There, she was given a hydrogen breath test, used to investigate intolerances to dietary sugars. It involves having small doses of common culprits, such as lactose (the sugar found in milk) and fructose (a sugar found mainly in fruit).
If the gut can't digest these, the food is fermented by bacteria in the gut, producing hydrogen that's expelled through the breath within two to three hours.
Symptoms of fructose malabsorption are similar to other intolerances, such as lactose, and include abdominal pain, bloating, flatulence and diarrhoea after eating culprit foods. It can affect both children and adults.
Fructose intolerance seems to occur as our bodies aren't especially well-designed to process large quantities of this type of sugar. The main sugar our body uses for energy is glucose, so we have dedicated proteins - "transporters" - to help effectively absorb it into our system. Fructose doesn't have these transporters, though, so has to hitch a ride with the glucose transporters.
However, this isn't an effective way to digest it, and so some fructose makes its way into the large intestine undigested.
"There, it ferments and feeds the gut bacteria, leading to formation of gas, bloating and diarrhoea in some people," says Peter Whorwell, professor of medicine and gastroenterology at the University of Manchester.
"We're not sure exactly why fructose causes symptoms in some and not others, but it's most likely differences in the sensitivity in the gut.
"Everyone passes some fructose to the large intestine undigested as our bodies aren't terribly good at dealing with it, especially in the quantities we eat these days, with year-round access to all kinds of fruits and huge amounts of added sugars in food."
Fructose malabsorption is not well-known, even with doctors, and it's not known how many may be affected, perhaps as it's often diagnosed as irritable bowel syndrome (IBS), says Professor Whorwell.
Fructose malabsorption can go hand in hand with digestive disorders including IBS and Crohn's disease, a more serious condition that is a form of inflammatory bowel disease.
"Around one-third of IBS patients will have fructose malabsorption," says Julie Thompson, a specialist gastroenterology dietitian for the NHS and founder of the Calm Gut Clinic.
"But it can also be a standalone condition."
Deniz had never heard of fructose malabsorption, and was surprised to find her problems were, in part, due to the fruit - at least three portions a day - she ate to stay healthy.
Her doctor advised her to cut down.
"I started doing some research and realised fructose is in more foods than fruit," says Deniz. But fructose doesn't need to be cut out altogether.
"It's the portion at each meal and the total amount you eat in a day that's important," says Julie Thompson.
She explains fructose is much better absorbed when eaten with equal amounts of glucose, as this stimulates the gut transporters. This means certain fruits and veg are better tolerated than others, depending on their fructose/glucose ratio.
Apples and pears, for example, have a much higher fructose content than glucose, while kiwis and berries are lower in fructose and have a more equal balance with glucose.
"Avoid large amounts of dried fruits and limit juices, too, as these have concentrated fructose levels," says Julie.
On a dietitian's advice, Deniz also started following the principles of the low-FODMAP diet, increasingly used to help IBS patients. FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols) are sugars such as fructose that have been identified as problematic for some people with IBS as they are poorly absorbed.
FODMAPs are found in specific foods, including artificial sweeteners, honey, dairy products, wheat, garlic and onions, as well as some fruit, such as cherries and plums, and some vegetables, such as artichokes, beetroot and mushrooms.
Deniz says: "Once I had changed my diet, the results were pretty much instantaneous. The pain, bloating, bad breath (caused by the fermenting fructose in my gut) and tiredness went in days.
"I'd always had dull, spotty skin - that cleared up, too. I limit myself to one small serving of low-fructose fruit a day and eat plenty of veg. I avoid onions, apples, pears, dates, wheat and most condiments."
As well as gut symptoms, fructose malabsorption has been linked to depression. As microbiologist Giulia Enders explains in her new book Gut: The Inside Story Of The Body's Most Underrated Organ, fructose absorption also effectively reduces absorption of the essential amino acid (a building block of protein) called tryptophan, which is found in foods such as oats, milk and eggs.
It's needed by the body to produce serotonin, a feel-good chemical messenger. Scientists believe high concentrations of fructose in the intestine react with tryptophan, making it less absorbable.
"Recent studies have confirmed a strong link between the state of the bacteria in the gut and the state of the brain," says Professor Whorwell.
"If something is disturbing this balance, it stands to reason that the brain may not be so happy."
This is borne out by Deniz's experience. "Before I was diagnosed, I used to get angry for no reason and couldn't work out why I felt so low. As soon as I reduced my fructose, my low mood lifted for good."
Even if your body tolerates fructose, it's worth considering your intake, as fructose is now used to sweeten so many foods that it's also being investigated as one of the key drivers behind the obesity epidemic.
This year, US scientists found fructose doesn't trigger the same fullness signals in the brain as glucose, leaving you thinking you're still hungry. The researchers say this is because fructose doesn't stimulate the body to produce as much insulin as glucose does - insulin signals to our brain that we have eaten.
Small amounts of table sugar can be tolerated by those with fructose malabsorption as it contains equal amounts of glucose and fructose.
But many, including Deniz, choose to cut it out entirely.
"I used to have a lot of sugary coffee and chocolate to combat tiredness," she says. "Now, I consume my tolerated amount of fructose from fruit and veg instead."
Deniz, originally from Germany, has found other ways to treat herself - including making coffee with coconut oil and using sugar substitutes such as rice syrup - and now shares fructose-free recipes on her online blog, Fructopia.
"My diet is so much healthier now," she says. "It may sound odd, but I'm actually glad that I was diagnosed with fructose malabsorption."
Ask your GP for this test
"Don't self-diagnose or self-treat," says dietitian Julie Thompson. Symptoms could be caused by a number of conditions, such as lactose intolerance (from milk and dairy), which causes very similar symptoms.
Ask your GP about a hydrogen breath test. After fasting for 12 hours, you are given solutions of glucose, lactose and fructose.
Your breath is tested after each for signs of hydrogen produced by fermenting sugars in the gut. This is a reliable way to confirm fructose intolerance, but not all NHS trusts offer it.
"A dietitian can diagnose you by taking a thorough diet history and starting a programme of excluding and reintroducing foods to see what causes symptoms."
But don't cut out food groups of your own volition.
"Fruit and veg provide vital nutrients and should not be reduced without expert guidance."
- Daily Mail
