Eat normally most of the time, except two days a week when you slash your calorie intake to about a quarter of recommended amounts. So on those days women eat 500 calories and men 600, never on consecutive days.
Assuming that you normally consume the recommended daily calories, just two days of fasting cuts out more than an entire day's food over the week. Aside from weight loss, there are other claims - like improvements in blood pressure and cholesterol, and that fasting could increase lifespan.
Does it deliver?
The largest trial in humans showed similar levels of weight loss between people put on a two day consecutive fasting diet and a regular calorie-controlled diet. They also had similar benefits in terms of blood pressure, cholesterol and other health markers, although the fasting group had slightly better improvements to insulin resistance. The study was in obese women though, so the results might not apply to everyone.
There is a long history of research showing that calorie restriction boosts lifespan. And animal research also suggests that fasting could boost brain cell growth and might protect the brain from the build-up of plaques seen in diseases like Alzheimer's. Research in people over 70 also found that intermittent fasting led to a 30% improvement in verbal memory after three months.
For some, sticking to a severe diet for just two days is more motivating than perpetual restriction, especially in the long term. But the jury is still out as to how strong the other effects are in people.
You'll be eating like a caveman - lots of meat, fruit and veg, nuts, and seeds, while avoiding modern food like grains, processed foodstuffs, dairy, pulses, salt and sugar. The logic is that there is a mismatch between the diets that we evolved to eat, and what we began eating after the advent of agriculture around 10,000 years ago. Proponents say our ancestors were full of vitality whereas we feel fat and sluggish. They say eating more protein and healthy fats instead of processed, gluten-rich foods, will sort you out.
Does it deliver?
Swapping processed for whole foods has advantages, says biochemist Kristian Le Vay, at the University of Bristol, "not least because processed food tends to contain a lot of added sugar". High sugar consumption is linked to obesity, heart disease, and diabetes. We also know that many of us eat too much salt, which is common in processed meats like bacon. But cutting out other food groups, especially dairy, could lead to deficiencies in nutrients like calcium. And carbohydrates such as pasta and grains are considered fine in the right amount, says Le Vay.
The idea that our genes have not evolved with our diets has been hotly disputed. For instance, several populations independently evolved the ability to digest milk as adults around the same time as humans started dairy farming, while some populations evolved extra copies of the enzymes needed to digest starch. It's also debatable that our prehistoric ancestors really were that healthy - they certainly didn't live too long.
Avoiding sugars and processed foods is wise, as is getting healthy fats from nuts and seeds, but you don't need to eat like a caveman to do that.
Sugar-free diets vary - cutting out all sugar includes dairy, fruit, and some vegetables. Most diets focus on added sugar: no more cakes, biscuits, sweet drinks and processed foods like cereals, bread and sauces. Oh and most booze. Some focus on particular sugars.
Replacing sugary foods with more protein and foods high in natural fats will keep you fuller for longer so you eat less overall and avoid that energy crash that leaves you craving yet another, sugary pick-me-up. There are other claims too - such as healthier skin, and increased energy.
Does it deliver?
We don't need sugar in the way we need essential fats, protein and carbs for our body to function so cutting down makes sense. The kinds of sugars often added to food, like high fructose corn syrup, are especially bad, argues Robert Lustig, at the University of California, San Francisco, in his book Fat Chance: Beating the Odds Against Sugar, Processed Food, Obesity, and Disease. That's because it is turned to fat by the liver, accumulates there, and leads to liver disease and diabetes. And high-sugar diets can cause us to eat even more. "Glucose gives us sugar highs that can take away hunger in the short term but leave us tired and hungrier once this has worn off," says Jane Ogden, professor of health psychology at the University of Surrey and author of The Good Parenting Food Guide. "Cutting down on alcohol, sugary drinks, cakes, and biscuits is an effective and healthy way to lose weight - it cuts saturated fat, sugar and calories," says Professor Susan Jebb, at the University of Oxford. But cutting down, rather than eliminating all sugar, should be easier to stick to and less likely to make us obsess over the foods we are avoiding. "Ultimately we will give in and binge on the foods we've been denying ourselves," says Ogden.
