Gary Alexander is a writer and author who lives in London. He suffered from secondary lactose intolerance for several months after a severe bout of gastroenteritis.

"I’d been ill for a few days with severe diarrhoea and abdominal pain, and was losing weight. I eventually went to see my doctor. Because the right side of my abdomen was tender, she thought it was appendicitis. Within minutes an ambulance had arrived and I was whisked to hospital. There, after a number of tests I was put on to a drip to counter my dehydration, and was diagnosed with gastroenteritis. I spent three days in hospital and two weeks recuperating at home, eating a very plain diet of rice and vegetables.

"I’ve always eaten a lot of dairy foods. I usually eat yoghurt and fruit for breakfast, take milk in my tea and coffee and eat cheese every day. But once I started to incorporate dairy food back into my diet, immediately I felt things weren’t quite right. I began to feel bloated and my tummy was rumbling badly. I had mild diarrhoea, and was concerned my gastroenteritis hadn’t properly cleared up, so went back to my doctor.

"She told me it was likely to be secondary lactose intolerance, and that I’d need to avoid all lactose for a while, especially in the form of milk.

"It was hard. At first, I tried to eliminate all dairy products and eat different foods. I experimented with soya milk and other milks, such as oat milk, which are all palatable, and soya yoghurt, which is not. I was missing cheese, so I reintroduced that gradually without any problems. My doctor had told me that most cheese is quite low in lactose anyway. Once I’d managed that, yoghurt came next, and that was fine too.

"Milk was not so easy. The first time I tried to introduce it I felt some familiar rumbles and knew my body wasn't ready for it. It took a few more weeks before I could reincorporate it gradually, with a little milk in tea and coffee.

"After about six months of gradual reintroduction and experimentation, I returned to normal. Now, I can eat all dairy again without any problem, but every now and then I use dairy substitutes, just for variety."