The traditional approach to weight loss is to count calories and try to reduce the number consumed each day. This is a time-consuming and error-prone process – often with disappointing results. Intermittent fasting – and the popular version known as time-restricted eating – could be a simpler option for people wanting to achieve a healthy weight.
But is intermittent fasting any better than calorie counting for losing weight? A new study, published in the Annals of Internal Medicine, aimed to provide the answer. It showed that the two methods could be equally effective – if undertaken with professional counselling.
In this year-long study, researchers at the University of Illinois Chicago recruited 90 adults with obesity, aged 18 to 65. The participants were randomly allocated to one of three groups:
- a time-restricted eating group who were required to consume all their calories each day between noon and 8pm
- a daily calorie restriction group, who were required to reduce their calorie intake by 25% by closely tracking their diet
- a control group who maintained their normal dietary patterns throughout the study.
The participants lost about 5% of their starting body weight on both diets in the first six months. The diets were then adjusted to help maintain this weight loss over the next six months.
The time-restricted eating group extended their eating window to ten hours (10am to 8pm) and the daily calorie restriction group increased their calorie intake to match their requirements, which was calculated based on their weight, height, age and activity levels. The control group maintained the same eating pattern.
The researchers hypothesised that participants focusing on reducing the number of hours they ate would achieve and maintain weight loss better than participants focusing on counting calories. The effects of these two diets on body composition (muscle, fat and bone mass), waist circumference, and a range of health markers were also assessed.
The study found that restricting the time during which you can eat and restricting the number of calories were equally effective for losing weight. Participants in both groups lost about 4% of their starting body weight after 12 months.
Both diets also reduced waist circumference and fat mass to a similar extent. Diet records revealed that calorie intake was reduced to a similar extent with both diets, despite the different approaches.
Neither diet showed any changes in health markers, such as glucose, insulin or cholesterol levels. One reason for this may be the use of a late time-restricted eating window (12pm to 8pm), which was considered to be more acceptable for participants.
There is evidence an early time-restricted eating window (8am to 4pm, for instance) can achieve greater weight loss and improve blood glucose regulation.
Scientists aren’t certain why this is the case. However, research suggests that our metabolism is more efficient earlier in the day, aligning with our natural waking and sleeping patterns. This means that the body may be better at using nutrients consumed early in the day.
These findings support previous studies that have found similar weight loss when comparing time-restricted eating and other popular versions of intermittent fasting (such as the 5:2 diet), to daily calorie restriction.
These studies all show that calorie restriction – whether achieved by reducing the time during which people are allowed to eat or counting the number of calories eaten – is the main thing that determines weight loss.
The new study shows that time-restricted eating can lead to weight loss without explicit instruction to reduce calorie intake. Another strength of this study was the racial diversity of the participants (79% were black or Hispanic), meaning these results can be applied more widely than most previous studies.
However, one important aspect of this study that makes it difficult to conclude that these interventions alone are enough to help people lose weight is the fact that participants in both dietary intervention groups received a lot of counselling during the study.
This included healthy-eating guidance and cognitive behavioural therapy (a type of talk therapy) to reduce impulse eating. This probably helped participants reduce the urge to eat high-calorie food after completing their fasting window.
Whether this study shows that time-restricted eating and daily calorie restriction are equally effective for weight loss, or whether professional support with healthy eating helps with weight loss, is debatable.
Interestingly, a recent study found that time-restricted eating without additional support did not lead to weight loss after three months.
There were also substantial differences in weight loss between individual participants on each diet. This suggests there may be factors that allow time-restricted eating or daily calorie restriction to be more effective for some people than others.
Dieting is difficult, regardless of the method used. This new study suggests weight loss can be achieved using intermittent fasting, but some people will probably benefit more than others. Why that is, we don’t currently know.
David Clayton has received funding from the British Nutrition Foundation and Society for Endocrinology to conduct research on intermittent fasting.