Food and diet

Is there an effective new alternative to the '5:2 diet'?

"Eating whatever you want between 10am and 6pm boosts weight loss after just 12 weeks," is the alluring headline from the Mail Online, reporting on a study into what's generally known as "intermittent fasting".

The most well-known intermittent fasting diet is the 5:2 diet, where you are encouraged to eat normally for 5 days a week and fast for the other 2. Researchers in this latest study looked at a variant form of intermittent fasting dubbed the "16:8 diet".

With the 16:8 diet, you fast for 16 hours a day and eat whatever you want in the 8 hours between 10am and 6pm. Researchers speculated this form of intermittent fasting may be more tolerable for some people.

This small study found that the group who followed the 16:8 diet for 12 weeks had an overall average reduction in body weight of about 3%, consumed on average 341 fewer calories a day and saw their blood pressure decrease.

Despite the positive findings, only 23 adults followed the time-restricted diet, which is not enough to draw reliable conclusions. The total amount of weight loss was also quite small: a 3% loss of body weight for an obese person amounts to around 3kg.

The NHS weight loss plan, which involves both diet and exercise, is designed to help people lose up to 1kg a week, so a total of 12kg over the 12-week study period would have been a more impressive result.

Read more about the NHS weight loss plan.

Where did the story come from?

The study was carried out by researchers from the University of Illinois, Indiana University and the Salk Institute for Biological Studies, all in the US. It was funded by the US National Institutes of Health and published in the peer-reviewed journal Nutrition and Healthy Aging.

The Mail Online reported the study fairly accurately, although it did say the control group was on a "different sort of fasting diet", which was not the case. This group was actually told to stick to their usual eating and physical activity habits.

What kind of research was this?

This study was a 12-week trial, using an intervention and control group.

Randomised controlled trials (RCTs) are the most reliable way of evaluating a treatment. However, this trial was not randomised, as the control group consisted of people who had taken part in a previous weight loss study, so we have no clear evidence that their data was relevant to the intervention group.

Carrying out an RCT that randomly allocated people to either group at the beginning of the study would be the only way to truly determine if there is a difference between the intervention and control.

What did the research involve?

In the intervention group, 23 obese adults followed an 8-hour time-restricted feeding intervention for 12 weeks. They could eat whatever they wanted between 10am and 6pm but then fasted and drank only water or zero-calorie drinks (such as black tea) between 6pm and 10am.

To be eligible, people had to:

  • have a body mass index (BMI) between 30 and 45
  • be aged between 25 and 65 years
  • be sedentary or lightly active (less than 7,500 steps a day)
  • have had a stable weight for 3 months prior to the beginning of the study (less than 4kg weight loss or weight gain)

People were excluded if they were diabetic, had a history of cardiovascular disease, smoked, worked shifts or took weight loss medications.

The researchers compared the intervention group with a control group from a previous weight loss study they ran from October 2011 to January 2015.

The 31 controls were of a similar age, BMI and gender distribution to the intervention group. They had been instructed to follow, and make no changes to, their usual diet. They were weighed on a weekly basis.

The intervention group completed a 7-day food record at the beginning and end of the 12-week study period. They had a 15-minute session with a dietitian at the beginning of the study that explained how to complete the diary.

The diaries were analysed using a computerised nutrition calculator that worked out each participant's daily intake of energy, fat, protein, carbohydrate, cholesterol and fibre.

The primary outcome measure was body weight, which was assessed to the nearest 0.25kg every week. The researchers also measured:

  • blood pressure and heart rate at the beginning and end of the 12 weeks
  • fasting plasma total cholesterol
  • low-density (bad) lipoprotein cholesterol
  • high-density (good) lipoprotein cholesterol
  • triglycerides (fats found in blood)
  • homocysteine levels – linked to occurrence of blood clots, heart attacks and strokes
  • fasting glucose, fasting insulin and insulin-resistance levels – linked to diabetes

The researchers then analysed differences between the groups at the beginning and end of the 12-week period to determine the effects of the diet.

What were the basic results?

Average body weight in the intervention group following the diet reduced from 95kg to 92kg, a loss of 2.6% (±0.5%). This meant their average BMI decreased from 35 to 34. Their calorie intake also reduced, by an average of 341 calories a day – the equivalent of about 1.5 chocolate bars.

The control group's average weight remained at 92kg, their BMI at 34 and their calorie intake at around 1,654 calories.

Systolic blood pressure also decreased in the intervention group by 7mmHg (±2mmHg) versus controls.

There was no difference in any of the other measured outcomes.

How did the researchers interpret the results?

The researchers said their findings suggest that 8-hour time-restricted feeding produces mild calorific restriction and weight loss, without calorie counting, and that it may also offer clinical benefits by reducing blood pressure.

They added that these early findings offer promise for the use of time-restricted feeding as a weight loss technique in obese adults but further studies, using longer-term and larger-scale RCTs, are necessary.


This study was a nice attempt at examining the effect of time-restricted feeding on body weight and risk factors for metabolic diseases that increase the risk of heart disease, stroke and diabetes.

However, there were a number of limitations.

The control group was from a previous study that began 5 years before the current study. This makes it more difficult to match the controls to the intervention group – indeed, the controls were lighter than the intervention group at baseline.

Because the participants reported it themselves, the measure of adherence to the diet might be inaccurate because people tend to forget or lie about exactly what they've eaten. Also, most of the participants were women, so the results may not be applicable to men.

As 12 weeks is quite short for a trial, it may not have been long enough to see a big difference in weight loss and also means it's unclear how sustainable such a diet may be in the long term.

Finally, these findings were based on adults who were obese but otherwise healthy. People who have a problem with their cholesterol levels, blood pressure, diabetes or heart health should always consult their GP before starting a fasting-style diet.

There are many ways other than fasting to lose weight – whether it's doing more exercise or making changes to your diet – that can hugely improve your health if you are obese. Read more about the NHS weight loss plan.

NHS Attribution