The diet recommends 60% of your diet should be "healthy" fat, 20% good quality meat, and the rest vegetables and a tiny bit of starch. It also promotes taking supplements. No calorie counting, eat when hungry and stop when you're full. No snacking between meals, and kick off the day with a bulletproof coffee - basically coffee blitzed with two tablespoons of butter.
The bulletproof coffee and accompanying diet were dreamed up by Silicon Valley entrepreneur Dave Asprey. He says he lost 45kg eating a high-fat diet, and claims he now eats more than 4,000 calories day and does no exercise. He says calorie restriction leads to energy storage but that "by eating more, you tell your body it's OK to burn fat."
Does it deliver?
There is no scientific explanation for how people could lose weight on an incredibly high calorie diet (with little exercise). "Fat is designed to be stored not burned," says Ogden. "Some fat is needed, but a diet low in fat but high in fruit and vegetables has got to be healthier," says Ogden. The no snacking policy, though, is good. "Learn to eat three meals a day and you start to associate food with a specific time and place which limits what you eat," she says, and it also helps you stop eating for other reasons like boredom or comfort.
The diet is based on the idea that much of the weight gain and general sluggishness experienced in particular by women as they age is caused by "broken hormones". And the best way to address these "imbalances" is through your diet. Depending on which hormone imbalances you are tackling, much of the advice involves cutting caffeine and gluten, eating lean proteins and grass-fed meat, avoiding GM food, cutting sugar and processed carbs. The diet also recommends certain activities, such as yoga, as well as taking food supplements.
Does it deliver?
It is true hormones, lifestyle factors such as stress, and our diets are interconnected. But some aspects carry more weight than others. Certain hormones involved in food metabolism, like insulin, are affected by what we eat - slow-release carbs help prevent the insulin peaks you get with sugar. As for fat loss, some hormones, such as leptin, are thought to regulate the amount of fat stored in the body, says, Madeline Burke at Bristol University. "However there has been no concrete evidence so far that shows by eating certain foods the levels of these hormones can be controlled." Avoiding hormone-fed meat makes sense though. "There is sound scientific basis to say if you have too much oestrogen-injected beef or what have you, that could influence your body," says Professor Giles Yeo at the University of Cambridge. Not so for GM. "There are many ways of genetically modifying foods, so to say GM items are bad is a naive, dangerous thing to say."
The idea is that we all suffer from some form of food intolerance, and that's what makes us fat. Using this diet, you will eliminate seven "high intolerance" foods - gluten, soy, dairy, eggs, corn, peanuts, sugars and sweeteners. That leaves mostly lean protein, high-fibre starch, and veg. It is claimed these foods also exacerbate your immune system causing inflammation, which messes with hormones like insulin and leptin, making you even more fat. Cut these foods out and in three weeks you will look and feel 10 years younger.
Does it deliver?
Studies do show that high-fat diets can lead to inflammation. And this in turn has been linked to all sorts of ills, from diabetes to dementia. But the idea that most people have a food intolerance is suspect. "We did a study a few years ago and found that most of the symptoms people believed were caused by food intolerance were actually reactions to an unhealthy diet," says Ogden. "Symptoms such as headaches, tiredness or bloatedness may be seen as a result of a food intolerance but are often a consequence of fizzy drinks, caffeine, snacks, fast foods and ready meals."
"Rather than cutting out these foods it is better to buy basic ingredients, cook simple food and eat plenty of fruit and vegetables," says Ogden. Cutting out so much of the diet will probably lead you to lose weight, but "the worry is that excluding whole food groups may lead to nutritional deficiencies which are not consistent with the best long-term health," says Jebb.
Top Photo: Paleo: the diet for the modern stone-age family Photograph: Pal Hansen/theguardian.com