I was slim, fit and a GP with a healthy diet — then I learned I had 'type 1.5' diabetes. Many people have never heard of it, or do not realise they have it: Dr Dawn Harper

After a long day of filming, I was exhausted and getting ready to head home when one of the researchers came over wearing a serious expression.

“I just wanted to flag, Dawn, that some of your test results came back a bit strange,” she said. “It’s probably a lab error, but you might want to get your blood sugar levels checked. According to this, you’re prediabetic.”

I was stunned. The Channel 4 series Embarrassing Bodies, which I co-hosted for nine years, was known for putting us through blood, stool and fitness tests so our own health could be measured alongside that of the patients we treated.

That day, I had undergone the same checks as the woman featured in our episode: she was 44, the same age as me, 5ft 4in tall, and had three children, just like I did.

The only real difference was our weight. She was morbidly obese, while I was a trim eight stone, with a BMI of just 19.5.

We had our fat cells examined under a microscope, our gut flora compared and our blood sugar measured with an HbA1c test. This test measures the amount of glucose attached to red blood cells and is one of the most reliable indicators of average blood sugar levels over the previous two to three months, as well as the risk of developing type 2 diabetes.

Most of the results were exactly what we would have expected.

My healthy diet, based largely on wholegrains, fruit and vegetables, along with regular exercise — weekly 30-mile cycles and daily walks with my dogs — meant my gut microbiome was in good shape and my fat cells were less than half the size of my companion’s.

So it seemed to make no sense that my blood sugar was a problem, leaving me in the grey area of prediabetes, where levels are higher than normal but not yet high enough for a type 2 diabetes diagnosis.

I was hardly the classic diabetes patient, and I was very busy, so I ignored it.

Fifteen years later, though, I got a phone call from another concerned and equally puzzled diabetes nurse. My husband and I had just moved house, and after registering with a new GP surgery I was asked to attend an NHS health check.

This time, the results were harder to brush off. “You need to take this seriously,” she told me, booking me in for a repeat blood sugar test three months later.

Even after I tried to improve my health further — including a yoga retreat and juice cleanse shortly before the second Hb1Ac test — the results were even worse.

What is diabetes, and why is it not just type 1 or type 2?

Diabetes is a lifelong condition that causes blood sugar levels to rise too high.

People with it cannot make or use enough insulin, the hormone produced by the pancreas that enables the body’s cells to turn sugar into energy.

Without enough insulin, glucose builds up quickly in the blood, causing intense thirst and frequent urination as the body tries to remove the excess sugar. If it is not treated, high blood sugar slowly damages blood vessels and nerves, increasing the risk of serious long-term complications including blindness, kidney failure and heart disease.

For years, diabetes was understood as having two main forms: type 1, in which the immune system attacks and destroys the cells that make insulin, and type 2, in which the body cannot make enough of the hormone or the cells stop responding to it.

More recently, research has suggested there are several other forms. Gestational diabetes develops during pregnancy because hormonal changes reduce the body’s sensitivity to insulin, while monogenic diabetes is caused by a single gene mutation and often runs in families. Another form can be triggered by steroid medications.

My blood sugar had risen to 6.5 per cent. Even though I was detoxed and glowing, and in the best shape of my life, I was officially diabetic.

During nearly three decades as an NHS GP, I have diagnosed countless patients with type 2 diabetes.

I know how serious the condition can become if it is not treated. Uncontrolled diabetes damages blood vessels, nerves and major organs, raising the risk of stroke, heart disease and even blindness.

It is potentially deadly, and it is becoming more common.

Estimates suggest almost five million people in the UK are living with diabetes. Another 1.3 million are likely to have it without knowing, while a further 6.3 million are affected by prediabetes. Many, like me, will have no idea.

The condition develops when the body becomes resistant to insulin, leading to dangerous levels of sugar in the blood.

In type 1 diabetes, the insulin resistance is caused by an autoimmune reaction, in which the body mistakenly attacks and destroys the insulin-producing cells in the pancreas. It usually develops in childhood or adolescence, symptoms appear quickly and severely, and immediate insulin treatment is needed.

Type 2 diabetes, which accounts for the vast majority of cases in the UK, is usually linked to lifestyle-related risk factors and tends to develop in adulthood.

Being overweight or obese, inactive, and eating a diet high in fat and sugar can all increase the risk, because excess body fat and inflammation reduce the body’s ability to process insulin.

Over time, the condition can show up as severe fatigue, excessive thirst, unexplained weight loss, tingling in the hands and feet, and dark patches of skin, among other symptoms. But some people have no obvious signs at all, or no visible risk factors.

As a doctor, I am trained to spot the early signs of diabetes. So how did I miss my own?

As it turned out, there was more to the story.

After further testing, I was diagnosed with a little-known but widespread form of diabetes that mainly affects slim people like me in middle age.

Called latent autoimmune diabetes in adults, or LADA, it sits somewhere between type 1 and type 2 diabetes and was only identified in the 1990s.

The condition happens when the immune system targets and destroys the insulin-producing beta cells in the pancreas. It can often be managed for months, and sometimes years, after diagnosis with diet and lifestyle changes alone, without needing insulin straight away, much like type 2 diabetes.

People often call it type 1.5 diabetes, and for that reason it is also the most commonly misdiagnosed form of the disease. Experts estimate that as many as 12 per cent of people diagnosed with type 2 diabetes actually have LADA — around 350,000 Britons.

The problem is that LADA patients often do not fit the usual picture doctors are trained to look for. We are more likely to be slim and fit, and our symptoms tend to be slow, vague or, as in my case, absent altogether.

As a result, many people with LADA go undiagnosed for years, leading to serious complications such as heart disease and diabetic ketoacidosis, in which the blood becomes dangerously acidic. Others may be given a diagnosis but the wrong one, told to lose weight or put on type 2 treatments such as metformin, only to find they do not work.

Researchers still do not know exactly what causes LADA, but as understanding grows, one thing is clear: it needs to be detected more quickly.

As my diabetes nurse rightly urged, people should take blood sugar readings that are above normal seriously, even if they feel perfectly well and look healthy.

In my own case, I have so far managed my diabetes through changes to my diet, although my doctor has told me that I will eventually need insulin.

I now use a continuous glucose monitor, or CGM, which checks my blood sugar every three minutes and has completely changed the way I eat.

Mine is from Yuwell Anytime, a company I now work with as a medical ambassador. It has been so useful that I think every diabetes and prediabetes patient should be offered one on the NHS for a few weeks, so they can see exactly how their diet affects their glucose levels.

For me, there was no obvious cause of the high blood sugar. But my CGM has since shown that bread is my weakness.

My daily lunch of a ham and cheese sandwich, which I ate for years while travelling for Embarrassing Bodies, had been steadily wearing down my insulin levels. Even my weekend treat of sourdough with Marmite was sending my blood sugar all over the place, to my annoyance, I discovered.

So I have cut out bread, and even reduced some fruit, from my routine — though I still allow myself the occasional treat.

I miss it, but it is a small price to pay for staying healthier for longer. Instead, I fill my diet with fresh vegetables, and when I do eat carbohydrates I choose wholegrain options.

What works for me will not suit everyone.

In fact, I think my experience shows that general advice does not go far enough.

You do not have to be overweight to develop diabetes, and exercise and healthy eating will not solve the problem for everyone, although they will usually help. But one thing is clear: ignoring the signs, as I did, can only do harm.