Fasting has been integral to human societies throughout recorded history. From our Palaeolithic ancestors who experienced cycles of feast and famine to medieval monks who practiced self-denial, to fast is to be human. Today, the science of fasting is an exciting and evolving field of research.
One popular intermittent fasting approach is the 5:2 diet, outlined in The FastDiet by the late Dr. Michael Moseley and Mimi Spencer. [1] This diet involves eating normally for five days of the week while significantly restricting caloric intake — around 25% of daily needs — on the other two days. The two days of ‘fasting’ can be on consecutive or non-consecutive days.
The authors claim that following this diet leads to weight loss and numerous other health benefits, including lower cholesterol, reduced blood pressure, and decreased inflammation.
Structure of The FastDiet
The book is organized into three main sections:
- The Science of Fasting: This section explains the theoretical foundation of the 5:2 diet.
- Practical Tips: This part includes guidance for starting the diet and a Q&A addressing common concerns.
- Menu Planning: This section provides recipes and a list of staple ingredients, along with a helpful calorie counter at the end.
In this review, we will examine the central claims of the book and evaluate them against existing research.
Central Claims of The FastDiet
The authors present two main claims:
- Fasting offers various health benefits related to obesity, diabetes, cardiovascular disease, and cancer.
- These benefits can be achieved through significant caloric restriction for just two days a week.
Additionally, they assert that:
- The advantages of fasting extend beyond simple energy restriction.
- Adhering to the 5:2 diet is easier than following traditional continuous energy-restricted diets.
As the authors state:
“This is a book about fasting, the voluntary abstention from food. The benefits go beyond simply eating fewer calories; our bodies are designed for intermittent fasting.” [1]
Evidence Supporting the Central Claims
The first two claims are generally supported by evidence. Large-scale reviews have found that the 5:2 diet is an effective strategy for weight loss, [2–6] and may improve biomarkers like fasting insulin levels, [7] waist circumference, [4, 5] and systolic blood pressure. [5]
However, these reviews often conclude that the 5:2 diet is not more effective than traditional continuous energy restriction. While it can be a viable alternative, it shouldn’t be regarded as superior to other diets that promote modest, daily caloric restriction.
The third claim, suggesting that fasting’s benefits go beyond caloric restriction, lacks strong support from randomised controlled trials. Most evidence indicates that the weight loss outcomes of the 5:2 diet are similar to those of conventional dieting. [2–7]
Future research may uncover specific benefits of prolonged caloric restriction, but currently, it’s challenging to definitively claim that the 5:2 diet offers significant advantages beyond mere calorie reduction.
Is the 5:2 Diet Easier to Follow?
The authors assert that the 5:2 diet may be easier to adhere to compared to traditional diets. Dr. Moseley explains that having set days for fasting can help people resist temptations, as they can look forward to eating normally on non-fasting days. [1]
However, evidence on adherence is mixed. The original 5:2 study by Michelle Harvie and colleagues found that participants on the 5:2 diet were significantly less likely to continue the diet after six months compared to those on a continuous energy-restricted diet. [8] Furthermore, another randomised controlled trial reported that participants assigned to the 5:2 diet experienced more hunger, adverse events, and less favourable changes in dietary habits. [9]
Overall, larger reviews suggest that adherence rates between the 5:2 diet and continuous energy restriction are not significantly different. [2, 7] While some individuals might find the 5:2 approach easier, it is not universally more compliant.
Concluding Thoughts
The FastDiet is an accessible and motivating read, offering valuable insights for those interested in the 5:2 diet. Regardless of personal opinions on fasting, sitting with hunger and learning it is not a linear process that increases over time, but instead comes in waves, can be beneficial. As Dr. Moseley notes, “Hunger passes,” challenging the misconception that it builds to an unbearable level. [1]
This understanding can be crucial for those trying to lose weight, as restricting energy intake, whilst keeping energy expenditure constant, will almost certainly result in increased hunger (even if that restriction is only modest). However, individuals with a history of disordered eating or those who are underweight should consult a healthcare professional before embarking on a 5:2 diet or fasting more generally.
Ultimately, while the 5:2 diet may not be a magic bullet solution, it offers a promising alternative for individuals who have struggled with continuous calorie restriction or consider themselves very ‘all or nothing’.