Prince Harry appears to be cutting a sharper, slimmer figure in the run-up to his wedding and speculation is rife that Meghan might have nudged him towards the London gym much loved by her future sister-in-law’s sister, Pippa.

He has been spotted coming in and out of KX (pronounced ‘kicks’), the exclusive Chelsea gym where Pippa famously trimmed and toned her slender frame in preparation for her wedding last year.

It could be the top-of-the-range gym equipment, expert personal trainers and disco-themed spinning studio that are transforming Harry’s beefy, beery body, but it’s very likely Meghan has put him on the special green juice diet that KX recommends.

The diet is the brainchild of nutritionists Aidan Goggins and Glen Matten, who have dedicated their careers to investigating the powerful natural properties of certain foods. Their ideas hit the mainstream when they were enticed into working together with KX gym.

Their aim was to devise a ‘fast-track’ diet that would guarantee maximum athletic performance and speedy weight loss. The result was the Sirtfood Diet book, which hit the bestseller lists when released in 2016.

The KX ethos is so Sirtfood-entrenched that much of the on-site restaurant is ‘sirtified’ with on-message Sirtfood salads (£20), green juice (£12) and energy bites (£3.50).

Prince Harry (pictured last month, opening the Greenhouse Sports Centre in London) is cutting a slimmer figure in the run-up to his wedding later this month

Despite Pippa’s obvious success with the regime, there’s nothing girlie about the Sirtfood Diet. Chocolate, coffee and red wine are allowed, and, unlike the intermittent fasting regimes so popular with many men, there’s no requirement to go hungry.

Goggins recommends the diet as a ‘performance enhancer’ for premiership footballer clients, top rugby players and Sir Ben Ainslie’s America’s Cup sailing team.

However, the diet’s creators have come under fire from dietitians for devising a regime that strays from government advice.

The initial seven-day kickstart suggests green juice as a meal replacement, which goes against the Food Standards Agency’s balanced EatWell Guide plate or the food pyramid that NHS dietitians typically fall back on.

But Goggins and Matten indignantly point to their eminent qualifications: both have a masters degree in nutritional medicine. Goggins is also a qualified pharmacist.

They hit upon the idea of Sirtfoods after following up on an investigation into the life-enhancing properties of a plant chemical called resveratrol, found in grape skin and red wine, which scientists speculated could hold a key to longevity.

The royal, 33, (pictured in 2015) has been spotted coming in and out of exclusive Chelsea gym KX, and may have been on the special green juice diet that the gym recommends

The duo discovered that other foods — such as walnuts, capers, red onions and dark chocolate — contain specific natural plant chemicals that appear to be able to activate genes in our bodies.

These genes are known as sirtuins — so Goggins and Matten named the foods with the most abundant natural chemical profile ‘Sirtfoods’.

Each Sirtfood provides a different combination of phytonutrients and supposed health benefits, but the pair found that combining Sirtfoods appears to enhance their active powers.

‘One food might contain a chemical which helps prevent fat formation and another helps increase fat disposal,’ explains Goggins, ‘you could eat a whole field of kale, but get nowhere near the benefit that a Sirtfood-loaded meal would bring.’

The authors were so excited about the proposed effects on blood sugar levels and fat metabolism that they agreed to a trial into the health impact of a seven-day intense boost of Sirtfoods.

Goggins says: ‘The trial was designed to measure how the diet stimulated rejuvenation and cellular repair.

Pippa Middleton trimmed and toned her slender frame at the KX gym in preparation for her wedding to James Matthews (pictured together) last year

‘We added a degree of calorie restriction, because studies show it is good for longevity, so we knew people would lose weight, but we had no idea the average weight loss would be half a stone.’

That’s one big reason the Sirtfood diet appeals to men.

‘The dieting culture has got stuck in a rut, with experts advocating eating less and less.

‘Suddenly, everything that you enjoy seems to be bad for you. People are getting fed up with conflicting messages and confusion. It’s such a shame,’ says Goggins.

