Caloric restriction beats fasting diets for weight loss, new trial finds

A trial led by researchers from the University of Bath is questioning the value of popular fasting diets, finding a simple calorie-controlled diet is more effective at reducing weight. The trial also found fasting can lead to greater losses of muscle mass.

Intermittent fasting is a popular dietary phenomenon that can take a wide variety of forms, from time-restricted feeding patterns (limiting food intake to a few hours each day) to alternate-day fasting (eating every other day, or fasting for a whole day once or twice a week). The science is far from settled on whether these kinds of eating patterns are beneficial for weight loss or even general metabolic health.

The few robust controlled trials testing these diets have delivered mixed results, often suggesting intermittent fasting is no more effective that a straightforward calorie-controlled diet. A 2019 trial testing alternate-day fasting found it was safe and at least as effective as caloric restriction, and a more recent time-restricted feeding trial reached similar conclusions.

This new trial set out to investigate whether fasting generates any specific weight loss or metabolic improvements compared to a consistently fed control group consuming the same overall caloric intake. A small cohort of lean, healthy volunteers was randomly separated into three groups: a control group tasked with eating a diet restricted to around 1,500 calories a day, an intervention group fasting every other day and eating around 3,000 calories on their feeding day, and a third group fasting every other day but eating two times the recommended daily caloric intake on their feeding day (around 4,000 calories).

Essentially, the interventions were geared to explore whether fasting without any caloric restrictions made a difference to ultimate weight loss. The findings revealed the third group, those fasting without reducing their overall caloric intake, did not show any significant weight loss after four weeks.

The other fasting group, consuming a similar caloric amount in totality to the dieting group, did register an average of 1.6 kg (3.5 lb) weight loss after four weeks. However, the simple caloric restriction group lost an average of 1.9 kg (4.1 lb) after four weeks. So, even though the restricted fasting group essentially consumed the same amount of calories as the control, they did not lose as much weight.

“Many people believe that diets based on fasting are especially effective for weight loss or that these diets have particular metabolic health benefits even if you don’t lose weight,” explains James Betts, lead researcher on the project. “But intermittent fasting is no magic bullet and the findings of our experiment suggest that there is nothing special about fasting when compared with more traditional, standard diets people might follow.”

Perhaps even more importantly, the researchers found about half of the weight loss seen in the restricted fasting group was due to lost muscle mass. This contrasts with almost all of the weight loss in the control group seen coming from reductions in body fat content.

It is hypothesized that the greater loss of muscle mass in the fasting cohort could be related to a reduction in overall physical activity observed on fasting days. The researchers note this behavioral change is largely subconscious, indicating fasting subjects may not even realize they are reducing their physical activity on fasting days.

“Most significantly, if you are following a fasting diet it is worth thinking about whether prolonged fasting periods is actually making it harder to maintain muscle mass and physical activity levels, which are known to be very important factors for long-term health,” adds Betts.

Of course, as with all of these kinds of studies, there are limitations in how broadly the results can be interpreted. The trial was conducted in lean and healthy subjects, for example, so it is unclear whether fasting obese subjects would register greater fat reductions compared to the muscle mass loss seen in this study.

The researchers also note alternate-day fasting still may be a preferable intervention for some as it has been considered an easier dietary pattern to follow. It is advised that countermeasures to protect lean muscle mass, such as resistance training, are adopted if one chooses to follow alternate-day fasting protocols.

The new study was published in the journal Science Translational Medicine.

Source: University of Bath