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The 5:2 Diet – Everything You Need To Know

Published: 17th July 2018. Last updated: 24th July 2019.

Shaun Ward MSc ANutr

Staff Writer

What Is The 5:2 Diet?

The 5 2 diet is currently one of the most popular types of intermittent fasting, created by author Dr Michael Mosley.

The diet is a form of ‘intermittent energy restriction’, where 5 days of the week are normal eating days, while the other 2 days of the diet severely restrict calorie intake to 500-600 per day.

Intermittent energy restriction is an alternative to continuous dieting, which usually involves a minor degree of food restriction every day.

Individuals can self-select the days they wish to severely restrict calories on the diet, but these 2 days must not be in succession.

It’s important to emphasize that eating “normally” means to continue prior eating habits and should not imply that an individual has an unlimited food allowance.

No instructions are given regarding the types of food that should be consumed on the 5:2 diet, with the sole focus placed upon counting and significantly reducing calorie intake on restrictive days.

Does The 5:2 Diet Work?

As the 5:2 diet implements 2 severely restrictive days for food intake, this will thereby promote weight loss by decreasing average weekly calorie intakes.

Although this would likely cause weight loss by itself, research suggests that intermittent energy restriction diets also cause a mild spontaneous restriction of energy on non-restricted days, which further lowers weekly calorie intakes.

Weight loss seen on intermittent energy restriction diets are:

  • 1lbs loss in 6 months [1]
  • 2lbs loss in 12 weeks [2]
  • 7lbs loss in 8 weeks [3]
  • 2lbs loss in 6 weeks [4]

However, the vast majority of evidence concludes that intermittent energy restriction diets does not produce superior weight loss results compared to standard energy restriction regimens.

Only one known study is available that suggests intermittent energy restriction diets results in significantly more weight loss than a standard weight loss diet - 4.1% [5].

The 5:2 Diet Health Benefits

While the 5 2 program is primarily used for the purposes of weight loss, there are other health benefits too. Below we've broken down the positive effects that are proven in scientific study:

It May Increase Longevity

In animal models, intermittent energy restriction can extend lifespans by 0-80%, depending on the species and age at the time of study.

The increased longevity is speculated to be superior to that seen by standard caloric restriction diets. In fact, intermittent fasting diets seem to be able to increase lifespan without the need for a decrease in calorie intake – although better effects will be seen with weight loss [6].

This is potentially due to an improved ability to resist cellular stresses that cause cell damage and death.

Specifically, intermittent energy restriction may impose a mild beneficial stress to which cells adaptively respond to by upregulating the expression of cell defense systems - increasing the production of antioxidant enzymes and cytoprotective proteins. This is known as “hormesis” [7].

It is theorized that the large alternating periods of anabolism and catabolism on an intermittent energy restriction diet plays a mechanistic role in triggering the increase in cellular stress resistance.

However, the rapid decline in the levels of IGF-1, a major growth factor in humans, seen on energy-restricted diets is another potential mechanism that could increase longevity.

High IGF-1 levels may inhibit apoptosis - the programmed cell death of abnormal cells – and create an environment in which cancer can develop and progress.

Just 5 days of fasting has shown to cause over a 60% decrease in IGF-1, mainly due to protein and insulin restriction [8].

It May Help Protect Against Cardiovascular Disease

Caloric restriction has repeatedly demonstrated to decrease LDL cholesterol levels (“bad” cholesterol) and vascular inflammation, and increase HDL cholesterol (“good” cholesterol). All these factors have beneficial effects on the function and integrity of blood vessel walls and improve cardiovascular health.

Caloric restriction also improves arterial vasodilation, via decreased activity of the sympathetic nervous system, which can lead to profound improvements in regulating blood pressure.

Specific mechanistic benefits for intermittent energy restriction diets on cardiovascular health are the increase in adiponectin levels and the decrease in resistin levels [9].

Resistin, a hormone excreted by fat cells, accelerates atherosclerosis by promoting the formation of fatty deposits on vessel walls (foam cells) [10].

On the other hand, adiponectin may protect against vascular complications by promoting an antiatherogenic expression of genes in vessel walls that can reduce plaque formation.

Importantly, markers of inflammation and stress seem to remain low even on days when individuals return to normal eating habits between fasting days. This indicates a maintenance effect of the cardiovascular improvements from fasting that is typically not seen on other types of eating plans.