‘When giving up food is routinely offered as the “solution”, surely something has gone very wrong.

‘Our seven-day plan is an optional kickstart. The most important message is to be adding Sirtfoods to your existing diet — the benefits come from the foods you eat, not from foods you have to avoid.’

THE DIET

What you should eat:

Buckwheat

Capers

Celery (as green as possible, eat the leaves, too)

Chilli (ideally bird’s eye or Thai chillies)

Dark chocolate (85 per cent cocoa solids)

Coffee (drink it black to get most nutrients)

Extra virgin olive oil.

Green tea (ideally matcha, but ceremonial grade is not necessary)

Kale

Garlic or lovage (available from garden centres, or buy seeds)

Medjool dates

Parsley

Chicory

Red onion

Red wine (ideally Pinot Noir, which contains more resveratrol than other grapes)

Rocket

Soy (miso paste)

Berries (cherries, strawberries, blackberries, blueberries, raspberries)

Turmeric

Walnuts

You should drink your coffee black as part of the diet, in order to get the most nutrients

FAST TRACK

This is the ‘hyper-success’ phase which promises you’ll lose 7lb in a week and significantly boost your health. It combines fasting with a super-boost of Sirtfoods. Repeat every three months.

DAYS 1-3

3 green juices

1 main meal (packed with Sirtfoods)

Max 1,000 cals per day

DAYS 4-7

2 green juices

2 main meals

Max 1,500 cals per day

Spread the juices throughout the day, and drink an hour before or two hours after meals.

No eating after 7pm.

Drink plenty of water, black coffee, green tea.

No alcohol (apart from in cooking) or fruit juice.

20g of dark chocolate (85 per cent) per day.

Berries, including strawberries, blackberries, blueberries, raspberries, are among the recommended foods

MAINTENANCE PHASE

Consolidates weight loss, at 1lb per week.

1 green juice per day (30 mins before breakfast or mid-morning).

3 meals per day packed with as many Sirtfoods as possible.

One glass of red wine with a meal 2-3 times per week.

No processed meat (sausages, salami, burgers).

Limit red meat to 500g a week.

Eat wholegrains.

‘Sirtify’ your meals.

Evening meal no later than 7pm.

DOES IT REALLY WORK?

Would the Sirtfood diet perform its slenderising miracles on my 54-year-old graphic designer husband, Jonathan Woods?

If we could both lose half a stone while still eating chocolate and drinking red wine, I was sure he’d be willing to give it a try.

So I added the 20 top Sirtfoods to my weekly shopping list (plus a cheap-as-chips juicer because you can’t do this diet without one). I struggled to find lovage — a nettle-like salad leaf. True Sirtfood fans grow their own from seeds, but green celery leaves are nutritionally close enough.

If you’re new to juicing, be warned, stuffing handfuls of kale, rocket and parsley into a narrow chute is a messy task. Repeatedly re-juicing the resulting pulp (as advised) to extract the powerful sirtuins sends things off the messy scale. I’m sure Harry will be buying his green juice at KX or getting Meghan to make it because my husband flatly refused to get involved in this daily mucky ritual.

Recipes to slim down your man THE GREEN JUICE Makes 1 serving 2 handfuls (75g) kale

Handful (30g) rocket

5g parsley

150g green celery (2-3 stalks), plus leaves

½ green apple

Juice of ½ lemon

½ level tsp matcha powder The green juice contains kale, matcha powder, green celery and half an apple Juice the greens, then the celery and apple and squeeze in the lemon. Makes 250ml juice. Add the matcha to each glass (whisking to mix it well) just before drinking it. Top up with water to taste. SIRTFOOD BITES A guilt-free treat made from Sirtfoods if you’re peckish between meals. Eat 1-2 per day. Makes 15-20 120g walnuts

30g dark chocolate (85 per cent)

