This feature on gut health starts with a love story. Our heroine is Grace Whittle, a 35-year-old civil servant from Dublin. Twelve years ago she was suffering what felt like endless bouts of food poisoning. She saw doctors, dieticians and gastroenterologists, and had many tests.

Her eventual diagnosis of irritable bowel syndrome gave little relief. Her horrible symptoms — daily and frequent abdominal pain and diarrhoea — persisted. She barely dared to eat bananas and dry toast.She left parties early, missed work, felt constantly tired and ill. “Honestly, I was simply not looking forward to the rest of my life,” she says.

Our hero, her newish boyfriend, Aonghus Shortt, a practical fellow with a PhD in electrical engineering, wasn’t having it. He read up on the low- Fodmap diet recommended for those with IBS. (Fodmap is an acronym for a range of highly fermentable carbohydrates such as lactose, fructose, sorbitol and inulin that can wreak havoc on a sensitive gut.)

“The principle was, if you could avoid certain foods that rapidly ferment in your gut, you could feel better,” he says. “But the key was to find which foods. It’s different for each person.”

Shortt discovered that hospitals used a hydrogen breath test machine to diagnose digestive problems — with levels indicating how much gas gut bacteria produced after a patient had ingested, say, fruit sugar. “I said, ‘OK, can I make one of these?’ ”

And he did: a basic version Whittle could use at home to find out, in conjunction with any symptoms, exactly which carbs she had trouble digesting. “You could see it on the breath readings,” he says.

There were many foods Whittle was certain she couldn’t eat. “But when she’d test with them, we weren’t seeing much fermentation,” Shortt says. She’d banned wheat, for example, as it caused bloating and cramps. But, courtesy of the device, starting with a slice of bread a week, she found she could tolerate up to two slices a day.

Romance fans may like to know that the couple are now married and expecting a baby. But there’s also good news for gut health enthusiasts. Shortt quit his job, and in collaboration with clinicians, developed the device into a medical-grade consumer product.

His company, FoodMarble, now sells an app-connected hand-held breath tester, AIRE. Anyone with a keen interest in their digestion can use it, although it’s primarily for those with food intolerances or digestive issues.The mission is, he says, “How do we get people to a) where they can manage their symptoms better, and b) where they can eat a broader range of foods?”

There’s often guesswork involved as to the cause of food intolerances.FoodMarble’s research with their users in ten countries found that dairy was the most commonly eliminated food in all of them — yet, compared with sorbitol, fructose and inulin, lactose had the lowest rate of intolerance (43 per cent of UK users).

Meanwhile, sorbitol, which is found in foods such as apples, pears and stone fruits, and used as an artificial sweetener, wasn’t well tolerated by 78 per cent of UK users. They also note that while many people assume they have a gluten intolerance, inulin — a slow-fermenting complex carb found in garlic, onions, many root vegetables and wheat — can be the culprit.

I’m fortunate in that I don’t have digestive problems. Even so, trialling the latest iteration of the device, AIRE 2, which measures hydrogen and methane on the breath, I learn a lot.(For example, that methane has been shown to slow down the passage of food in the gut and is linked to constipation.) I now know more about my gut, and what it thinks of my diet, than any navel-gazer could dream of.

Initially it’s a faff. I download the FoodMarble app and pair the breath tester with my phone. The device is palm-sized, with a button you hold down to take a breath test — exhaling for five seconds into its mouthpiece. The app then calculates your fermentation score (from zero to ten), displayed on your phone screen within seconds.

When you are tired or stressed, your digestion is less optimal

I select “optimising digestion” as my “motivation” rather than “suspected food intolerances” or “manage IBS symptoms”. I log any foods I suspect are problematic. I note my most common symptoms — the list includes bloating, constipation, flatulence, diarrhoea, heartburn, cramps, fatigue, nausea or other. I add “burping”. (Ha, they didn’t think of that one.)

Then I begin collecting data. There’s a lot of it. I log every meal so my breath scores — starting an hour after every meal, then taken every hour for three hours, to track fermentation levels — and any symptoms (also logged) can be matched to my food and drinks. I also record other factors that impact digestion — sleep and stress levels. I also enter “poop” frequency and solidity. The focus on faeces is a bit much, and I stop logging it.

The food bit, though, is fascinating.On day one, my breakfast is peanut butter on sourdough toast. Normally, my system wouldn’t blink, but perhaps because the weekend was unusually riotous, it’s reactive. I feel bloated, burpy, tired. Two hours after eating this food I consume regularly without so much as a rumble, my fermentation level is 7.8 — bizarrely high — though it’s down to 1.1 an hour later.

