Chris Pratt is one of Hollywood’s reigning kings of physical transformations (second to Christian Bale’s numerous body modifications). He’s no stranger to trying out different diets, but his latest one is taking him to church—literally. Pratt decided to start 2019 off Old Testament-style, revealing on his Instagram story that he was three days into “The Daniel Diet” on January 3.

“OK, hi, Chris Pratt here. Day three of the Daniel Fast, check it out, it’s 21 days of prayer and fasting,” he said in a video, according to People. He went on to say how this extreme fasting may impact his upcoming press tour. “It’s gonna coincide also coincidentally with the Lego Movie 2 junkets, so by the time you see me, I’ll probably be hallucinating. Stay tuned,” he said.

During a Feb. 8 interview on The Late Show with Stephan Colbert, Pratt also announced that he had just finished his fast and felt “actually amazing.”

The diet is based on the Book of Daniel in the Old Testament, and involves 21 days devoid of animal products and preservatives, and inclusive of fruits, vegetables, whole grains, legumes, nuts, and seeds, according to the U.S. National Library of Medicine.

In the Bible, Daniel limited his diet to vegetables and water to be able to cleanse his body and better focus on a spiritual connection to God, leaving behind distracting indulgences like meat and wine. The story goes that Daniel avoided the temptations surrounding him for 10 days. This is all unrelated, by the way, to Daniel surviving a night in the lions’ den (which Colbert admitted is the only part he remembers from the Book of Daniel). Despite the fast’s ancient roots, it didn’t gain modern attention until The Daniel Fast blog was launched in 2007; it’s since grown in popularity among Evangelical Christians and churches throughout California.

The diet has similarities to veganism in that you can’t consume any meat, eggs, or dairy products. To add another level of difficulty, followers of the Daniel Fast can’t have any sugars or sweeteners, processed foods, soda, or alcohol. So, what can you eat? According to Pratt, you can only eat “foods grown from seed.” For those of us a little less fluent in Old Testament phrasing, that means fruit, vegetables, and grains—specifically unleavened bread. Sounds intense—even Chris Pratt agreed, referencing his love of alcohol. “You know how much I drink,” Pratt joked on Colbert.

Pratt stated his reasons for doing the diet are two-fold. The actor admits that he had been struggling to find motivation to exercise and eat properly recently, especially this past December over the holidays. “These last couple of months, I have just not been that motivated to work out. And I’ve been eating,” he said on an Instagram story in December according to USA Today.  “And I know it’s winter and it’s holiday season. This morning I woke up and I’ve been determined that I’m going to get on it. And I don’t want to, but I’m going to do it, I’m going to start it today.”

The second reason Pratt chose this bare bones, Bible diet is, because he was inspired by his pastor. Pratt has been involved with religion for most of his life but has become more vocal about his faith in recent years. Pratt told Colbert, “I was inspired by my pastor. It’s kind of like our Lent, you know, give something up.”

Many followers of the Daniel Fast include 21 days of prayer and spiritual practice alongside their fast. Pratt went on to describe how his faith helps him be the best version of himself for his friends, family, and fans—citing a quote from his pastor: “If the spotlight that’s shining on you is brighter than the light that comes from within you, it will kill you.”

Liz Weinandy, a dietician at the Ohio State University Wexner Medical Center recently broke down the Daniel Fast. She said the diet is lacking in “all kinds of nutrients” like essential fats and proteins and could be potentially dangerous for individuals with pre-existing conditions. While Weinandy supports intermittent fasting, she believes the long duration of the Daniel Fast makes it unhealthy and unsustainable.

According to the U.S. Department of Health and Human Services (HHS), too much restriction in a person’s diet is harmful. “Diets that severely restrict calories or the types of food you eat usually don’t lead to lasting weight loss and may not provide all the nutrients you need,” read the report from the HHS.

Richard Bloomer, the dean of the University of Memphis’ School of Health Studies, conducted multiple small studies on the Daniel Fast and spoke to TIME about the diet’s logistics. He said that after three weeks on the Daniel Fast, subjects can begin to lower risk factors for high blood pressure, cholesterol, and reduce oxidative stress. The Daniel Fast does not explicitly restrict the number of calories followers consume, but Bloomer says that participants end up losing weight because they are filling up on nutrient- and fiber-dense whole foods instead of meat, dairy and processed products.

Dr. Wayne Jonas, a family physician and executive director of Samueli Integrative Health Programs at the University of California said that most healthy people should be able to complete the Daniel Fast without a problem. People with chronic health conditions, especially those requiring dietary monitoring, should consult a doctor first.