No diet is bulletproof; there’s always the temptation to veer off track. Yet, a weight-loss plan called the Bulletproof Diet has recently gone viral with the help of reports of celebrities, like Harry Styles and Shailene Woodley, being followers.

The plan involves drinking a modified version of black coffee, called Bulletproof Coffee, and adding 2 tablespoons of butter and 1 tablespoon of medium-chain triglycerides (MCT) oil to a cup. That drink serves as the dieter’s breakfast. Other meals must be gluten-free and avoid sugar. All foods are classified as “bulletproof,” “suspect” or “kryptonite," Agence France-Presse reported.

But it’s not doctor certified or nutritionist approved. In fact, the creator of the diet plan hails from Silicon Valley and got his start in information technology and developed cloud computing. The self-proclaimed Bulletproof Executive, Dave Asprey, worked at several technology companies before creating Bulletproof Coffee in 2009.

Asprey has claimed that the drink and diet plan helped him lose 100 pounds, dropping from 300 pounds while in his 40s. He had tried doctors' recommendations of restricting calories and working out, but those methods did not provide sustainable results. His own plan did, Asprey says. “Your brain has energy that doesn't come from sugar. You didn't want sugar in your coffee and you lose the craving and you sort of have freedom,” Asprey told AFP.

The plan is spelled out in Asprey's book “The Bulletproof Diet: Lose Up to a Pound a Day, Reclaim Energy and Focus, Upgrade Your Belief,” published in 2014. A hardcover copy sells for $16.45 on

The 2-year-old fad, which drew headlines across news outlets and was the subject of morning talk shows in 2014, is again hitting the mainstream. And again, doctors and nutritionists are putting it on their do-not-attempt lists. The British Dietetic Association added it to its list of 10 celebrity diets to avoid for 2016, the Huffington Post reported.

Concern lies in the absence of nutrients, which could extend to deficiencies, and the broader ability for a diet such as this to work in the long term. “Whilst the idea of minimizing alcohol and processed food is positive, the classification of foods is at odds with health recommendations and lacks evidence,” the association wrote.

UCLA Medical Center nutritionist Amy Schnabel told AFP that the diet plan could spur weight loss in the short term. "Any diet that has you restrict large food groups does result in some weight loss," Schnabel said.