Lactose intolerance is a genetic problem. You are born with a gene that tells your body whether or not it will eventually shut off the lactase-making mechanism after the age of weaning.
You hear that everywhere. I say it myself fairly often. And it's true.
Up to a point.
You can be lactose intolerant and have the gene that never shuts lactase down. Both. That's because the lactase-making mechanism is delicate and can be damaged by any number of things. Diseases, drugs, surgery.
And the effect can be permanent.
Andrea McLean, a British television personality (she's one of the Loose Women), wrote about her experiences with secondary lactose intolerance for the Daily Mail.
It started, innocently enough, with a severe bout of food poisoning 20 years ago. I was in my 20s, backpacking around India.
The culprit was a bottle of water - I noticed the lid came off a bit too easily but I drank it anyway. Later I walked past the stall I had bought it from and saw a man filling identical bottles from a tap.
Within 24 hours I was hideously ill. I spent the next few weeks groaning and clutching my stomach on a mattress in a darkened hostel dorm room, before finally the symptoms began to subside.
And a year later, when I got back home, things still weren't quite right. A couple of times a week, I had to rush to the loo. I felt tired and lethargic.
My GP referred me to the Hospital for Tropical Diseases in London, where I spent what seemed like the next year having blood test after urine test. Was something hideous lurking inside me?
It was nothing so exciting. The tests all came back negative. Instead, I was told to try cutting certain foods from my diet, starting with dairy.
Within a week I felt back to my old self. I had developed secondary lactose intolerance.
Secondary lactose intolerance is treated exactly the same way as the kind that is genetic, known as primary lactose intolerance. Take lactase pills or avoid dairy.