Find out about the singer's dramatic weight-loss transformation and what a nutrition expert thinks about her diet plan.

Adele recently posted a photo on Instagram standing in front of a beautiful floral arch and thanked everyone for wishing her a happy birthday. In the photo, Adele is wearing a form-fitting black dress, heels and a big smile—and looks fitter than ever. She reportedly lost 100 pounds by following the Sirtfood Diet and ramping up her exercise routine with cross-training and pilates.

People reports the singer's transformation was never about losing weight, but getting healthier for her son, Angelo. However, as Adele began cutting back on booze and upping her intake of whole foods, she began to feel more confident, energized and inspired to keep going.

Social media users can't stop talking about Adele's dramatic weight-loss transformation, leaving many wondering what the Sirtfood Diet is. (It's OK, we didn't know either.) We're also interested in knowing how healthy this diet plan is, and if there is any real science to back it up. Find out everything you need to know about the Sirtfood diet, below:

What Is the Sirtfood Diet?

The Sirtfood Diet is a weight-loss plan created by nutrition researchers Aidan Goggins and Glen Mattern. They were inspired by a 2013 study out of the University of Kiel in Germany that identified a list of "sirtuin-activating foods that show to promote optimal health." Sirtuins (SIRTs for short) are a family of proteins responsible for regulating functions such as lifespan, inflammation and metabolism, and research shows certain antioxidants found in plant foods could activate these proteins.

Goggins and Mattern set out to conduct research on their own, holding a small trial study of 40 members from the gym where they worked. These participants were put on a week-long diet of three green juices and a Sirtfood-centric meal every day. 39 of the participants lost an average of seven pounds in the first week "after accounting for muscle gain." The pair say the participants not only lost weight without losing muscle mass but felt more energized, experienced better sleep and had healthier skin. Goggins and Mattern say these "sirtfoods" will turn on your body's "skinny genes" and help you lose weight quickly.

This three-week diet plan has two phases. The first phase lasts seven days and is more calorie restricted with the intent of helping you lose seven pounds in seven days. Sirtfood dieters can only consume 1,000 calories for the first three days in the form of three juices and one meal. This meal can be chosen from The Sirtfood Diet book. Caloric intake increases to 1,500 calories on the final four days of phase one, where you drink two juices and eat two Sirtfood diet-approved meals.

Phase two is a two-week period of "maintenance," but dieters are still expected to lose weight during this time. One can consume three meals from The Sirtfood Diet recipe guide and one juice per day. Dieters are encouraged to either continue the cycle until they have achieved their goals or simply add more sirtfoods into their diet every day—including a green juice.

What Can I Eat on the Sirtfood Diet?

The 2013 study that helped Googins and Mattern create the Sirtfood Diet discovered 16 "sirtfoods" that promote longevity and overall wellbeing. The researchers from this study advise following a "MediterrAsian" diet plan based on the following foods for optimal health and longevity:

  • Tofu and other soy products
  • Wine and grapes
  • Strawberries
  • Apples
  • Persimmons
  • Shallots
  • Olive oil
  • Cruciferous vegetables
  • Beans
  • Onions
  • Capers
  • Green tea
  • Oily fish
  • Turmeric
  • Seaweed

Other sirtfoods that can be enjoyed on the diet include:

  • Bird's eye chili
  • Buckwheat
  • Celery
  • Cocoa
  • Coffee
  • Kale
  • Lovage
  • Medjool Dates
  • Parsley
  • Red chicory
  • Arugula
  • Walnuts

Some of the Sirtfood Diet recipes one can expect to eat include a miso-marinated baked cod with greens, strawberry buckwheat tabbouleh and shrimp and vegetable stir-fry with buckwheat noodles. Sirtfood dieters will also be drinking at least one Sirtfood Green Juice every day. This juice is made from kale, arugula, parsley, celery, green apple, ginger, lemon and matcha powder.

Is the Sirtfood Diet Healthy?

There are three major concerns with the Sirtfood Diet. While all of the "sirtfoods" on this list make healthy additions to your diet, there are plenty of other nutritious foods that aren't on this list. All fruits, vegetables, nuts and whole grains should be up for grabs—not just a select few. Other protein sources like lean meats, additional forms of seafood, eggs and dairy can also be part of a healthy weight-loss plan.

We're also not big fans of consuming 1,000 calories a day—even if it's for the short-term. Jessica Ball, M.S., R.D., EatingWell's assistant digital nutrition editor, says very restrictive diet plans like this that only allow for specific foods can be hard to follow. This is especially true if you're attempting this diet for more than a few weeks. It can also lead to intense cravings, yo-yo dieting and weight gain.

"Some of the rapid weight-loss claims made by dieters are likely due to water weight loss rather than actual fat loss," Ball says. "Any eating pattern should fit your lifestyle and not feel like a chore."

Finally, there just isn't enough research to back up this diet plan. While there is plenty of evidence to show the health benefits of the Mediterranean and flexitarian diets, we can't say the same for this one. These two diets are similar to the Sirtfood Diet in that they prioritize plant-based foods, but they allow more wiggle room for enjoying the foods you love (and aren't calorie-restricted). While we can get behind a diet that encourages eating more veggies and an occasional glass of red wine, we think Adele should choose an eating plan with a less-restrictive eating pattern.