After Adele revealed a transformation on Instagram, fans began buzzing about the singer’s weight loss.

For years, it’s been rumored that the star follows a meal plan based on the book “The Sirtfood Diet.” The regimen includes multiple phases and focuses on “cellular wellness at a genetic level,” according to Aidan Goggins, one of the creators of the diet. The Duchess of Cambridge’s sister Pippa Middleton and boxer Connor McGregor have also been said to use the diet.

“The Sirtfood Diet,” by Aidan Goggins and Glen Matten

According to Goggins, the Sirtfood diet was created as “the antidote to conventional dieting.” He and his partner, Glen Matten, were both “diet skeptics” because of their history as nutritional medical scientists. Instead of focusing on weight loss, the Sirtfood diet is based on activating sirtuin genes. Sirtuins are proteins that are members of a family of enzymes. They activate and are typically utilized in response to stress and metabolism, explained Kristin Kirkpatrick, a registered dietitian.

“The weight loss is not the primary goal, but as a consequence of rejuvenating our cellular wellness at a genetic level, which essentially resets our metabolism,” said Goggins. “Secondly, in contrast to other popularized diets where the focus is on cutting out foods, with Sirtfoods we can only reap the benefits through eating, and this means indulging in your favorite foods, not restriction.”

Kirkpatrick, who works with the Cleveland Clinic in Ohio, said that she’s had patients come to her asking about the Sirtfood diet multiple times, and while there is some science to support the effects of the diet, she said many of the studies and research are very new.

“I think the diet is harmless, but we don’t have data showing that it’s got long-term sustainability,” she said. “The whole theory behind the diet is that certain foods are going to activate these sirtuins, which are related to proteins in the body.”

Kirkpatrick noted that the Sirtfood diet operates similarly to intermittent fasting diets, which have also been shown to help with weight loss.

Goggins said that when he and Matten worked in a high-end private health club in London, they became “increasingly concerned” about “increasingly extreme diet trends” and the foods that people “villainized.”

“You’d be afraid to eat,” he said. “And it all became about reducing calorie intake.”

When creating the diet, they wanted to make sure that people felt free to eat some of their favorite foods while still getting important nutrients.

“It is important that we not only get plenty of these foods in our diet but also ensure our meals contain a diverse array of them as it is the synergy of their combination in meals and juices where the real benefits come from,” Googins said.

The real goal of the Sirtfood diet is to pack one’s meals with as many “sirtuin-activating nutrients” as possible. In lab settings, sirtuin genes have been activated, but Kirkpatrick noted that despite a pilot study by Goggins and Matten that analyzed 40 people on the diet, there haven’t been any other studies in humans.

“Sirtfoods are all readily available and accessible plant foods,” Goggins said. “Some of the top Sirtfoods include leafy greens like arugula, kale and parsley, strawberries, walnuts, extra virgin olive oil, dark chocolate, curry spices, green tea, red wine and coffee.”

On the website for the diet, Goggins and Matten recommend trying to “Sirtify” meals where possible and adding Sirtfoods to favorite meals or substituting regular ingredients with a Sirtfood alternative.

Kirkpatrick praised the food choices the diet recommends, but said that there’s nothing showing that the foods in the diet will trigger the genes as advertised.

“We don’t know if these very specific foods, if you eat just these foods and nothing else, that these things will be triggered,” said Kirkpatrick.

The diet has two phases. For the first three days on the diet, one should consume “three Sirtfood green juices and one full meal rich in Sirtfoods” daily, for a total of just 1,000 calories per day.

On days four through seven, people should increase their intake to 1,500 calories a day by consuming “two green juices and two meals.” According to Goggins, people lose 7 pounds in seven days during this phase, though Kirkpatrick noted that anyone eating just 1,000 calories per day would see weight-loss effects no matter what they were consuming.

The second phase lasts for two weeks and is called a “maintenance period.” Intended to help people lose weight steadily, dieters can eat “three balanced Sirtfood-rich meals every day, plus one green juice.” The two phases can be repeated “whenever you’d like for a fat-loss boost.”

Kirkpatrick said that the phases of the diet made her unsure of its sustainability.

“The actual foods are good,” said Kirkpatrick. “The first phase is very much dependent on green juices and ‘No more than 1,000 calories’ … Anything with 1,000 calories and pretty severe caloric restriction is going to lead to weight loss, but the body will bounce back. The body’s searching for alternative fuels. Once you get into those later phases and you start relaxing the motivation you had in weeks one and two, you’re more likely to gain weight.”

However, Goggins said that once the phases have ended, the diet focuses more on what you’re eating instead of how much.

“While the initial one-week phase has a calorie restriction, there is no counting for the maintenance,” he said. “We now eat as we always were intended, and did throughout history.”

Kirkpatrick advised combining the positive parts of the diet, like its focus on green foods, and mix it with other things known to be tied to weight loss, like stress management, sleep and exercise, and things like eating in moderation and trying to adhere to a healthier eating plan overall.