Gilles Souteyrand arrives at our photography studio in northwest London at 9am on the dot. Last night he was out celebrating his birthday over a seven-course meal at one of the city’s most famous restaurants with his girlfriend, yet he’s looking as fresh as a daisy.

As he whips off his T-shirt for the test shot and reveals his lean, muscular physique, he mentions it was a “big birthday”. He looks great for 30, I tell him. “No, it was my 40th,” he says with a smile. I assume something’s got lost in translation, even though the Frenchman has lived in London for the past 20 years – but no, I heard him right. He’s 40, and the assembled crew can hardly believe it.

I want to pepper him with questions to find out the secrets of his age-defying body, but first we need to get the cover shot in the bag. It doesn’t take long. As you might expect of a model of 20-plus years’ experience and the creator of the Body By Gilles fitness and nutrition plan, he aces it with the perfect pose within the first few frames.

Which is good news for our photographer’s index finger – and even better news for me, because it gives me more time to quiz him on how he (and you and I) can slow the advance of Father Time.

So, Gilles, what’s your secret? Is there a picture of you in an attic somewhere?

[Laughs] There is no secret! I just love training well and eating well. I can’t imagine a life without training: I do it every morning at 7am. After some lemon juice, I stretch for ten minutes then do a 30-minute workout, never any longer. If it lasts any longer I haven’t been working hard enough. I work out at home so I use mainly bodyweight moves with some kettlebells, TRX and resistance band work thrown in. Ring pull-ups and dips are just brilliant for making all your muscles work together, through all the planes of movement.

So you never hit the heavy weights?

No. You can build a lean and muscular physique without heavy weights, using just your bodyweight as resistance. Gymnasts are the perfect example: floor, bars, vault, everything bodyweight – and they never bench press but look at their physiques! They are carved out of wood, hardly any fat, super-flexible and mobile, with a huge range of motion. Fantastic!

RECOMMENDED: Olympic Gymnast Nile Wilson’s Bodyweight Workout

What benefits does bodyweight training offer?

There are unlimited movements and you work your entire body in millions of ways – the only limit is your imagination. They allow you to be inventive, challenge yourself, build strength and muscle mass… You can do them any time and any way, and you don’t need a lot of knowledge to start. It allows you to move as nature intended, moving your whole body as a single unit. There’s no switching off of muscles, which can happen on resistance machines, and you have to maintain tension and squeeze your muscles hard, otherwise you can’t do the moves. It means your core is constantly challenged so you build a fitter, more functional body.

Are flexibility and functionality more important to you now you’ve hit the big 4-0?

Flexibility is hugely important and I work on it a lot. Mobility too. What’s the first thing cats and dogs do after waking up? They stretch and we should too, especially because we spend so many hours a day sitting down, hunched over and tense.

And that’s the theory behind your new Body By Gilles training plan?

Absolutely. It’s a 12-week plan you can download as a PDF, designed to help men and women transform their bodies for the better. It’s three high-intensity workouts per week, done on Mondays, Wednesdays and Fridays, and on the other days – except Sunday, which is a rest day – you do your own form of low-intensity workout. This change between high and low intensity keeps your body guessing and burning more fat. There’s also some direct abs work to help define the core.

EXCLUSIVE: Souteyrand’s 10-Minute Bodyweight Abs Finisher

What’s your approach to nutrition? And has it changed as you’ve got older?

As we get older, our metabolism slows down, so portion control becomes very important. Each meal I have is one fist-sized amount of protein, one of unprocessed carbs and one of vegetables. Use as much fresh produce as possible and cook it yourself – learning to cook gives you control of your diet.

My personal approach to nutrition, and the one I use in my plans, is that it should not be about restriction but about balance. Do treat yourself, but don’t turn a cheat meal into a cheat day. In my guide there are 52 recipes for breakfast, lunch, dinner and snacks that help you come in at 2,000 calories per day. The approach is healthy but not restrictive, and emphasises the importance of portion control.

Your Instagram account suggests you travel a lot. How do you find the time to eat well and train when you’re almost constantly on the move?

I live in London but my girlfriend [Instagrammer and Bikini A Day founder Natasha Oakley] is based in LA, and she has to travel a lot for work. So we have to be very organised if we want to spend any time together! You have to find the time for what’s important to you, so I make the time to train. That’s the main thing I’ve learnt from Tash: if you are organised, you can do pretty much anything you want and if you want success you have to constantly work at it – whether that’s business or fitness. Find something you’re passionate about and then never stop working until you get it.

Natasha’s day job requires her to spend a lot of time in bikinis… does she follow your plan?

Yes! But I have never trained her. She is a very busy woman, so instead of training her myself I just let her get on with it!

Gilles Souteyrand’s Health Tips

Follow our cover star’s advice to start building your best-ever body

Get back to basics

“If you are new to training or coming back from a spell on the sidelines there’s no better option than doing bodyweight training,” says Souteyrand. “Using just your bodyweight as resistance better enables you to understand how your muscles are supposed to move, and teaches you to keep your joints aligned, your core tight and your muscles always engaged, which will lay a fantastic foundation to build upon – whether that’s more advanced bodyweight training, like TRX or kettlebells, or moving on to dumbbells, barbells and other heavier gym kit.”

Move like a unit

“The only equipment I ever use is are kettlebells, TRX and resistance bands, which are basically more challenging and advanced versions of bodyweight training, in that they still require your body to move and perform as a combined unit, which isn’t the case with weights or resistance machines. I’ve never really been into heavy weights and have always preferred more functional training. And this approach really can build a lean and defined physique without overly stressing your joints and other connective tissue.”

Eat with your head

“I spend a lot of time travelling and recently spent a whole month in Italy. And I love Italian food! But my approach is simple and it’s something anyone can adopt – I eat with my brain, not with my eyes. If I have a huge bowl of pasta for lunch, I simply eat a smaller dinner, such as a chicken salad or fish and vegetables. Balance is important, but so is not allowing one big meal to turn into a big day, or big weekend or big week! And always try to eat fresh produce whenever possible – it will be higher in essential vitamins and minerals, and it’s almost impossible to overeat.”

Body By Gilles is available for download now, $69.95 (around £54). Visit bodybygilles.com