What do Mick Jagger, Kim Kardashian, Kobe Bryant and Alec Baldwin have in common? At first glance not a lot, it would seem, but if we are to believe the American gossip press then the musician, the celebrity, the athlete and the actor all follow what is called the 'keto diet'.
This diet, which in its most extreme version involves an extremely low consumption of carbohydrates and a high ingestion of fats, seems to be very effective for rapid weight loss although the loss is difficult to maintain long-term.
Lose weight by eating fat? It seems to go against everything we have ever been told, but there is an explanation. Agriculture and farming were only invented about 10,000 years ago - the day before yesterday, in the history of the evolution of man - and until then, for hundreds of thousands of years, humans used to hunt and collect everything they had to hand.
This is explained by chemist Luis Jiménez, author of the book and blog Lo Que Dice la Ciencia Para Adelgazar (What Science Says About Slimming). "A good hunting season meant that people ate practically nothing but meat for a while; and if it was a bad season they were forced to rely on berries, fruits, roots and other vegetables. It is very probable that these variations were interspersed with long periods of shortages and need," he says.
It is in our DNA: the human metabolism has different ways of obtaining the energy it needs, and the easiest and most direct is by transforing carbohydrates into glucose. The keto diet 'forces' the body to enter into ketosis, in other words to use the fat instead of the glycogen to stay alive.
The Inuits are a present-day example of this: obliged by circumstances, they have traditionally eaten meat and caribou fat (a type of reindeer), polar bear, seal, walrus, whale and fish, and have consumed very few vegetables and cereals, fundamentally because they don't grow in the ice. And their rates of illness linked to obesity are much lower than in the western world. Or were, because many Inuits put on weight when they stopped hunting and started eating junk food.
The deceptive pyramid
"In the faculties of Medicine, ketosis is associated with diabetes and kidney disease, but man has always lived in ketosis," says Dr Ángel Gutiérrez, professor of Physiology at Granada University.
The "perversion of the food train," says this sports doctor, became consolidated when the United States, at that time the world's biggest producer of cereals (it is now the second, after China), decided to get rid of its surpluses by means of a true marketing success: the food pyramid and its fervent recommendation that for a 'balanced' diet people should eat generous helpings of bread, biscuits, breakfast cereals, pasta and rice.
The claim that a healthy diet should include plenty of carbohydrates is now starting to be questioned. The panel of experts who assess the health system in Sweden concluded that for people who are obese, a diet low in carbohydrates is the most effective way of losing weight in the short term, rather than the traditional low-fat diet.
The keto diet restricts the ingestion of foods which end up converted into glucose, high in flours, starches and sugar, and encourages consumption of fruits and vegetables with low hydrates such as lettuce, spinach, asparagus, avocado, tomato and courgette. Although the most effective way of going into ketosis is occasional fasting, this programme achieves it by reducing the portion of carbohydrates - at least at the beginning - to less than 20 - 50 grammes a day, in some versions. To give you an idea, a 33cl bottle of beer has 13, an apple 18 and a plate of pasta 90.
Once the body's glycogen reserves are used up - which normally occurs after two or three days with these restrictions - it activates 'Plan B': the fat in the diet is transformed in the liver into the so-called ketonic bodies, which serve as fuel for the cells. Once in ketosis, the body also easily burns the fat which the body accumulates as a reserve, and loses weight.
As well as the radical version, there are other variants of 'low-carb' diets based more on proteins and not as much on fats. The oldest is the Atkins diet, created by the doctor of the same name in 1970, who recommended starting with a quite radical restriction of carbohydrates, (20 grammes a day) and gradually increasing them as you achieve your objectives. This is very similar to the more modern Dukan plan, which in the early stages recommends entire days with no fruit or vegetables.
A different perspective is that of the 'paleodiet', which proposes a return to the primitive menu of the hunter-collectors, comprising meat, fish, eggs, roots, fruits, fresh vegetables and nuts and excluding cereals, pulses, dairy products and refined sugars.
Accustomed to identifying fat with the devil, many people don't believe this can work, but in fact an increasing number of experts are now warning that the real enemies, the true cause of the obesity epidemic which affects the world, are processed foods which are high in sugar and flours - which end up being converted in the same way: glucose - and not lipids.
"According to the studies, the ketogenic diets or those which are very low in carbohydrates tend to be more effective than others to lose weight in the short to medium term," says Luis Jiménez. The problem, he points out, is that a strict ketogenic diet means that many foods have to be excluded and it is not easy to maintain that for very long. And everybody knows what happens when you return to 'normality' after a period of containment: the 'rebound effect' is likely to occur.
Is it healthy to give up carbohydrates to lose weight? Starting from the premise that obesity is even less healthy - in Spain obesity is, after smoking, the second cause of preventable death from diabetes, cancer and cardiovascular illness - the option is not to be entirely discounted. However, precautions are advisable.
"If it is a complete diet with all types of nutrients, a large quantity of fruit and vegetables which are low in carbohydrates and good quality meat and fish, nuts and olive oil, and you do it for a relatively short period of time, there is no evidence that it will do you any harm," says Luis Jiménez. He admits, however, that no studies have been done into the safety or effectiveness of this type of diet over the medium to long term.
Obviously, a diet with 'nothing green', dominated by bacon, cured meats, sausages, milk, cured cheese and butter would present serious risks. The 'good fats' are found in oily fish, olive oil, nuts and avocado, for example.
A controversial question is whether the ketonic diet is useful for athletes. Those who say it is point out that metabolic flexibility is also a blessing for people who do sport: when it comes down to it, why depend on a glycogen reserve of about 2,000 calories (stored half in the liver and half in the muscles) or use glucose gels or drinks, when you can have a reserve of more than 100,000 calories, the energy accumulated in just ten kilos of fat?
The 'keto diet' can cause side effects for the first few days, until the body adapts to its new metabolic regime: bad breath, dizziness, nausea, constipation, reduction in sports performance and irritability. However, the studies show that after that there is an improvement in the levels of triglycerides, cholesterol and glucose in the blood. There is also a reduction in appetite.