Earlier this year, one of Tom Watson’s local Labour party members emptied out his wardrobe and donated a load of clothes to the deputy leader so he would have something to wear for prime minister’s questions the following week. The keen-eyed member had noticed that Watson, who at his heaviest weighed 140kg (22st), had started shrinking inside his suits. In total, he has now lost 44kg (7st) and has reversed his diagnosis of type 2 diabetes.

The reversal has been relatively fast – it has taken him just over a year. But it has taken more than 25 years to gain control over his diet and exercise. Watson was diagnosed with type 2 diabetes in late 2015. It wasn’t a huge shock – there had been warning signs, such as his increasing weight and high blood pressure – for some time, but it was still a blow. “The overwhelming emotion was shame,” he says. “I felt frightened and ashamed that I had come to this point, and guilty. I’ve only admitted it publicly now, even though I’ve been in remission for a year, and I’ve felt quite nervous. I think particularly men find it difficult to talk about health.”

There are about 3.7 million people in the UK with diabetes. After his diagnosis, Watson started taking medication, but did little else to address his condition. “I guess it was a combination of lack of knowledge and fear; for a year or two I was in denial. Then I started to read a lot more and started to realise that if I got my weight down, it would affect my insulin level and blood pressure.” Two books helped – Aseem Malhotra’s The Pioppi Diet, which advocates a low-carb regime, and Michael Mosley’s The Fast Diet. For about six months, Watson sought out the scientific papers and research referred to in the footnotes in the books. “They contradicted some of the public health advice that was available and the marketing messages, [particularly] about low-fat products.”

Sugar, Watson realised, might be the real problem. So, the first thing he did was cut out all refined sugar, and the processed foods that often have hidden sugar in them. He went through his kitchen cupboards and threw out all the tempting snacks and products such as pasta. “I would avoid, slightly more controversially, a lot of starchy carbs,” he says. “I do have some brown rice and occasionally pasta when I’m out. If I have bread, I have it made with almond flour.

“I started to feel really, really good quite quickly. My sleep improved after about a week and a half.” Watson’s blood glucose levels were normal within a month – he was still on medication – and after three months, his long-term blood sugar levels were also within normal range. Two months ago,he came off medication. “I consider myself diabetic and a reformed sugar addict because I know that if I take sugar in again, the condition will come back. But I’m liberated. I don’t get tired, I don’t get the thing called ‘brain fog’ when your mental acuity is deadened a bit. All of that is gone. I feel more energetic.” When he went for an eye test, his sight had improved.

Around the same time as cutting out sugar, he started exercising, which wasn’t easy because he was so heavy. “I set myself little goals to start with. The first was I decided to use whatever stairs I came to, rather than the lift. I’d forgotten it was 36 steps to my office.” He laughs. “The first time I went up the steps, I felt I would probably need oxygen at the end of it.

“The office would always laugh at me because I would cling to the wall when I got to the top. I started monitoring my daily steps and tried to do 10,000 steps a day. It was quite hard and I would feel very tired after the first 5,000.”

He got a rusty bike out of the shed, had it refurbished and started to ride around London. In the park, he would attempt press-ups. “It was incredibly humiliating to start with. I looked pathetic. I remember trying to do a press-up on the side of a park bench and I could barely push two out without feeling lightheaded. I would be really tired to start with, but exhilarated at the same time.” In January this year, Watson joined a gym and started weight-training.

Watson in June 2016. Photograph: Ian Forsyth/Getty Images

“Nobody really noticed I was losing weight, but by February this year my suits started to hang off me,” he says. He now does two cardio workouts a week, such as running on the treadmill or doing 5km outdoors, or a boxing session; does weight-training a couple of times a week; and walks a lot. “I hate going to bed at night not having done 10,000 steps.”

Long hours and travelling can make it difficult when it comes to healthy eating, he says, but he gets around it by planning what he can eat and where. If he is on a train without many options, he will just eat the chicken out of a sandwich, and it is usually easy to find a bag of nuts to snack on. Does he miss cake and biscuits? “No. The first week, I was withdrawing from sugar and if you gave me a packet of chocolate Hobnobs I’d eat one, and then I’d have to eat the whole packet. Now I really don’t think about it. Sometimes I’ll have a little bit of dark chocolate. Once I came off sugar and was very clear about my physiology and the impact of it on my personal health, my resolution became stronger. I believe I am a sugar addict and if I eat it again I will get ill. My will is very strong on this; it doesn’t really challenge me.” He doesn’t count calories, but tries to loosely follow a diet with low-glycaemic-index foods – berries over bananas – and has even developed a taste for cauliflower rice.

