The 5:2 diet, also known as The Fast Diet, became famous a few years ago in a BBC Horizon documentary screened in 2012. This was followed by the book The 5:2 Fast Diet by Dr Michael Mosley (a famous TV doctor in the UK).
Since then, the approach has continued to evolve. Dr Mosley and his partner, Dr Claire Bailey, have been involved in several clinical trials investigating the efficacy of similar approaches for weight loss, type 2 diabetes, and heart disease.
The diet is also becoming popular with dieters following a keto or low-carb diet.
Is the 5:2 diet good for you? Yes, it can be, but as with most diets – there are downsides.
The foundation of the 5:2 diet is to eat a lower-carbohydrate Mediterranean-style diet for five days a week (your normal days) and only eat 800 calories for two days a week of your choice (your fasting days – this has been increased from the original 500 calories set in the book).
On low-calorie days, it’s recommended to choose busier days when you’re not socialising so you’re not thinking about food or tempted by social offerings and to mainly eat protein, fat, fibre, and low amounts of carbohydrates to help you feel full.
While this has been termed a type of fasting, as you’re only consuming fewer calories on the two ‘fasting’ days, the technical term for this approach would be intermittent calorie restriction.
The 5:2 diet has only become known recently in popular culture, but intermittent caloric restriction is not a new concept and has been researched since the 1980s to some extent in humans and more so in animals – with promising results for physical health.
The 5:2 could be seen as a healthy diet. It recommends eating a lower-carbohydrate Mediterranean-style diet (not too dissimilar from the nutrition guidelines at Second Nature), prioritising whole foods, is rich in essential nutrients, and doesn’t restrict fat.
The diet restricts calories to less than 800 on two days of the week where it’s likely you’d need to count calories, and it recommends using MyFitnessPal or other calorie intake trackers.
However, during the other five days, it doesn’t recommend calorie counting and prefers people manage their portions and listen to hunger and fullness signals.
The 5:2 diet encourages the consumption of:
- Non-starchy vegetables (leafy greens, broccoli)
- Full-fat dairy
- Olive oil
- Nuts and seeds
- Lower amounts of high-fibre carbohydrates and whole grains
- Homemade soups
The 5:2 diet discourages the consumption of:
- Refined carbohydrates such as cereal, white rice, and supermarket bread
- Ultra-processed foods
- High amounts of starchy carbohydrates, such as pasta
There’s good evidence that this approach to nutrition can bring health benefits.
Various types of intermittent fasting diets, like alternate-day fasting and time-restricted eating, have been shown to support weight loss, lower blood sugar (glucose) levels, and reduce the risk of cardiovascular disease.
What are the downsides?
As is possible in all types of diets and fasting where there are rules and restrictions, there’s the risk of people falling into the ‘all-or-nothing’ mindset if they fall off the wagon or slip up and stray from the diet; they’ll feel like they failed.
This can lead to the ‘what-the-hell-effect’ where people feel like they’ve had something they’re not supposed to; they’ll tend to overindulge and overeat that food because they feel like they’ve ruined everything anyway, so what the hell.
This phenomenon could also be present with the 5:2 diet, whereby people feel like they can eat what they want on non-fasting days because the consecutive days of fasting ‘make up for it’.
Fasting and caloric restriction can also be challenging practices to maintain and sustain in the long run.
The typical adult requires anywhere from 2,000 to 3,500 calories a day (or more) depending on their requirements and physical activity demands.
Women may need to apply more caution
Women, in particular, seem sensitive to caloric restriction, and studies have shown that severe caloric restriction and strict dieting can lead to menstrual irregularities and lower fertility.
Restricting calories down to 800, even if it’s just for two days a week, may be too challenging for many people to maintain. At the same time, others may find it more manageable, particularly if their current activity levels are low.
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Otherwise, keep reading as we look at three lines of evidence behind the pros and cons of the 5:2 diet.
1) The 5:2 diet can support modest weight loss
Research into intermittent calorie restriction dates back to the 1980s. There were promising signs this approach could support weight loss, blood sugar levels, and other markers of chronic health conditions.
More recently, more extensive human trials have shown promising results.
A randomised controlled trial in 300 participants compared the effect of a 5:2 diet with different levels of support (either group support or self-led) or a standard brief advice session (called the control group) with information on health and nutrition lasting 20 minutes.
