I will be 50 in a few months and like many middle-aged men I have started to worry about the declining state of my body. For most of my adult life, I have been in pretty good shape, looking trim in a suit and able to comfortably jog a few miles around the local park.

Despite being greedy, I have always managed to just about burn off all the ice cream and glasses of rosé. But over the last couple of years, the jogs have got a bit shorter — with me blaming a persistently bad back — and the calories have not reduced.

A few months ago, I stepped on to the scales and discovered I had gone over 12 st (76 kg). Then my 16-year-old daughter, Celia, called me ‘chubby’. This was at the dinner table as she tried to shame me as I picked off her plate. My wife did not demur.

The following day, football superstar Jude Bellingham was unveiled as a new model for the new range of men’s underwear. It was not any old pants brand, it was Skims, a company co-owned by reality TV star Kim Kardashian, and Bellingham looked pretty amazing. Women around the country gasped in delight, while men could only gaze enviously at a physique that looked as chiselled as Michelangelo’s David.

‘If only you stopped scoffing chocolate,’ my daughter said mockingly, ‘you could look like him.’

‘That’s not true. For starters, he’s only 21. And, more importantly, he’s a professional athlete with a team of coaches, nutritionists and trainers to ensure he is at peak physical fitness so he can do his job. My job is basically typing.’

Her comment made me think, however. Could I get a six pack like him at the age of 49? It seemed not only an impossible task but also impossibly vain, although I did know it would do me good to hit the gym properly. Doctors increasingly advocate that middle-aged people lift weights to build up muscle strength and bone density. A large-scale study in the British Journal of Sports Medicine in 2022 found that people who lifted weights aged 55 and over considerably reduced their chances of dying earlier.

Plus, so many middle-aged actors — from Brad Pitt, 60, to Jason Statham, 57 — have the gym-honed bodies one used to associate with professional athletes half their age.

Next month, I am off on a holiday to France we take every year with the same group of three other families. It has become completely unacceptable to comment on a woman’s physique or to even compliment her on getting ‘beach body ready’ — even among good friends — but for some reason it is still okay to say a man has a ‘dad bod’ or to joke that he has ‘eaten all the pies’.

Judging from the reaction to actor Theo James in a tiny pair of white Speedos earlier this month — and the ripped physique of Paul Mescal revealed in the trailer for Gladiator II — it is apparently also acceptable to shamelessly objectify men’s bodies.

Well, I was determined that come the first day by the pool I was not going to be the butt of anyone else’s ribbing. I was going to get a six pack. Before it was too late.

Week 1: Humiliation

I knew I could not do this on my own. So I enlisted the help of a company called Roar Fitness, a team of personal trainers who combine gym sessions with a diet to transform your body.

They were recommended by a friend, who had lost an astonishing nearly 4 st (25 kg) over 18 weeks and, crucially, looked great. ‘Abs aren’t made in the gym, they’re made in the kitchen,’ he told me at a party, saying he had never felt so good.

Sure enough, at my initial assessment, Matt Lindsay, 41, who is in charge of the nutrition aspect of the programme, laid out the ground rules.

I was to have three one-hour sessions a week with a personal trainer, but more importantly I was to eat a diet that consisted of just 1,750 calories a day, made up of no sugar (even fruit is not allowed), no gluten, no dairy, no caffeine and no alcohol — and lots of protein.

Surely, considering I am only a little bit ‘chubby’, I just need to convert my slight excess body weight into muscle, don’t I? I thought I was going to have to bulk up, not slim down.

‘Athletes are first and foremost lean,’ said Lindsay, 41, a former British speed skater, explaining why I needed to lose weight to look muscly. ‘The analogy I would use would be if you imagine something written down in black ink and I put ten sheets of tracing paper over the top. The ink is your muscles. If you take three sheets away, it’s a bit of blur. If you take away another two, you might see a bit more.’

In short, I needed to shed the tracing paper and lose fat. Otherwise, I would remain a blurry blob. Humiliatingly, my body fat was measured in the most old-school way possible: callipers.

‘It measures the subcutaneous fat, so essentially the thickness of your skin,’ Lindsay said, explaining it was the best way to track progress. Around my umbilical cord area, I had 26 mm of fat, around my supra-iliac — what a 1980s dieting advert once called ‘love handles that nobody loves to handle’ — I had 22.6 mm. And I weighed just over 12 st (76 kg).

Most of Roar’s clients sign up to an eight-week, 12-week or 18-week programme and Lindsay warned me that four weeks was too short a time to see any significant progress. ‘A diet should be a race, not a sprint,’ he said, but added if I stuck strictly to the regime, I might stand a chance.

Crucially, absolutely no alcohol must cross my lips. ‘Alcohol is the enemy of body transformations. It’s not just the calories — if you’re trying to get lean, that is taxing on the body. Adding another stress on to the body, you’re stacking the cards against you.’

Week 2: Hunger

My diet is not just calorie-controlled, it is designed to have the right amount of protein, fat and carbohydrates. The latter comes entirely from vegetables; I have no starches.

