Diets: is there any science behind the latest fad regimes?

The 5:2 to the Paleo, the Bulletproof and the Virgin and more claim they will make you thin, healthy and happy. We look at the facts.

The Observer, 5 December 2014

Photograph: Pal Hansen/

Photograph: Pal Hansen/


The promise

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.



The promise

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.


The promise

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 promise

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.