If you want to lose weight, add a pat of butter to your coffee.

It sounds like a cruel joke á la "Mean Girls," right?

"Butter coffee," or "fat black," has become the new trend in U.K. cafés - a double espresso blended until frothy with a tablespoon each of butter and coconut oil, according to the U.K.'s Daily Mail.

The U.S. diet fad of sugar-is-worse-than-fat crossed the pond and has taken up residence in the United Kingdom. Many are using the beverage as a meal substitute, claiming the energy and fat make a perfect drinkable breakfast that keeps you focused, burns calories and keeps your tummy full until lunchtime, according to Daily Mail.

Sounds like a miracle, right? Before you sign up, be aware that the texture is an acquired taste and first timers often have a serious bout of nausea and diarrhea. (Perhaps that is why the breakfast-in-a-cup is also called "bullet coffee?")

U.S. entrepreneur Dave Asprey was inspired by the beverage he had been served while trekking in Tibet - tea with yak butter, according to Daily Mail. Mountain climbers drank the tea for high-fat, low-carb energy. Asprey took the idea home with him and devised what he claims is "the bullet-proof diet" that will help you drop weight, gain energy and ward off chronic diseases.

Ketosis is the process your body goes through when there are no sugars to burn. Like the theory applied for the Atkins diet, if you cut carbohydrates, your body will burn fat for fuel. According to Daily Mail, many experts agree that low-carbohydrate diets aren't worth the headaches, cramps, weakness and intestinal issues.

Suzie Walker, a 33-year-old nutritionist, started following a Paleo diet when she breastfed her daughter, who is now 3-years-old.

"I like to start the day with a bullet coffee," Walker said. "It fills me up, keeps me focused and I don't have to think about eating again until lunchtime."

Walker makes her own butter coffee at home. There are starter kits available online, but for $50, you might want to taste one first and decide if it is for you.

Asprey suggests organic coffee (he believes the toxins in regular coffee are what gives people the jitters, according to Daily Mail), butter from grass-fed cows and organic coconut oil. Extreme fitness fans, like those who do CrossFit, and people who follow the Paleo Diet tend to gravitate toward the thick coffee drink. Asprey claims that the caffeine speeds up your metabolism, but the fat slows your digestion, causing the energy to come on gentler and last longer than it would from a plain double espresso.

We should probably mention: One cup of butter coffee has 400-500 calories (more than a bacon sandwich, according to Daily Mail) and almost all of your daily allowance of saturated fat (50 grams out of the allowed 70 grams).

"Ketosis is only triggered when the liver glycogen stores are depleted, so if you enjoyed a carb-heavy takeaway the night before, your morning cup of bullet coffee won't be enough to make the switch," dietitian Sarah Schenker said, according to Daily Mail.

"If you are trekking in Tibet you need concentrated calories because you are expending so much energy," Schenker said, "but you can't just transfer the drink to our more static Western environment and expect it to aid weight loss: it just won't work."

She suggests something way more radical and recently unheard of: ordering a plain coffee with just basic cream and sugar.