Watson’s diet is very much self-created and, while it works for him, it may not work for everyone (nor is his admission that he often skips breakfast necessarily good advice). But he questions whether one-size-fits-all public health advice on nutrition works. “I think we need to move to an era where GPs are trained to give bespoke advice, and that’s a bit harder and requires quite a lot of change in the NHS.”

He says the food-labelling system in Britain “is absolutely broken. I hate the fact that people are judged for being overweight [when] there is a whole load of hidden sugar in processed foods.” How, he asks, does Coca-Cola get away with selling cans containing 35g of sugar “when the government is telling people the recommended daily limit is 30g. And that’s the adult amount. Kids are buying this stuff.”

Treating diabetes, he points out, costs 10% of the NHS budget – not just in medication, but for complications such as limb amputation, sight problems, kidney problems and stroke – and this is predicted to almost triple to £39.8bn a year by 2035/36, if trends continue.

“Apart from anything, that will sink the NHS. You look at the scale and you realise we’re in a public health crisis.” Watson points to the 26% rise, revealed this week by Labour research, in diabetes-related foot and toe amputations over the past three years. It is a tragedy, he says, when for many people the disease could be reversed.

“We’ve shown that type 2 diabetes is simply a disease of too much fat inside the liver and pancreas,” says Roy Taylor, a professor of medicine and metabolism at Newcastle University, who has spent four decades studying the disease. “If we manage to shift that, the diabetes will go away, provided the [condition] has not been there too long.” Watson’s success is a prime example, he says. “Had it not been dealt with, you would be looking at terrible possibilities of effects on eyesight, early heart attacks, even loss of limb. He has taken action and the good news is if he keeps his weight down, it won’t come back. That is something I can say from observing individuals over the last decade or more. It’s very clear that, provided people keep their weight down, what used to be thought about type 2 diabetes is not correct; everyone has been told it’s lifelong and steadily gets worse. But it doesn’t. You can switch off the basic problem.”

Such action is most likely to be successful for people, like Watson, in the early stages of type 2 diabetes, although Taylor says he did hear from someone who reversed the condition after 24 years. “But the chance of being able to do it steadily decreases with time, so it’s not a guarantee. In the first few years, probably nine out of 10 people will be able to do it, but after 20 years we’re probably looking at something like one out of 10 [people who are able] to get rid of it.”

The Diabetes Remission Clinical Trial (Direct) into a low-calorie diet, funded by Diabetes UK and led by Taylor, has results that look promising. Participants, supervised by doctors, were put on an 800-calories-a-day diet for about three months, losing on average about 16kg (2.5st), before going back to more “ordinary” eating, although with smaller portions and fewer starchy foods such as potatoes, rice and pasta. “Life goes on and we’re looking at helping people enjoy life, within the aim of keeping their weight steady,” says Taylor. Nearly half – 45.6% – of people were in remission from type 2 diabetes within 12 months.

Emma Elvin, a senior clinical adviser at Diabetes UK, says: “Some people with type 2 diabetes can find it useful to reduce the amount of carbohydrate they eat to help with weight loss, but it is important to note that carbohydrates are the main source of energy in a person’s diet, and can also provide important nutrients such as fibre.” She adds: “The amount of carbohydrate a person needs varies depending on an individual’s weight, activity levels, age and weight-management goals, so there is no hard-and-fast rule about the amount a person should eat each day.”

Elvin points out that drastically reducing calories can mean missing out on nutrients, and the low-calorie diet in the Direct study “was designed to be nutritionally complete. It is also important to note that Direct participants had the support of a nurse or dietitian both during the weight-loss phase, and while reintroducing normal food. It is incredibly important that anyone considering a low-calorie diet speaks to their GP before starting – a very low-calorie diet can be dangerous, particularly if someone is on insulin or taking certain medications.”

Should more people be on these calorie-restricted diets to control the disease? Taylor would prefer patients wait for the NHS to roll out a programme, but he adds: “What everybody ought to know is that the old idea of type 2 diabetes being a life sentence, and in many cases a death sentence, is not true and it’s possible for people to escape.”

For Watson, it has been life-changing. On holiday with his children this year, he was able to play in the pool with them for a couple of hours. “It was absolutely fantastic.”

Sugar is no longer a temptation. “I feel like I’m looking at a former life sometimes,” he says. Controlling his condition “has been such a significant thing in my life. Every day, I wake up happier and more relaxed, clearer-thinking. I can see there are millions of other people with this condition who may not have the knowledge or support networks, and I feel it’s my new mission to try to give people the joy of getting their lives back – like I’ve had.”