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Where does it come from?

Back in the 19th century, the French biologist Claude Bernard discovered that changing the diet of rabbits from herbivore (mainly plant) to carnivore (mainly meat) turned their urine from more alkaline to more acid. Excited by his discovery, subsequent scientists built on his findings, which eventually led to a bunch of loosely related diets (other names include the alkaline ash diet and the acid alkaline diet), whose popularity has recently taken off after the celebrity take-up.

What’s the theory?

Our blood is slightly alkaline, with a normal pH level of between 7.35 and 7.45. The theory behind the alkaline diet is that our diet should reflect this pH level (as it did in hunter-gatherer days when we ate fewer acid-producing foods such as grains, fish, meat, poultry, dairy and salt).

Proponents of alkaline diets believe a diet high in acid-producing foods disrupts this balance and promotes the loss of essential minerals such as potassium, magnesium, calcium and sodium as the body tries to restore equilibrium. This imbalance is thought to make people prone to illness and gaining weight. The ultimate aim is to eat 70 per cent alkaline foods and 30 per cent acid foods, meaning you can still have a little of the “bad” stuff such pasta and rice, although things can get slightly complicated. The way you cook your vegetables, for example, can have an impact. Raw spinach is alkaline, but when you cook it, it becomes acidic.

What do the experts say?

“The theory of the alkaline diet is that eating certain foods can help maintain the body’s ideal pH balance to improve overall health. But the body maintains its pH balance regardless of diet,” says British Dietetic Association spokesperson, Rick Miller.

What’s more, while there is evidence that alkaline diets may help prevent the formation of calcium kidney stones, osteoporosis, and age-related muscle wasting, there isn’t any proof that an acid-producing diet is the foundation of chronic illness.

Mind you, says Miller, you’re unlikely to do yourself any harm. “The diet’s premise is to increase alkalizing foods (such as fruit and vegetables) and reduce your intake of acid foods (such as meat, salt, and refined grains). Well, that’s pretty much what we consider as healthy eating anyway and if you’re overweight, of course it will probably help you shift some pounds.”

How punishing is it?

Fans of this diet say the fact that it’s un-faddy makes it easier. “It doesn’t cut out food groups or involve counting calories, which is probably why it’s the first diet I’ve ever got on with,” says Alison Heldon, 24.

But most people agree that the fact that the regime warns against any “very acid-forming” foods, such as starchy grains and vegetables – such as pasta, wheat and beans – all dairy products, meat, fish and shellfish, means it’s challenging. You can also forget about indulging in coffee, tea, sugar, fizzy drinks including soda and tonic water and alcohol. “I defy anyone with a normal family life to sustain it in the long term – I think I did well to last six weeks,” says Suzanne O’Shea, 33. Monitoring your pH levels isn’t very glamorous either – you have to test your urine.

The 5:2 diet

What is it?

Intermittent fasting, basically. So you eat normally for five days and severely restrict your calories for the other two – 600 calories for men and 500 for women. It’s up to the dieter how they divide them up – so you might, for example, have scrambled eggs with ham and a black coffee for breakfast (300 calories) and a lunch or dinner of grilled fish or meat with vegetables (300 calories). The rest of the time, you eat what you want.

Where does it come from?

The diet virtually became an overnight sensation after featuring in a BBC2 Horizon documentary last summer by health journalist Dr Michael Mosley. After just five weeks, Mosley lost nearly a stone, reduced his body fat by about 25 per cent and improved his blood-sugar and cholesterol levels. A book, The 5:2 Diet Book, has become a bestseller. Advocates of the diet also claim it can help us live longer and decrease the risk of cancer, diabetes and Alzheimer’s. Gwyneth Paltrow, Jennifer Aniston and Jennifer Lopez are said to have tried it.

What’s the theory?

The rationale behind it centres on the effects of fasting on levels of a hormone called IGF-1 (Insulin-Like Growth Factor 1). Although the hormone is essential in early life, when rapid new cell growth is advantageous, high levels in adulthood increase your risk of cell divisions such as those found in cancer. Restricting the calories you consume, so the theory goes, lowers blood levels of IGF-1, protecting you against some major diseases. Mosley fell upon the particular success of cutting back for two days a week virtually by default. Having tried various patterns of intermittent fasting, he eventually discovered that severely restricting food for two days a week was both achievable and had the best results. The sheer simplicity of the diet has no doubt had a hand in its phenomenal success.

What do the experts say?

Despite the claims that it helps people lose weight, increases their lifespans, improves cognitive function and protects against conditions such as dementia and Alzheimer’s, the general medical consensus is that there isn’t actually any firm evidence.

“Yes, there’s some work on the effects of intermittent fasting on ageing and cognitive decline, but almost all these studies involve rodents, not humans, and the work on preventing diseases took place in laboratory conditions, with no guarantee of successful real-world outcomes,” says British Dietetic Association spokesperson, Rick Miller.

He warns that if you’re only eating a quarter of the calories you need, you may suffer low blood-sugar, as well as digestive problems, and that unless dieters increase their water consumption on their restricted days, they could suffer from constipation.

“I used to eat like the 5:2 diet in my teens and early 20s and it was called bulimia,” says Zoë Harcombe, author of The Obesity Epidemic book, who is concerned that this diet carries a huge risk of encouraging disordered eating in people who are prone to it.

Sarah Schenker, spokesperson for the Nutrition Society, agrees: “The reality is that the 5:2 diet is how a lot of people manage their weight naturally. You eat all you want on the weekend, then have a day or two when you cut right down – and it does work for people. Also, there’s something to be said for giving our systems a rest, particularly if we eat a lot of sugar and carbohydrates. So I don’t think it’s a fad and I’m certainly not dismissive, although I think we need to do more research.”

Some experts believe that on the eating days, there’s a risk of over-indulging and putting on weight, although a study by the University of Illinois found that people only eat about 100-110 per cent of the calories they needed. It’s quite hard to fully make up for the lack of food on the restricted days.

How punishing is it?

People report that the diet is both easy and hard. On the fasting days, it’s hard not to give into snacks, but the rest of the time, you can eat what you want so there’s no guilt. “On the restricted days, I have very little energy, which can affect my work, and I’m usually quite irritable by the evening, but then I just remind myself that I can eat what I want tomorrow and that cheers me up,” says Susan Kemp, 34.

Others report that it’s not sustainable. “I lost 8lb in just over two weeks and I did manage to keep it off for a few months, but if you think about it, the restricted days are just slightly shy of 30 per cent of your week. Who wants to be starving for 30 per cent of their life?” says Alan Mitchell, 40.