Intermittent energy restriction

It is often claimed that intermittent energy restriction helps shed pounds, improves metabolism, prevents cardiovascular diseases, reduces inflammation, boosts fitness, and extends life. However, many of these claims are either unproven or insufficiently supported by scientific studies.

When studies do exist, they often suffer from several common issues:

• The number of participants is too small, making the results less reliable.
• The follow-up period is too short to assess the risk of the yo-yo effect.
• A high dropout rate, where participants who struggled with fasting may have left the study, potentially skewing the results.
• Different types of intermittent fasting are studied, making comparisons between studies difficult.
• Often, there is no control group with a standard diet, so it is unclear how intermittent fasting compares to regular dieting.
• Conflicts of interest may exist, as some studies investigate products being marketed for sale.

Some of these problems can be mitigated when several studies on intermittent energy restriction are combined in a so-called meta-analysis. Simply put, in a meta-analysis, for example, three different studies with 30, 50, and 70 participants are combined and evaluated using special statistical methods as if it had been one large study with 150 (=30+50+70) participants. In 2022, such a meta-analysis on the effects of intermittent energy restriction on body weight was published. Sixteen studies were included in the main evaluation.

Per study, between 8 and 68 participants took part in intermittent energy restriction, which was carried out between 2 weeks and 6 months. All studies had a control group with a calorie-reduced diet. Weight loss in intermittent energy restriction across the 16 studies ranged between 0.5 kg and 13.9 kg, and in calorie-reduced diets between 1.9 kg and 11.8 kg. Weight loss with intermittent energy restriction was not statistically significantly different compared to a calorie-reduced diet. In addition, there were no statistically significant differences in weight loss between 1:1 fasting, 5:2 fasting, and time-restricted eating. Maintaining intermittent energy restriction became increasingly difficult for participants over time, as is also typical for calorie-reduced diets.

The key results of this meta-analysis were confirmed in two subsequent high-ranking published studies.

In a study by Lin and colleagues, time-restricted eating (mTRE) was compared with a calorie-reduced diet (=25% reduction in calorie intake). Each group included 30 participants who were followed for 12 months. Most participants completed the study successfully. Weight loss after 12 months was 4.6 kg in the time-restricted eating group and 5.4 kg in the calorie-reduced diet group. However, the difference of 0.8 kg between the two groups was not statistically significant. The authors concluded that time-restricted eating over a period of 12 months is not more effective than a calorie-reduced diet.

In another study, Liu and colleagues investigated whether time-restricted eating (eTRE) over 12 months in addition to a calorie-reduced diet (= energy intake between 1200 and 1500 kilocalories per day for women or between 1500 and 1800 kilocalories per day for men) is more effective for weight loss than a calorie-reduced diet alone. In total, 139 participants were equally divided into the two groups, and most participants completed the study successfully. The average weight loss after 12 months was 8.0 kg in the time-restricted eating and calorie-reduced diet group and 6.3 kg in the group with calorie-reduced diet alone. However, the difference of 1.7 kg between the two groups was not statistically significant. The authors concluded that time-restricted eating in addition to a calorie-reduced diet does not offer a significant advantage for weight loss.