WHAT ON EARTH IS BULLETPROOF COFFEE?
The drink was invented in America, perhaps unsurprisingly, by 43-year-old Silicon Valley entrepreneur Dave Asprey after he "felt amazing" when drinking a traditional yak-butter tea on a trek in Tibet.
On his return to the U.S., he made his own version of the high-fat drink using coffee, a special oil and butter - and found that by drinking the mixture instead of eating breakfast, he lost weight easily.
That's despite the fact that one cup of Bulletproof coffee contains up to 500 calories and around 51g of fat (women are recommended to consume 36g to 62g of this a day), of which more than 70 per cent is saturated - the most harmful to our health.
Asprey claims it works thanks to the coffee boosting your metabolism, while the oil forces the body into a fat-burning mode which melts away a muffin-top or unwanted tummy quickly.
So committed to the drink is he that he gives his two young children an espresso-cup measure every morning "because the oil turns off their hunger". He has now reportedly shed around 7st by drinking the coffee.
Asprey was quick to see the potential in his invention after a recipe he posted online became wildly popular. He now sells coffee beans (at around £40/kg) and specialised oil so people can make their own "official" Bulletproof coffee, and boasts millions of fans, from school-gate mums to Silicon Valley millionaires in search of a quick mental boost.
The stuff has appeared in coffee shops all over the U.S., including in Los Angeles and Chicago, and has featured on the menus of a few trendy coffee bars in London, too, such as the chain Crussh.
It's big business: Asprey recently secured investment of more than £14 million from venture capitalists to keep building his Bulletproof brand.
CAN I MAKE IT AT HOME?
The recipe is certainly simple, if decidedly strange.
First, Asprey recommends combining a cup of freshly brewed coffee (from beans, as they have more antioxidants than instant powder) with two tablespoons of unsalted butter, which contains brain-boosting omega-3 and butyrate, a type of fatty acid that keeps us fuller for longer.
He then adds two tablespoons of an oil that is rich in a compound known as medium-chain triglycerides (MCTs).
Many devotees use coconut oil here as it comprises up to 60 per cent MCTs, though health shops also sell pure MCT oil, which some say is even better for you and which Aspray recommends.
Stick the lot in a blender and whizz until it has a foamy top layer - just stirring will leave oily lumps - and, hey presto!
Asprey claims it will be the "creamiest coffee you've ever tasted". Fans say it has the flavour of a rich latte and leaves a pleasant, oily feeling in the mouth.
Critics, however, find it revoltingly rich and just too much to handle at breakfast.
Certainly, Vicky Charles, a mother of one from Wiltshire, wasn't a fan at first. "I read about it several years ago and attempted to make my own version by stirring normal butter into normal coffee," says training consultant Vicky, 36. 'But I didn't like the taste.
"Then, about two years ago, I started drinking Bulletproof whenever I needed to drop a few pounds for a special occasion or an important business event."
She recently lost 10lb in two months "with very little effort".
"I've never been a fan of low-fat diets and have always drunk high-quality, organic coffee with cream.
"I've tried every silly diet going over the years, but drinking this for breakfast and then eating normally for the rest of the day is so much easier."
IS THERE ANY SCIENCE BEHIND IT?
Yes, mostly in the MCT-rich oil - though many of the claims are disputed.
Devotees say this oil's chemical make-up means it's broken down so quickly by the liver that's it is speedily accessible to the brain as energy. That's why Bulletproof drinkers say they feel clear-headed, and also the reason the body doesn't get the chance to build up extra fat.
"At almost 500 calories a cup, you're certainly not consuming fewer calories than you would eating a normal breakfast," says Dr Trudi Deakin, a consultant to the NHS on the prevention and management of diabetes.
"But because Bulletproof doesn't contain carbohydrates, it means that your body doesn't trigger an insulin response which tells the body to store fat."
Put simply, without access to sugar or carbohydrates, the body will instead burn existing fat stores for energy - think muffin-top or saddlebags - a process known as ketosis.
This effect continues until you consume carbohydrates - potentially not for several hours if you don't eat again until lunch.
Fans believe the caffeine in coffee helps the process by speeding up our metabolisms, while the butter keeps you feeling full.
This will be ringing bells with anyone who has tried the Atkins diet or intermittent fasting, as both of these regimens are based on the same principle of a high-fat, low-carbohydrate diet.
CAN IT PUT MY HEALTH AT RISK?
Doctors are less certain the science stacks up.
"It's important to remember that buttery coffee is a trend and has created a lot of marketing opportunities," says Dr Deakin.
"But it should never replace eating nutrient-dense real foods, which we know provide all the micronutrients that promote good health and are key to any weight-loss strategy."
Scientific evidence as to the benefits of MCTs are also inconclusive.
Researchers at Columbia University in the U.S. concluded MCTs don't in fact give you energy, they just burn away as they aren't useful to the body - though that means they may help with weight loss if you manage to consume them alone, with no carbohydrates.
What's more, specific research around Bulletproof coffee is nebulous and many sceptics claim it's nothing more than a money-spinning exercise.
One of them is Tom Sanders, emeritus professor of nutrition and dietetics at King's College, London. "MCT-rich oil can make you feel sick and gives you abdominal pain and cramps, while coffee has both a laxative and stimulant effects, leading to the urge to purge," he says.
"This idea of consuming coconut oil with coffee has been around for several years and there is no evidence to support the weight-loss claims, unless you count feeling sick after consuming the stuff.
"Neither is there any for enhanced brain function, although the caffeine may give you a slight lift. The claims are hokum."
Vicky says that she has suffered in the past from drinking it.
"There have been a few times when I've put in too much oil, which has left me with an acidic and unpleasant stomach," she says.
"But it still beats faffing around counting calories and points and living off kale and quinoa for days on end - plus the weight's falling off me."
Meanwhile, Tracy Parker, a heart health dietitian at The British Heart Foundation, warns that the high fat content of Bulletproof could have dire consequences - particularly in those with high cholesterol.
"Eating a diet high in saturated fat can lead to weight gain, higher levels of bad cholesterol and an increased risk of heart and circulatory disease," she says.
"A coffee that has in it two tablespoons of butter contains 70 per cent of your recommended daily intake of saturated fat alone, before any food has been eaten."
WILL IT REALLY KEEP POUNDS OFF?
According to leading nutritionist and author Dr Zoe Harcombe, the coffee could actually lead to you piling on the pounds.
"Based on scientific evidence, butter is not dangerous, but neither is it particularly nutritious," says Dr Harcombe, who is an expert in public health dietary guidelines. "So Bulletproof coffee isn't unsafe, but I'd still urge caution.
"What people need to realise is that if you start adding extra fat to your diet, you had better make sure that everything else you consume is very, very low in carbohydrates.
"If you think you can have a Bulletproof coffee and then devour a muffin without any consequences, forget it, you will start to gain weight. Basically, if you add butter and coconut oil to your coffee like this, you should also be eating low-carb foods equivalent to two cups crammed with green salad leaves or you'll see an adverse impact on your waistline.
"The bottom line is that although fat is satiating and thus people may feel fuller after drinking coffee spliced with butter and oil, if you want to lose weight, eating fewer carbohydrates, not more fat, is what makes the difference."