After six weeks, the 5:2 diet with group support led to a weight loss of 2.3kg, the self-led 5:2 diet of 1.5kg, and the control of 1kg. Suggesting the 5:2 diet with group support was over twice as effective as the control group in the short term.
However, after 12 months, there weren’t any significant differences between the groups, with the 5:2 diet losing 2.6kg, the self-led 5:2 diet 1.9kg, and the control 1.8kg.
This is likely due to poor retention rates in all groups. At 12 months, only 48% of people in the control group were adhering to their programme, 56% in the 5:2 self-led, and 46% in the 5:2 with group support.
In contrast, there was a difference in the number of people achieving more than 5% weight loss. 28% of people in the 5:2 diet with group support achieved more than 5% weight loss, compared to 18% in the self-led 5:2 diet group, and 15% in the control group.
This suggests that this approach can work for the right individual, but it may not be the most sustainable option for many as they might struggle to adhere to it.
This is demonstrated in a recent meta-analysis (a study that compares the results of many studies) that, on average, the 5:2 diet only seems to produce modest weight loss. It’s around 9% more effective than standard interventions, but with a big range of weight loss between different studies.
The analysis reported that weight loss on 5:2 diets ranged from 1.7% of body weight to 7.97%.
This suggests that the approach’s success will depend on many factors, such as the individuals chosen, the approach people take to food on non-fasting days, the support given alongside the diet, and the length of time the studies report weight loss (you’re more likely to see a positive effect in the short term).
- Research investigating the impact of intermittent calorie restriction, like the 5:2 diet, dates back to the 1980s when promising results on weight loss were seen.
- More recent trials show contrasting results, with some studies showing a large effect (more than 5% weight loss) and others showing a very modest effect (less than 2% weight loss).
- Although, a randomised controlled trial suggested the 5:2 diet helped more people achieve more than 5% weight loss compared to standard diet intervention.
- Whether the approach will be a good weight management tool depends on many factors.
- Research suggests adherence to the diet is no better than standard diet interventions.
2) The 5:2 diet may lower your risk of chronic disease
Beyond body weight, markers of metabolic and physical health such as blood sugar levels (reducing your risk of type 2 diabetes), blood pressure, and liver fat are vital indicators into how healthy an individual is.
Research on 5:2 and fasting has suggested it can be a valuable tool to improve these markers.
A randomised controlled trial in 74 living with non-alcoholic fatty liver disease (NAFLD – a condition of very high liver fat levels that leads to poor liver function and requires a liver replacement in extreme cases) compared the impact of a low-fat diet, a 5:2 diet, and a low-carbohydrate diet on liver fat levels.
As the liver is one of the crucial organs that manage our blood sugar levels and other vital functions (like filtering our blood), having low liver fat levels is a vital marker of our health. Reducing liver fat levels can also reduce your risk of developing type 2 diabetes.
The results showed that after 12 weeks, the 5:2 diet led to a reduction in liver fat levels of 50.9%, the low-carb group by 53.1%, and the low-fat diet by 16.8%.
Participants in the 5:2 and low-carb diets also saw more significant reductions in weight, insulin resistance, and blood sugar levels compared to the low-fat diet.
Similarly, an annual review on the impact of intermittent fasting, calorie restriction, and the 5:2 diet on cardiovascular risk factors analysed five human trials using the 5:2 diet as an intervention.
It highlighted that 5:2 diets typically improve blood sugar levels, insulin resistance, triglycerides (how much fat is transported in the blood), and blood pressure.
This suggests that when adhered to, the 5:2 approach can be an effective tool for people to reduce their risk of chronic disease and improve their physical health.
However, the authors highlighted that as most human trials completed on 5:2 diets are relatively short (typically less than 6 months), it’s challenging to know their impact on long-term health and how sustainable an approach they are.
- 5:2 diets improve markers of physical health, such as blood sugar levels, blood pressure, and insulin resistance.
- However, the trials completed on 5:2 so far have been relatively short, and it’s challenging to know what effect they’ll have on long-term health.
- The challenge for individuals would be whether they’re sustainable or whether they can transition into a more sustainable approach after using a 5:2 diet as a short-term intervention.