250 Medjool dates, pitted

1 tbsp cocoa powder

1 tbsp ground turmeric

1 tbsp extra virgin olive oil

1 tsp vanilla extract

1-2 tbsp water A guilt-free treat made from Sirtfoods if you’re peckish between meals. Eat 1-2 per day Blitz walnuts and chocolate in a food processor. Add all the other ingredients except water and blend until the mixture forms a ball. Add water drop by drop as needed. Roll into bite-sized balls (cover in cocoa or desiccated coconut if you like) and refrigerate in an airtight container for an hour before eating. They keep for a week in the fridge. SUPER SIRT SALAD Serves 1 The super sirt salad contains avocado, celery, capers and walnuts 50g rocket

50g chicory/ radicchio leaves

100g smoked salmon slices/tinned tuna/cooked chicken or 100g tin of cooked green or Puy lentils

80g avocado, peeled, stoned and sliced

40g celery, sliced

20g red onion, sliced

15g walnuts

1 tbsp capers

1 large Medjool date, pitted and chopped

1 tbsp olive oil

Juice of ¼ lemon

10g parsley, chopped

10g Lovage or celery leaves, chopped Place salad leaves on a plate, then mix all remaining ingredients and serve on top. Advertisement

It takes a full bag of kale, a bag of rocket, a bag of parsley and a whole head of celery with an apple, lemon juice and matcha powder to produce four small bright green glasses of juice. Ten minutes to make it, 15 minutes to clear it all up.

The juice, which fans call ‘rocket fuel’, is something only the truly committed could honestly enjoy. It’s like liquidised salsa verde: bitter, piquant and best knocked back like a shot of tequila than a soft lovingly-sipped smoothie.

Jon took one slug and immediately spat it out, muttering: ‘Disgusting!’ But no pain, no gain, so he persevered (grumpily), swilling his mouth afterwards.

For the first three days, you must survive on three glasses of juice and one meal (albeit enhanced by Sirtfoods).

Without a personal trainer snapping at his heels, Jon found the process tough. But he said the acrid bitterness of the juice made it an effective appetite suppressant.

It turns out the key to ‘sirtifying’ any meal is no more complicated than throwing in as many of the top 20 Sirtfoods as you can fit on the plate.

So a Sirtfood salad is like any other salad except you add radicchio leaves with rocket, parsley, walnuts, dates and scatter it with tiny capers. We both agreed it is delicious — a cornucopia of health-giving foods in a bowl.

Coffee is a Sirtfood, so that helps ease hunger pangs, and a little dark chocolate is allowed. Plus, the matcha green tea powder you stir into the green juice has a caffeine-like effect which keeps you buzzing. And, let’s face it, the 1,000 calories a day limit is still more than the 600-800 you’re allowed on a 5:2 Diet fast day.

On day four, the protocol changes to two daily juices and two Sirtfood-enhanced meals (up to a 1,500-calorie max), so life did begin to ease back to a healthier version of normal.

Already we’d both noticed our clothes were a little looser. At the end of the week we were thrilled to find we had both lost the promised half-stone and — joy! — much of it from our middle-aged middles.

After this kickstart you are advised to move on to the ‘maintenance phase’ of the diet (explained above) where you whizz up one green juice a day and pack as many Sirtfoods into your normal meals as you possibly can.

We still eat a lot of kale (rendered palatable when roasted in salt and olive oil or tossed in a pan with butter and garlic) and I make a batch of green juice most weeks.

I sprinkle berries and walnuts over my breakfast, substitute buckwheat for many rice and flour-based dishes (buckwheat pancakes, buckwheat risotto) and the only onions I buy are red (not white).

Jon’s diet is marinated in Sirtfoods whether he likes it or not, but I can’t persuade him to join me in that juice. He says that it’s a Sirtfood step too far, and he’s convinced he’s getting all the Sirtfoods he needs in his glass of red wine.

Recipes and text taken from The Sirtfood Diet and The Sirtfood Diet Recipe Book, by Aidan Goggins and Glen Matten, both published by Yellow Kite.