Gabriela Peacock, who counts several of the young royals as clients, believes that even the smallest of changes can have huge results.

“Biochemistry is so powerful — you can really change someone’s well-being in just a week,” she tells PEOPLE, but adds, “It has to be easy otherwise it’s not sustainable.”

That’s the premise behind her new book 2 Weeks to Feeling Great, which advocates for intermittent fasting, supplements and minor tweaks to what you eat and when you eat it.

Beatrice, who is godmother to Peacock’s daughter Iris, is such a fan, she has even endorsed
the book saying it’s principles “are effective and realistic guidelines to follow no matter what you are doing in the day.” Peacock says the royal mom-to-be, who announced her pregnancy last month, has never looked better. “She looks amazing, just super happy, and they are all fantastic — it’s great news!”

Though Peacock hasn’t put the princess on a special maternity diet regiment, she says that, “like all friends, I try and be helpful, no matter what kind life changes they go through.” She adds, “I talk about nutrition, all the time, so I think it slightly rubs off on my friends!” Can’t get enough of PEOPLE’s Royals coverage?

The Czech-born nutritionist, who studied for more than nine years before working with Queen Elizabeth’s physician Sir Tim Evans, said she felt compelled to write the book after she could no longer keep up with client appointments during the first lockdown last year.

Peacock advocates intermittent fasting — whether the 4:3 approach (eating 500 calories on alternate days with one blowout “magic day” for treats) or the 16:8 plan (which involves fasting for 16 hours overnight).

She says it has been scientifically proven to be beneficial to the whole body, not just for weight loss: “It’s anti-inflammatory, it prevents cardiovascular diseases and it’s anti-aging, which is fantastic. And it really works for my clients’ lifestyle.”

Aside from fasting, the nutritionist is a firm believer in trying to balance blood sugar levels. “Never eat fruit on an empty stomach because it will spike your blood sugar levels, as will coffee,” she says. “It’s fine to have them, but eat with a portion of protein too.”Dubbed “the royals’ secret weapon,” she also believes results can be achieved quickly with the right attitude and supplements (she even has her own brand, GP Nutrition Supplements).

Peacock encourages protein-based snacks such as yogurt with nuts and seeds, nut butter and apples or a protein bar for an afternoon snack, setting you up for a healthy dinner, which should be your lightest meal of the day.

And since Peacock herself has three children, her book — which contains healthy-yet-indulgent recipes such as flourless chocolate chip cookies and “cheats pizza” made with corn tortillas — is all child-friendly.

She would advise mom-to-be Beatrice or second-time dad Harry to start their children on the healthy-eating track as soon as possible: “Introduce them to different flavors and vegetables at a young age, and if they don’t like something, chop it up very small and mix it into something like brown rice and chicken, then gradually make the chunks bigger.” But occasionally, “Rewards are the way to do it.

My kids are perfectly bribable with dried mango, oat biscuits and then sugar-free lollies from Whole Foods if you are really desperate!” And she follows one key rule of thumb for both clients and friends, young and old: “It’s all about balance — life is for living!”