What is the 5:2 diet? The 5:2 diet is a fat loss plan that relies on intermittent fasting to reduce overall kilojoule intake.
The diet was developed by Michael Mosley, a British doctor and author, to control his diabetes. It was the subject of a 2012 BBC documentary called "Eat, Fast and Live Longer" and a spin-off book called "The Fast Diet", which has gone on to become an international bestseller, rivaling the success of Dr Atkins' New Diet Revolution. The diet has received a boost from the endorsement of a host of celebrities including J Lo and Miranda Kerr. It is taking the world by storm.
Intermittent Fasting - How?
The recommended regime is to mix five days of unrestricted eating with two days of fasting. On fasting days, dieters are allowed to consume a quarter of their recommended daily energy intake, which is about 500 calories (2092 kJ) for women and 600 calories (2510 kJ) for men.
Whilst dieters are not restricted in what they consume on normal eating days, they are instructed to eat sensibly. The expected weight loss, about 0.45kg per week for a woman, and slightly more for a man, is calculated using the recommended daily energy intake, which is around 2000 calories (8368kJ) for an average female, and 2400 calories (10042kJ) for men.
Fasting days can be consecutive or non-consecutive, and on the fast days there are no restrictions on the way the kilojoules are used. Some people will choose to have one meal, while others will choose to spread it out over a number of small snacks. A number of cookbooks full of interesting ultra-low calorie recipes designed for fast days have been published recently.
Everyone will have their own strategies for picking their two fast days, but some good general advice is to choose a busy day, so there is plenty happening to distract from thoughts of food, but not a day so busy that a lack of food is likely to impact performance or cause extra stress. If you're not feeling well, postpone the fast. Your body needs energy for a quick recovery, and the last thing anyone wants to do is prolong an illness.
Mosely named the diet after the 5:2 ratio that was optimal for him, but also says that this may be used as a starting point. Whilst most people will experience steady, healthy weight loss with two fasting days, he suggests a third fasting day may be added if desired, bringing the ratio to 4:3. By the same token, he suggests dropping back to a 6:1 ratio to maintain weight loss once the goal weight has been attained (14).
The 5:2 diet for Weight Loss
There is no doubt that reducing energy intake causes weight loss. A review undertaken of the scientific studies show that intermittent calorie restriction, like the 5:2 diet, is just as effective for weight loss as a traditional diet where calories are restricted every day, and may even be more beneficial in preserving lean body mass (3). There is research showing that an intermittent fasting regime is easier to stick to than a traditional calorie-restricted diet (4). Greater compliance means a greater chance of success. The 5:2 diet has been widely praised for its flexibility and lack of social restrictiveness – you can slot the fasting days in whenever is convenient and don't have to worry about eating out. It's often called "The foodie diet" because it still allows the dieter to indulge their passion for the food of their choosing, at least for five days out of seven.
A diet where you can eat anything you want sounds great, but to lose weight, energy intake must remain less than energy expenditure. Whether or not people following the 5:2 diet compensate for the decrease in energy intake on the two fasting days by eating more food, or choosing more energy-dense options than usual because they're "allowed to eat anything they want" on the non-fasting days is a big question that has not yet been answered.
Paleo, Atkins and the 5:2 diet
As the runaway success of Paleo and Atkins illustrate, diets that are thought to mimic the eating habits of our ancestors have become very popular in recent times. The thinking behind this is that human metabolism has evolved, or adapted at a slower rate than the way in which we consume food - processed food, refined carbohydrates, even the idea of eating regularly - and that our body is not optimised for the modern way we eat.
The 5:2 diet is thought to reflect the diets of our hunter-gathering ancestors, who fasted intermittently when food was not available. The belief is that short periods of fasting can trigger an increase in metabolism, as the body's instinct is to expend more energy to find food, and runs contrary to the widely held belief that food deprivation suppresses the metabolism. One study performed on people of normal weight showed neither of these circumstances to be the case, recording no changes to the baseline metabolism over three weeks of intermittent fasting (1), whilst another showed a slight decrease in resting energy expenditure (2).
This is not a miracle diet. Any weight loss during the 5:2 diet is going to be because the body has taken in fewer kilojoules than it has burned, the same mechanism behind traditional kilojoule controlled eating. In terms of weight loss, the 5:2 diet is just a different way of eating less food.
The 5:2 Diet Benefits
Intermittent fasting regimes like the 5:2 diet are thought to confer a number of additional and unrelated health benefits. Surprisingly, this is the case even when the fasting days do not reduce the overall number of kilojoules consumed (10).
Probably the best studied benefit is the role of fasting on longevity. Fasting is known to decrease the levels of a chemical called Insulin-like growth factor-1 (IGF-1) (5). The way IGF-1 works is almost identical in all species all the way from single celled yeasts to humans. Worms, mice and other lab animals with low levels of IGF-1 have shown great increases in their life spans, and there is some evidence to show that humans with low IGF-1 may live longer and experience lower levels of cancer and diabetes. Scientists agree that IGF-1 is definitely involved in the process of human aging (6, 7).
Intermittent fasting is known to increase the sensitivity of our bodies to insulin, which 5:2 diet inventor Dr Mosley claims to have experienced, reversing the progression of his own type II diabetes using the diet. Through its effect on insulin, the process of fasting has been shown to improve the blood sugar levels, lower blood cholesterol and offer some protective effect against heart disease in humans (4, 8).
Studies in mice have shown that following an intermittent fasting diet long-term may improve brain function and enhance mood (9).
While these studies are promising There is a lot more direct evidence on the health benefits of fasting in animals like mice than humans. Animal research is sometimes the best source of information we have, but mice are not humans, and there have not been many direct studies of people on intermittent fasting diets which look at the health benefits secondary to weight loss. Since this is such a new diet, there are no studies on the long term effects specific to the 5:2 diet.
Bodybuilding and the 5:2 diet
Intermittent fasting alone does not result in loss of fitness or condition in athletes when a normal training schedule, macronutrient and energy intake are maintained (12). There is a lot of anecdotal evidence in support of the safety and effectiveness of using this diet to cut weight whilst in training. In spite of this, it is probably a good idea not to fast on a training day. A workout not backed up with protein and amino acids can be inefficient for lean gain, there's a risk of muscle breakdown without proper nutritional support, in addition to faster onset of fatigue. Fast days would tie in very well with rest days during the cutting phase.
Side Effects of the 5:2 diet
The most common reported side effects of the 5:2 diet are headache, irritability and tiredness on fast days. Many people say that these disappear as the body becomes used to going without food for longer periods of time. Studies have shown that intermittent fasting can affect fertility in rats, via a mechanism that is also present in humans (13). The effects of poor nutrition on fertility in humans, particularly in women, are well documented. The modest overall decrease in energy intake recommended by this diet, which is just over 20%, is not likely to be sufficient to cause reproductive issues in otherwise healthy people.
There are some groups of people for whom the 5:2 diet is not recommended, including children, the elderly, people with type I diabetes, the underweight, sufferers of eating disorders, and people with certain types of chronic illness. Talking to a doctor is a good starting point for anyone thinking of embarking on the 5:2 diet.
5:2 diet – The Verdict
The 5:2 diet has become very popular over the past year, which suggests it is yielding some good results. It is not a miracle – it operates on the simple premise that consuming less energy than you burn will cause you to lose weight, but it has certainly proven easier to stick to than a traditional reduced kilojoule diet for some people. There may be added health benefits to an intermittent fasting regime, but these benefits are only backed up by a small amount of evidence at the moment. The popularity of this diet will hopefully open this area up for more research in the future, which is something to look forward to.
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