Food and diet

No proof 5:2 diet prevents cancer

"Could 5:2 diet help to ward off cancer?" is the question posed by the Mail Online after the publication of a study into experimental diets.

An honest and accurate answer to the question, based on the study, would be "we don't know".

The Mail reports on a study that gives an overview of the evidence surrounding low-calorie diets and intermittent fasting, and whether they are beneficial to our health.

But this study does not provide new evidence on the 5:2 diet or whether fasting diets ward off cancer.

As the piece didn't report the methods used to find the evidence, it is unclear whether any cherry-picking took place. That is, evidence supporting the authors' opinions may have been included, but conflicting evidence may have been ignored.

The article says we need more good-quality research on issues such as whether certain types of diet can have a wider beneficial impact on health. We would certainly agree with this view.

The best way to reduce your cancer risk is to eat a balanced diet that includes plenty of fruit and vegetables, and is low in red and processed meat and salt. 

Where did the story come from?

The study was carried out by researchers from many different collaborating US universities, as well as one UK and one Belgian University.

It was funded by the US National Institute on Aging Intramural Research Program and the Glenn Foundation for Medical Research, the US National Institutes for Health, the European Union's Seventh Framework Programme MOPACT, Genesis Breast Cancer Prevention (UK) and the Belgian Foundation for Scientific Medical Research.

The study was published in the peer-reviewed Proceedings of the National Academy of Sciences (PNAS).

The media reporting was generally true to the facts of the research, which focused on the speculation that a fasting diet might reduce the risk of cancer. 

But neither the Mail nor The Daily Telegraph made it clear to their readers that this study was not a systematic review, which would have given more weight to its findings. Rather, it was more of an expert opinion piece.

The Telegraph did include a useful quote from Tom Stansfeld of Cancer Research UK, who said more research was needed looking into the long-term effects of intermittent fasting.

He added: "Decades of research tell us the best way to reduce the risk of cancer through nutrition is to eat a balanced diet with plenty of fruit and veg, and low in red and processed meat and salt."

What kind of research was this?

This was an evidence-informed "perspective" article describing the physiological responses of people and animals to controlled variation in meal size, frequency and timing of meals, and the impact on health and disease.

The study group described how research efforts have largely ignored the importance of the frequency and timing of meals, and potential benefits of periods of no or very low energy intake. Hence, they sought to describe some of the evidence around this grey area.

They argue eating three meals a day is abnormal from an evolutionary point of view. They also describe how the habit of eating three meals a day appears to have begun when humans switched from being hunter gatherers to farmers around 12,000 years ago.

The rationale is that our bodies, which have evolved incrementally over millions of years in the context of periods of fasting, may not be best suited to this relatively modern dietary switch.

Some studies suggest restricting energy intake for as little as 16 hours can have health benefits. They say the mechanisms that mediate this benefit are metabolic shifts to using fat as an energy source, and the stimulation of cellular responses that prevent and repair molecular damage.

What did the research involve?

It was not clear how the evidence to inform this "perspectives" piece had been searched for, selected or synthesised, as no methods were described.

As no systematic methods were described, as would be the case with a systematic review, we cannot discount the potential influence of bias on the evidence selection, sifting and synthesis. These biases have the potential to influence the content and conclusion of the article.

What we do know is the piece considered evidence on three broad experimental diets:

  • caloric restriction (CR) – where daily calorie intake is reduced by 20-40% and meal frequency is unchanged
  • intermittent energy restriction (IER) – this involves eliminating (fasting) or greatly reducing daily food and caloric beverage intake intermittently; for example, two days a week, as is used in the popular 5:2 diet
  • time-restricted feeding (TRF) – this involves limiting daily food intake and caloric beverages to a four to six-hour period

This article also reportedly incorporated information from a workshop on eating patterns and disease. Those with a particular interest in experimental diets may find the video of the workshop interesting, though we should warn you it is more than six hours long.

The Mail Online said the IER 2-Day Diet described in the article is the forerunner to the 5:2 diet. It involves two days of eating just 600 to 1,000 calories of low-carbohydrate foods. On the other five days, the dieter eats a healthy Mediterranean diet. Women usually need 2,000 calories a day, while men need 2,500.

What were the basic results?

There are no clear new results presented in this article, as it presents a flowing, evidence-informed description of the state of knowledge around the timing and frequency of eating and its potential influence on health. The media picked up on the description of the IER diet section around cancer.

The research said: "IER/fasting can forestall and even reverse disease processes in animal models of various cancers, cardiovascular disease, diabetes and neurodegenerative disorders", citing a single source on the molecular mechanisms of fasting.

It then goes on to describe four general biological mechanisms by which IER might protect cells against injury and disease.

It also suggests future directions for research and society-wide implications, highlighting how recommendations for healthy patterns of meal frequency and timing may emerge as more evidence gathers consensus.

How did the researchers interpret the results?

The researchers indicated that, "If sufficient evidence does emerge to support public health and clinical recommendations to alter meal patterning, there will be numerous forces at play in the acceptance or resistance to such recommendations."

These, they said, included the cultural tradition dictating three meals a day, the food industry's vested interest in making people eat frequently, and the ability or willingness of health systems to emphasise prevention through lifestyle modification, overtreatment and medicalisation.


This evidence-informed article presents an overview of, and perspective on, the potential mechanisms through which low calorie or intermittent fasting diets may be beneficial to the body.

The information provided by the authors is certainly interesting. But this study does not provide new or compelling evidence proving that fasting diets actually lead to a lower risk of disease or postponement of death.

This does not appear to be a systematic review, where the authors would search the global literature to identify all relevant evidence on the effects of different eating patterns on health outcomes.

As the piece reported no methods, we do not know how evidence for the article was searched for, selected or synthesised, and it therefore has the potential to be biased. 

The main contribution of this study is as a discussion starter. From the evidence included in the piece, it seems clear there is relatively little definitive evidence pointing to the best pattern or timing of meals. In this void of evidence, there may be misinformation.

For example, the researchers say that despite equivocal and even contradictory scientific evidence, breakfast is often touted as a weight-control aid, but recent evidence has suggested it may not be.

In addressing or clarifying potential misinformation, the article says we need more clarity about these still grey issues through more and better research.

The authors also say we need to ensure that the best available evidence is informing public guidelines and knowledge on these topics. It is tough to argue against this.

Intermittent fasting diets such as the 5:2 diet may not be suitable for pregnant women and people with specific health conditions, such as diabetes or a history of eating disorders.

NHS Attribution