A lunch of spinach and lentil soup causes medium fermentation levels (4.4 and 5.3). My dinner of French onion soup plus cheesecake and prunes (don’t judge) results in drama — 9.0, then 7.4 and back up to 8.6.There’s methane as well as hydrogen. I still feel bloated. My screen, as well as displaying the fermentation levels, also has a neat chart showing my gas levels throughout the day, against the meals.

Helpfully, if I touch the onscreen entry of any of my meals, the app lists the ingredients and the Fodmap content and basic nutrition. The app makes suggestions for lower-Fodmap alternatives. It also has a surprisingly impressive low-Fodmap recipe list (“One of the guys on the team is a chef,” Shortt says).

Yet I’m confused. The high fermentation scores, which are in red, make me feel as though I’m doing something wrong. The FoodMarble nutritionist (and Aonghus’s sister) Dr Claire Shortt clarifies. “We don’t want people to think every time they see red, ‘Oh, can’t eat that, can’t eat that,’ ” she says. “Onions are full of fermentable sugars — that’s likely why you got a red score, but if you felt good, that’s fine. If you can tolerate eating those kinds of foods — a lot of people would struggle — they’re great. You’re feeding your gut and you’re feeling good.”

Everyone will get a range of low, medium and high scores. What matters is whether you get symptoms.“The key thing is, how much gas can one person tolerate before they experience discomfort?”

If you do have high scores, plus symptoms, Shortt says, look at what you’re consuming — eg milk — then cut down the next day and see if it makes a difference. “It’s really about identifying what can create a lot of gas for you, and how much gas is too much — because you’ll find over time you can tolerate a certain amount on certain days, but when you’re tired orstressed, or after exercise, your digestion is less optimal,” she says.

While AIRE 2 includes a six-week programme in which you identify, restrict and slowly reintroduce any food causing an issue, Shortt stresses, “It’s never about entirely eliminating foods. Even if you’re lactoseintolerant, you can still eat a certain amount of lactose in a day.”

The ultimate goal is to help people to make their diet as diverse as possible. (With IBS, there will of course be limits.) A food intolerance, she notes, is different to an allergy. You can still eat that food, “but the portion amount is much lower”.

After my shaky start — I blame lack of sleep — normal cast-iron digestive service resumes.Over three weeks, my usual midday snack of Greek yoghurt, fruit and nuts leads to low fermentation (0.9, 0.5, 1.1). My gut bacteria don’t even get out of bed in response to my peanut butter on sourdough habit — 0.2, then 0.0. In fact, apart from a mild rise after red meat (sausage and tomato pasta — 4.5), nothing I eat creates excess gas or discomfort.

Now, even though the app’s weekly report tells me my daily average Fodmap consumption is high, especially in the prebiotic fibre inulin, I worry that my gut bugs are hungry, they seem so inert. I pig out at a birthday dinner — fried courgetteswith parmesan, sole meunière plus chips, spinach, red wine, chocolate pudding and more chocolate — and expect to see some action, but two hours later my score is just 2.9.No symptoms either.What’s a girl gotta do to get some fermentation around here?

I consult Dr Hiroshi Mashimo, chief of gastroenterology at the VA Boston Healthcare System andassistant professor of medicine at Harvard Medical School. “What you’re describing is not only that you have a healthy gut in termsof your own cells and enzymes, but that you have an assortment of bacteria that is well tuned to the diet that you have decided to keep,” he says. (He suspects, correctly, that I favour a “plant-forward” diet.)

The focus on faeces is a bit much, and I stop logging it

Mashimo, who has no financial involvement but has worked with the team, is a big fan of the AIRE, while noting that “not all food intolerances are of gas production”.

He likes that it gives those with digestive issues instant feedback without the need for protracted hospital tests and that you can share findings with your clinician. “It’s always nice for the end user to start understanding their symptoms and what the cause is — the reversible cause. I say reversible because food is always a choice.”

Incidentally, he rarely advises patients to cut out a food for good, despite intolerances, because it invariably leads to too many dietary restrictions. “It’s a matter of quantity,” he says, “It’s the old saying, ‘Poison is a matter of dose.’ ”

And to this end, Mashimo uses the AIRE himself, in restaurants. He’s Japanese and well aware that he’s lactose-intolerant. However, “I have a weakness for cheesecake. What I really want to know is, can I get away with half a slice . . . ?”