diets.’

You might also not be reacting to the food as a whole, but one small part of it. ‘As an example, if you know you get symptoms after you have a bowl of cereal for breakfast with milk, it is likely that something in that breakfast (either the cereal or the milk) caused the reaction,’ says Biana.

‘To test this, the next morning maybe you try a different meal like scrambled eggs and a glass of milk. If the same reaction occurs, it is likely there is an issue with milk.

But the next question would be whether that is an issue with the carbohydrate portion of the milk (lactose) or the type of milk protein (see more about how this might affect you in our post on A1 vs. A2 milk),’ explains Bianca.

Each of these can be fixed with a different alteration to your diet (namely buying slightly different forms of cow’s milk) – but neither of them sees you having to give up milk totally.

This advice is particularly important if you think that sugars called FODMAPs might be to blame for your symptoms. FODMAPs are found in a lot of food and trying to eliminate them without professional advice can make your diet unnecessarily restrictive as many people only react to one or two specific types of FODMAPs.

Elimination Might Not Be Essential

And even if you have found your trigger food correctly, that doesn’t mean you need to eliminate it entirely to stop having any reaction.

Unlike allergies, where even the tiniest exposure to a food can trigger symptoms, when you have an intolerance or food sensitivity, it’s often possible to eat small amounts of a food without any issues.

As we talked about in some of our posts on why chickpeas cause bloating preparing foods in a different way can also minimise the chance of a bloat reaction.

Don’t Go It Alone

All of the above is why, even if you keep your own food diary, it’s still important to seek help from a dietitian to help you interpret it correctly. ‘Another one of the biggest mistakes people make is not reaching out to a dietitian or nutritionist earlier in the process,’ says Bianca.

‘I’d say come and see us before you start eliminating any major food groups from your diet,’ says Molly.

When to Speak with Your GP

‘It is important to acknowledge that some bloating after eating is normal. This is just part of digestion and gases being released during this process. However, if this bloating is constant, uncomfortable, or accompanied by pain, then it is time to seek some answers,’ says Bianca.

‘Even if you think you have a good idea of what food/s causes your symptoms, my first tip is to go to your GP to discuss your concerns. As when diagnosing intolerances, there are overlapping symptoms with irritable bowel syndrome (IBS) and other conditions that it’s good to get checked out.’

Also in case the case of bloating it’s very important to see your GP if the bloating doesn’t go away or is accompanied by symptoms like quickly feeling full.

Bianca says some of the other red flags to look out for include unexplained weight loss, a family history of bowel diseases – rectal bleeding – bowel motions overnight -persistent daily diarrhoea – vomiting – a fever or any symptoms that become more severe or progressive.

So, there you have it, our guide to keeping a useful food intolerance diary – don’t forget, if you think it’s going to be helpful, download our templates to make it easier to remember all the points our experts suggested.

Who is The Wellness Nerd?

My name is Helen Foster, and I’m a health journalist and wellness author. Publications I’ve written for include Women’s Health, Reader’s Digest, Body and Soul, Good Health at the Daily Mail, and more. I have also written 16 books on health and nutrition.

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