I am given a recipe booklet and can choose any of the breakfasts, lunches and dinners to cook. A typical breakfast is remarkably indulgent, such as a bacon and three-egg omelette; a salmon fillet with poached egg and avocado; even steak and spinach.

Lunch is very high protein and little else — a large grilled chicken breast with salad or mince with red kidney beans. Dinner is joylessly low-fat and consists of turkey mince meatballs with cauliflower mash or a small pork chop with steamed greens.

The food tastes fine. I am allowed to season my meals with as many herbs and spices as I like, but everything has to be measured out exactly, which is tedious. It is cleverly designed, however, because though I spend the whole time being low-level hungry, I never have cravings thanks to the high-fat start to the day.

Each morning I step on to the scales and can see the weight steadily falling.

The lack of carbohydrates, however, saps me of energy in the first few days. Earlier in the year, bullied by my children to take my fitness more seriously, I had signed up to a 10km race around Regent’s Park in London. It occurred on a very warm Saturday morning and even though I tried to take it as slowly as possible, I felt shattered after just the first few hundred yards.

By the end, my wife said I looked like the Italian marathon runner in the 1908 Olympic games, famously recorded on cine film staggering in a delirious zig-zag as he fainted over the finishing line.

Week 3: Dinner party

Though I am finding the diet hard work — the endless weighing, chopping, eating separate meals from all my family — I am starting to enjoy the personal training. All my sessions are in the gym, weight lifting.

I am assigned Luke Grahame, an impossibly young-looking 42-year-old father of two, who has a gentle manner despite being a former professional heavy metal guitar player. At no point does he raise his voice or sound disappointed in my efforts. However, at the start he did make it clear I was fairly clueless about lifting weights.

I thought I could easily lift 12 kg in each hand doing a ‘bicep curl’, when you lift your arm up and down by the side of your body holding a dumbbell. I was deluded.

I had been relying on momentum and widely swinging the weights with almost zero effect on my muscles. When I do it properly — as in, slowly and calmly — I can barely manage 7.5 kg. He teaches me how to position myself, how to ensure I don’t damage my back or exacerbate a trapped nerve in my shoulder and gets me to steadily push myself harder, logging each session meticulously on an app.

In the mirror each morning, I can see I am making progress. Is that a faint abdominal muscle I can see there?

The downside is going to a couple of parties during my project — including a friend’s 50th birthday — and being stuck on nothing but fizzy water. I may wake up without a hangover, but my wife claims I have become grumpy and am losing my temper more often.

A particularly low moment is when I’m invited to dinner with some close friends. I am reduced to bringing my own freezer box of mince, which I munch on while everyone else tucks into lasagne and views me with pity.

Week 4: Chest wax

Grahame has moved me on to properly big weights. Without really realising it, I’ve become significantly stronger. With a ‘single-arm row’ — where you kneel with one leg on a bench and lift a dumbbell to your chest —– I have gone from lifting 12.5 kg at the start to 32.5 kg.

I am also now allowed a small amount of carbohydrates with my lunch, such as 70 g of brown rice or a roasted sweet potato. This makes me less exhausted and grumpy. My wife even starts to parade me around like a prize heifer, trying to get me to display my new abs.

To really go the full Jude Bellingham, however, I have decided I need to embrace the Gen Z aesthetic: a waxed chest. My wife is appalled, but warns me I need to splash out on a hot wax, rather than a more painful strip wax.

Wise advice. Eva, at Strip, my local waxing salon, says that she’s depilating far more men.

‘Having no hair is fashionable at the moment. It’s like going to the gym and getting buffed up,’ she says, expertly stripping the wax off my stomach while watching The Real Housewives Of New Jersey on the television screen.

It is mostly painless, except when she yanks the hairs around my nipples, which are tender for the next few days.

The results

After 12 gym sessions and four weeks of lots of chicken breast it is time for my final weigh-in and calliper measurements with Lindsay. I have lost nearly a stone (6 kg), but more importantly the callipers show that the fat around my ‘love handles’ has halved from 22.6 mm to 11 mm, and nearly halved around my belly button, from 26 mm to 14.8 mm.

‘I think this is really impressive, considering the time frame,’ says Lindsay. He points out that even the swelling around my knees has gone down. ‘That’s the benefit of an anti-inflammatory diet.’

Looking at myself in a mirror, I am surprised by how much I enjoy seeing muscles. The project was initially prompted by vanity. But I don’t think there is any shame in wanting to look better if the side-effect is having made myself stronger and healthier.

Will I keep up the regime? I enjoy alcohol and food too much to switch full-time to fizzy water and turkey mince. But I am determined to keep up with the three times a week gym sessions, now that I know what I am doing. As Grahame told me after my final stint with him: ‘If you can keep up 10 per cent of the habits, I will have done my job.’

• To learn more about Roar Fitness and their eight to 12-week body transformations visit roar-fitness.com