A year ago, after I resigned from my Cabinet job as Brexit Secretary, I had a routine medical. The doctor frowned at my blood test results and recommended a liver scan.

His instincts were right. The scan showed that I had non-alcoholic fatty liver disease (NAFLD), caused by a build-up of fats called triglycerides that can progress to advanced liver damage. If I didn’t change my ways, I would be on the way to type 2 diabetes.

NAFLD can be almost imperceptible until it is too late. Many of our modern illness blights — metabolic syndrome, type 2 diabetes, heart disease, strokes — seem to have their origins in this silent killer.

I was not entirely surprised by the diagnosis. A government minister’s lifestyle is never healthy. Being driven everywhere rather than walking, long days without time for exercise along with endless dinners and cocktail parties made me put on 25lb.

So how to fix it? My first decision was to get rid of the bulk I’d put on. I set a target of losing that 25lb before Christmas.

I started an exercise regimen — a leisurely ten-mile run on alternate days, with 3½ miles of sprint intervals on the other days. It added about 40 miles a week. It failed miserably. I was constantly hungry and did not lose weight.

Before: A government minister’s lifestyle is never healthy. Being driven everywhere rather than walking, long days without time for exercise along with endless dinners and cocktail parties made me put on 25lb.

My next experiment was to cut out alcohol. Going teetotal lost me 6lb in two weeks. But the effect plateaued. Failure number two.

By then, I’d begun to study the science. I do not like dieting. I have almost no willpower with food. I’ve always viewed diets as failure-prone fads. But I’m a scientist by training and I scoured the internet for every paper on the effectiveness of weight-control regimens.

I thought I was familiar with food science. I’d spent most of my business career in the food industry, as an executive of the sugar giant Tate & Lyle. That was some 30 years ago. I had a low opinion of the quality of the food science back then.

So I WAS astonished to find that over the past couple of decades, vast numbers of high-calibre studies have been performed on the biology of food, digestion and exercise — and how metabolic diseases are caused.

I used the data to compare success rates of the two main regimens — low-carbohydrate diets and low-fat diets.

While both can bring benefits, going low-carb appears to be significantly more effective. The theory is that carbs, in the form of sugar and also starchy foods such as bread and pasta (which the body breaks down into sugars) trigger the release of insulin. Because insulin is a fuel-storage hormone, it makes your body build fat from the calories you consume.

There are lots of exotically named low-carb diets, such as the carnivore, Atkins, Pioppi, ketogenic and Warrior diets. On Saturday, the Mail launches its major low-carb series by NHS GP Dr David Unwin.

I cherry-picked my way through these to find an approach that suited me and this proved surprisingly easy to maintain. I had lots of carbs to cut, thanks to my sweet tooth. I had been taking four sugars in tea. Then there was my taste for potatoes, rice and bread. All the carby white foods had to go.

Cutting out carbs might have seemed daunting, but I did not feel tired from lack of energy nor did I feel hungry. My diet still allowed me to enjoy fats such as butter, cheese and meat, and fats and proteins are more filling and satisfying than the instant hits of sugar and refined carbs.

And because I wasn’t hungry, I didn’t find myself falling into the trap of substituting more calorific fatty foods to make up for lost carb calories. There was really nothing to test my feeble willpower.

I passed my target by 2lb the week before Christmas and could allow myself the luxury of turkey and roast potatoes, Christmas pudding and cake. And I felt pretty fit. My resting heart rate had always been low, at 48 beats per minute (compared to the male average of between 60 and 100) but it had dropped to 42.

After: A low-carb approach may not work for everyone but for those it does, the results can be pretty astonishing.

My wife — who never comments on my appearance — said that I looked a lot fitter than the previous Christmas. A large number of my friends and colleagues also noted my slimmer appearance.

A low-carb approach may not work for everyone but for those it does, the results can be pretty astonishing. I was more than happy with my own results but I was put in the shade by my old friend, the Labour deputy leader Tom Watson.

While I lost 27lb in five months, he lost 100lb after going low-carb in August 2017.

We had both succeeded using very low carbohydrate diets but our approaches were not identical: Tom starts his day with bulletproof coffee, a concoction of coffee and butter thrown together in a blender. I stick to more conventional zero-calorie black coffee.

Nevertheless, our views of public health policy are near identical. We both think it a disaster. For once, it’s no fault of the politicians. Much of the blame lies with Ancel Keys, an American researcher whose work purported to show that saturated fats cause heart disease.

He based this on inaccurate data, compounded by other scientific errors. It heavily influenced Western government health policy from 1980 onwards — causing the food industry to substitute saturated fats in processed foods with sugars and industrially produced oils.

Some fats are actively harmful. For every two per cent of your diet made up of trans fats, your risk of heart disease jumps by nearly a quarter. Unsurprisingly, many countries have banned trans fats, though not yet the UK.

The misconception about saturated fat — and the increase of carbs and sugar in the diet — was just one element of the health policy problem. The trend in metabolic ills — obesity, diabetes, heart and liver disease and strokes — has steadily accelerated. All have their roots in bad diet.

Non-alcoholic fatty liver disease: it is estimated up to 1 in every 3 people in the UK has early stages of NAFLD

More than half of all adults in the UK are overweight, and almost a third are obese or morbidly obese. This is strongly linked to NAFLD and type 2 diabetes. In 1996, 1.4 million UK citizens had diabetes; today, that is more than 3.5 million. The figure will rise to five million by 2025. Diabetes is a massive risk factor in heart disease, strokes, inflammatory illnesses, even some cancers.

Each of these illnesses costs the country billions and causes untold misery. If allowed to continue unchecked, in a generation the cost to the NHS will be mind-boggling.

However, it is, to a very large extent, preventable. We need to take action. The brilliant Professor Dame Sally Davies, Chief Medical Officer for England, recently announced a new review into childhood obesity. This urgent review will look at what can be done to meet the Government’s own target of halving childhood obesity by 2030.

It is not a moment too soon. It’s thought that ultra-processed foods (such as chicken nuggets, ice cream and breakfast cereals) could be targeted, alongside added sugar in baby food and high-calorie foodstuffs such as pizzas and cakes.

This shares much with the tax on sugar in soft drinks that was introduced in April last year.

It’s great to see that in fast food restaurants there is a range of zero-sugar soft drinks on offer. Moreover, since the tax was announced, 45 million kilograms of sugar a year have been removed from the national diet.

We need active measures for dealing with obesity in school-age children. This could take the form of summer camps to help children acclimatise to healthy foods and educational programmes for the parents.

We also need to have a really hard look at the science of manufactured food to discover the health effects of all the ingredients — and then start to design laws that will make producers formulate convenience products in ways that have far fewer bad effects.

But people are not going to stop eating processed food so I think that we need still more taxes on foods to deter manufacturers from producing things that put them in danger of developing diabetes and other metabolic diseases. I am opposed to the nanny state, but the alternative is stark. If we do not stop this growing epidemic of dietary diseases, the costs of the NHS will double in a generation, and millions of our youngsters will grow up to have shorter and more miserable lives.

And as for me? I’ve lost a significant proportion of the internal, visceral fat that sits around the abdominal organs and is linked to NAFLD. I lost 2in (5cm) from my waistline. And I feel pretty fit.

I am still the weight I was before last Christmas. Since I am 70 and not getting any younger, the issue now is what they call sarcopenia, the tendency to lose muscle mass as you get older.

The method for dealing with that is specific sorts of exercise but that, as they say, is a story for another day.