The 17/20 nutrition principle is the simple approach a Hollywood trainer uses to get A-listers in shape

• Magnus Lygdbäck is a personal trainer and nutritionist who's worked with Gal Gadot and Ben Affleck.
• He has a unique, simple approach to healthy nutrition, which means no restriction or food guilt.
• Every 17 out of 20 meals should be "on point" — the other three can be whatever you want.

Magnus Lygdbäck is the personal trainer and nutritionist responsible for some of Hollywood's biggest stars' physiques.

The Swedish coach trained Alicia Vikander for "Tomb Raider," Gal Gadot for "Wonder Woman," and Ben Affleck for "Justice League." He's worked with Alexander Skarsgård, Katy Perry, and Harry Styles.

As well as putting A-listers through their paces in the gym, Lygdbäck ensures his clients are eating right for their goals, and his approach to nutrition is unique and refreshingly simple.

It's called the 17/20 system, and involves no calorie tracking, no forbidden foods, and no extreme restriction.

Every 4 days, 3 meals can be whatever you want

Lygdbäck's food philosophy says that 17 of every 20 meals should be "on point" - the other three can be whatever you want to eat.

By on point, Lygdbäck means that ideally those meals would be made up of "a good protein source, good fats, and slow carbs, and vegetables." Slow carbs are complex carbs, such as oats, rice, whole-wheat bread, and potatoes.

And for the other three meals, you "enjoy life."

"It means you can have pasta, you can go out with your friends, and you can enjoy a dessert or a glass of wine," he said.

Lygdbäck works in four-day cycles of five meals a day (three meals and two snacks), which means that every four days, your 20 meals start over again.

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Portion your meals in fistfuls, not calories

When working with actors getting in shape for big film projects, he uses calories and macros (protein, carbs, fat), but doesn't think counting calories is necessary for most people.

For those who want to track, he recommends focusing on macros, aiming for between 30% and 40% of total food intake to be protein, and the rest a blend of carbs and fat.

But Lygdbäck thinks an easier approach is to keep portion size in check using your hands as a guide.

"For lunch and dinner, I do a fistful of protein, a fistful of fat or carbs or a combination, and two fistfuls vegetables," he said.

It's not a one-size-fits-all approach, but a starting point, and you can adjust based on your goals (fat loss or muscle gain), body type, and activity levels.

The approach is designed with 'happiness and balance' in mind

While some celebrity trainers and transformation coaches tell people to cut their calories drastically and give up all their favorite foods, Lygdbäck takes a more sustainable approach.

"There are so many diets and so much misinformation out there, so people don't really know what to do," he said. "I see too many people taking shortcuts to get something they want and in the process doing the wrong things and they're miserable.

"So I just think that we need to work much more on balance and happiness, and that's why I developed my system."

A post shared by Magnus Lygdback (@magnuslygdback)

It's the system he's been using himself for two decades.

"I love food, I love a good glass of wine, I think that we should enjoy food as an important part of life," Lygdbäck said. "I don't believe in restricting, taking out foods, and telling people they're not allowed to eat something."

Read more: 7 dietitian-approved high-protein breakfasts that have helped me lose fat and gain muscle

The idea is that you cannot fail and should never feel food guilt.

"I hate when we have a guilty conscience after eating something," he said. "It's so easy to walk around and feel bad for eating food that's good. I want to get rid of that feeling entirely."

How strict you are is up to you

What would Lygdbäck say if someone were to have four rather than three more indulgent meals in 20?

"I wouldn't beat myself up about the past, and I'd focus on the fact that I had 16 meals that were on point - that's pretty amazing," he said. "It happens."

Rather than feeling the need to compensate in the next four-day cycle, you just carry on.

"The system is not there to punish you. It's there to provide you with structure without forcing you to eat certain things or take out foods," Lygdbäck said.