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3) The 5:2 diet may not be sustainable for many people
While the physical effects of the 5:2 diet are favourable for those that can adhere to it, research suggests that many people struggle to maintain the approach over the long term.
A randomised controlled trial in 300 individuals (the one discussed above) measured retention and adherence to the diet over the course of 12 months.
After six weeks in the 5:2 diet with group support, 86% of the participants were still following the diet. At 6 months, this dropped to 69%, and after 12 months, it was down to 45%. The results in the self-led 5:2 diet were similar (87%, 77%, and 56%).
These results mirror a similar study comparing intermittent fasting, the Mediterranean diet, and the paleo diet. The results showed that after 12 months, only 54% of the individuals on the fasting diet followed the approach.
Other potential side effects to be mindful of
Alongside potential issues with adherence and sustainability, some research suggests that more severe approaches to fasting and very low-calorie diets, while promoting effective weight loss, may lead to other health complications.
Research in post-menopausal women compared the effect of severe or moderate calorie restriction on weight loss and bone mineral density after 12 months.
The study showed that severe calorie restriction was twice as effective for weight loss, but it also led to a significantly greater decrease in bone mineral density in the hip.
As we age, our bone mineral density is a critical marker of our health – and lower levels can increase the risk of fracture and further health complications.
However, this wasn’t a 5:2 approach, so it’s unclear whether intermittent calorie restriction would have a similar effect. We have no long-term data on individuals following a 5:2 diet, so it’s hard to say either way.
Pre-menopausal women also seem particularly sensitive to changes in energy intake. Severe caloric restrictions harm essential sex hormones regulating fertility and the menstrual cycle.
This was demonstrated in a clinical study where 22 women were placed on an 800-1200 kcal diet during their menstrual cycles (between 24-38 days). The results indicated a significant drop in the sex hormones regulating the menstrual cycle and fertility.
For example, FSH (follicle-stimulating hormone, essential in maintaining the production and growth of ovarian follicles) dropped by 13.5% and LH (luteinising hormone, necessary in the release of the egg and production of progesterone) by 38.5%.
These changes were outside the typical transient changes observed in these hormones during a regular cycle, where energy availability is higher.
Again, this wasn’t a 5:2 approach, so it’s difficult to know whether intermittent calorie restriction will have the same effect.
But if you experiment with the 5:2 diet, be sure to be mindful of any changes or irregularities in your menstrual cycle, particularly during the luteal phase and seek medical advice if required.
- Human trials in fasting and the 5:2 approach have suggested it’s not sustainable for many people in the long term.
- Research on severe calorie restriction has also shown that bone mineral density can be negatively affected, despite the positive effect on weight loss.
- Premenopausal women are particularly sensitive to caloric restriction as it can harm their menstrual cycle and fertility.
- However, as there is no long-term data on 5:2 diets, it’s difficult to know the lasting effects, and we can only speculate on what might or might not happen.
Take home message
Adherence and sustainability is where most diets fall down. The first 3-6 months look very promising, then after 12 months or more, the ability to stick to diets becomes more challenging, and weight is typically regained.
To maintain a diet plan in the long term, it’s essential to reflect on the bigger picture and approach your lifestyle from a more balanced perspective.
You can’t be perfect; you’re unlikely to stick to rigid rules or restrictions over the long term, and the more you try and restrict something specific, like food, the more likely you are to think about it and overdo it.
Fasting and intermittent caloric restriction diets like the 5:2 diet are no different. You might tell yourself, “I can’t eat more than 800 calories today”.
But then what do you think about? You think about food, probably more than you would if you were enjoying a healthy balanced diet that provides your body and mind with all the nourishment it needs.
This is called the ‘white bear effect’. It just so happens that the brain tends to work against us when trying to restrict something. If you tell yourself you’re not allowed to, the thoughts of that thing become accessible in your mind, and the cravings for them can become too great.
Healthy eating is about much more than what you choose to add to your diet. It’s also about your relationship with the foods you consume.
So if fasting and other diets have failed you in the past, a more holistic and sustainable approach to food – one that removes rules and restrictions and leaves you feeling preoccupied about food all of the time – might be the best way forward for your health and wellness.
Want to know what this feels like and achieve a healthy weight? Click here to try an NHS-trusted Second Nature meal plan.