"Fasting diets do not make much difference to weight loss compared to a traditional calorie-controlled diet," The Daily Telegraph reports.
A study found people on an "every other day" diet (where they restricted their calorie intake to around 500 calories every other day) lost no more weight than those on a normal diet plan.
The year-long study, carried out in the US, involved randomising 100 overweight people to one of three options:
Both dieting groups lost weight compared to the control group, but there was no difference in weight loss between the two diets.
Though this may suggest that the two diets are equivalent, there are several important limitations to the trial. A third of participants dropped out – mostly from the alternate day fasting group, which suggests that some people may find that regular fasting is difficult to stick to.
Another point to consider was the study population was mainly made up of black people from one region of the US – so the results may not be applicable to others.
The best way to lose weight is to avoid gimmicks, and instead make permanent changes to the way you eat and exercise. You can do this by making some healthy changes, such as eating more wholegrain food, fruit and vegetables, eating fewer calories and doing more exercise.
These principles underpin the NHS Choices Weight Loss Plan which is designed to promote safe and sustainable weight loss.
The study was carried out by researchers from the University of Illinois, University of Alabama, Stanford University and Pennington Biomedical Research Centre, Louisiana, all in the US.
The study was funded by a grant from the National Heart, Lung and Blood Institute and grants from the National Institute of Diabetes and Digestive and Kidney Diseases. One author declared receiving an advance for the book "The Every-Other-Day Diet: The Diet That Lets You Eat All You Want (Half the Time) and Keep the Weight Off".
The UK media reporting of the study was generally accurate.
This was a randomised controlled trial (RCT) aiming to compare the effect of two dieting interventions – alternate-day fasting and daily calorie restriction – on body weight and reduction in risk of cardiovascular disease. Both were compared with a control group of no intervention.
Intermittent fasting diet plans, such as alternate day fasting, or the 5:2 diet, have increased in popularity in recent years.
Alternate day fasting is where individuals consume around 500kcal with normal days of full calories in between. However, whether this actually results in weight loss has not been assessed before.
This type of study is good for looking at the effects of different interventions as the randomisation process should control for other factors, such as exercise levels, that could otherwise vary between individuals. This trial carefully controlled the types and quantity of food given to each group to assess the specific effect of an alternate-day diet and a calorie restriction diet.
This was a randomised controlled trial of 100 overweight and obese participants from the US, who had no history of cardiovascular disease or diabetes. The aim was to compare the effects of alternate-day fasting and daily calorie restriction on weight loss, weight maintenance and risk of cardiovascular disease.
Participants were randomised to either the dietary intervention group or a control group for one year. During the six month weight loss phase, the alternate-day fasting consisted of 25% of energy needs, all at lunch, on a fasting day and then 125% of energy needs on the alternate "feasting" day, split between three meals. Daily calorie restriction involved consuming 75% of energy needs every day, so both groups had the same number of calories over the course of the trial.
For the first three months, the diet groups were given their meals to attempt to control for consumption and make sure intake of fat, carbohydrates and protein were in line with government guidelines. Following this, they were in control of their own diet but had weekly meetings with a dietician.
All participants were asked not to change exercise habits. The control group were asked to maintain their weight throughout trial, but received no dietary instruction.
Participants were assessed for change in body weight and cardiovascular disease risk indicators such as blood pressure and heart rate.
Overall dropout across all groups was 31%. The dropout rate was highest in the alternate-diet group, with 13 out of 34 participants (38%) dropping out, compared with 29% in the calorie restriction group and 26% in the control group.
The study authors conclude that "the alternate-day fasting diet was not superior to the daily calorie restriction diet with regard to adherence, weight loss, weight maintenance, or improvement in risk indicators for cardiovascular disease."
This study finds no difference between alternating-day fasting diets and daily calorie restriction diets in terms of weight loss and cardiovascular disease indicators.
So this does not show that fasting diets don't work – people in this group did lose weight compared to the control group, just that they are no different to a calorie restriction diet.
While this seems good evidence that one is not better than the other, there are some important things to consider before taking the findings at face value.
Sample size and drop-out
The drop-out rate was high. Ideally in a trial you would hope to see at least 80% of participants completing the trial to give reliable results. This trial saw around a third drop-out, which is particularly relevant given that the overall sample size was fairly small.
After drop-out just 21 people in the alternate-day fasting group and 25 in the daily calorie-restriction group remained. A larger sample size could have given better evidence and might have shown more of a difference between groups.
Lack of adherence
Adherence to the assigned diets wasn't good. Participants in the alternate-day fasting group ate more on fasting days and less on "feasting" days than prescribed, making their diet more like the daily calorie-restricted diet. This means it wasn't a very reliable comparison of two diets, which might explain the similarity in outcomes of the two groups. Participants were also in control of their own diet after the first three months and might not have reported their consumption accurately, leading to further adherence bias.
Participants were all overweight and obese but without diabetes or cardiovascular disease. The results might not be generalisable to others, including those with those diseases or those who are following a 5:2 approach to maintain a normal body weight. The majority of participants were also classified as of a black ethnic background from a particular area in the US. The results might not be generalisable to people of all other ethnic backgrounds or other areas in the world.
Ultimately, you are more likely to stick to a diet if you enjoy it (or at the very least, don't find it intolerable). While some people may respond well to a fasting diet plan, it would appear that it is not suited for everyone.
Also, when it comes to weight loss, it is important not to just focus on calorie restriction and ignore the role of exercise, which as well as helping shed the calories, also brings a range of additional health benefits.
The NHS Weight Loss plan is designed to help you lose weight at a safe rate of 0.5kg to 1kg (1lb to 2lb) each week by sticking to a daily calorie allowance and taking regular exercise.