It May Improve Insulin Sensitivity

At least in animal studies, intermittent energy restriction appears equally or more effective than traditional isocaloric energy restriction for improving insulin sensitivity.

In fact, intermittent fasting diets have even shown up to ~40% lower fasting glucose and insulin levels compared to standard caloric restriction diets [11].

The superiority of intermittent energy restriction in this area appears to be due to a modest relative increase in adiponectin - a protein hormone that regulates glucose levels - which has a pivotal role in insulin sensitivity [12].

However, most of the beneficial effects on insulin sensitivity will be due to the weight loss, which has demonstrated to decrease average daily levels of insulin secretion and increase insulin receptor activity.

The diets influence on insulin sensitivity has allowed intermittent energy restriction to be named an effective alternative diet strategy for improving glycemic control and reducing the incidence of type 2 diabetes [13].

Is It Better Than Normal Dieting?

Both the 5:2 method and a regular calorie-restricted plan will achieve similar results for weight loss, insulin levels, insulin resistance, adiponectin, inflammation, cholesterol, and blood pressure. There are also similar improvements in a number of risk markers for cancer, diabetes, and cardiovascular disease.

However, there is emerging evidence that implementing very low-calorie days, or prolonged periods of fasting, may have additional benefits for longevity and long-term glycemic control, independent of weight change. This suggests possible metabolic advantages for the 5:2 diet, although more human research is needed to confirm this theory.

At this time there is not enough convincing evidence to suggest a superiority of intermittent energy restriction over standard energy restriction.

The best dietary approach for each individual will come down to which one they can sustain in the long-term.


People may find this way of eating to be easier to stick to than a traditional calorie-restricted diet, although differences are likely explained by personal preferences rather than scientific reasoning.

Overall, similar levels of dietary adherence have been found, with a ~25% dropout rate reported by intermittent energy restriction protocols; most people will find the diet is easier to stick to!

Levels of hunger also do not differ between intermittent energy restriction diet plans and standard weight loss diet plans after ~3 months, although there is usually a marginally greater amount of hunger experienced during the first restricted week of intermittent protocols [1]. This indicates that individuals may struggle initially with the change in meal timings but will quickly adapt to the restriction to make it sustainable.

In addition, contrary to popular belief that the diet is unsustainable as it may encourage ‘binge’ eating following restrictive days, research actually suggests that there is a continued spontaneous calorie restriction on non-restricted days, although to a much lesser extent.

Safety and Side Effect Information

There are few cases where side effects occur on intermittent energy restriction diets. <15% of individuals have mentioned they experience either irritability, low energy, or hunger on the diet, although more long-term studies are needed to evaluate any other potential concerns.

Of note, intermittent energy restriction diet plans can lead to deficiencies in magnesium, iron, zinc, calcium, fiber, plus vitamins A and D. These deficiencies are commonly seen on most weight loss diets and are not independent to intermittent fasting approaches.

However, a specific problem with the 5 2 diet is the possible short-term deficiencies of water-soluble vitamins, B and C, on restrictive days. These vitamins are quickly excreted from the body and therefore need to be consumed in adequate amounts every day to replenish their stores. As the 2 days of food restriction are very extreme on the 5 2 diet, it is extremely likely that individuals will become acutely deficient in these vitamins on these days.

It is not known what issues an acute micronutrient deficiency will cause, if any, as negative effects only tend to be noticed when deficiencies persist for a number of months.

This being said, it is still recommended to include foods dense in vitamin B (leafy greens, nutritional yeast, meat) and C (fruit, peppers) within the ‘calorie limit’ on restrictive days to avoid nutrient deficiencies.


The 5 2 diet is a form of ‘intermittent energy restriction’, where 5 days of the week are normal eating days, while the other 2 days of the diet severely restrict calorie intake to 500-600 per day.

Weight loss is common on intermittent energy restriction diet plans, with similar results compared to standard energy restriction regimens.

The diet can protect against cardiovascular diseases, improve insulin sensitivity, and increase longevity.

Sustainability does not seem to be an issue on the 5:2 diet, as individuals tend to adapt quickly to the drastic change in utilizing very-low calorie days.

This form of dieting has also shown to be a safe approach to weight loss and improving health, although it is advised to ensure adequate consumption of foods high in vitamins B and C on restrictive days to avoid acute deficiencies.

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