Alternate-day fasting has no specific health or metabolic benefits over standard daily diet

Reviewed by Emily Henderson, B.Sc.Jun 16 2021

An alternate-day intermittent fasting schedule offered less fat-reducing benefits than a matched "traditional" diet that restricts daily energy intake, according to a new, 3-week randomized trial involving 36 participants.

The study, which is one of the first to tease apart the effects of fasting and daily energy restriction in lean individuals, indicates that alternate-day fasting may offer no fasting-specific health or metabolic benefits over a standard daily diet. However, the authors caution that longer studies with larger groups are needed. Intermittent fasting, which involves cycling through voluntary fasting and non-fasting periods, has become one of the most popular approaches to losing weight.

There are many different intermittent fasting schedules, ranging from fasting for part of each day to the popular 5:2 diet (eating 5 days a week and fasting 2 days) to alternate-day fasting (eating one day, fasting the next). Many participants report that fasting schedules are relatively easy to adopt and stick by, and theories suggest that fasting can trigger beneficial changes in metabolism that encourage weight loss. However, few studies have examined the fasting-specific effects of intermittent fasting or compared its effects to diets that simply reduce daily net calories.

Iain Templeman and colleagues recruited 36 lean participants and split them into 3 groups of 12, who followed different diets for 3 weeks. The first group followed a restricted, alternate-day fasting diet (eating 150% of their habitual daily energy intake only every other day), the second group followed an energy-matched non-fasting diet with 75% daily energy intake, and the last group followed an alternate-day fasting diet with no restriction in energy intake (200% daily energy intake every other day).

After 3 weeks, the second group showed the greatest losses of both weight and fat, with an average fat loss of 1.57 kg. Meanwhile, the first group of alternate-day fasters lost weight but lost fat less effectively (an average of .74 kg), and the last group showed no significant drops in either weight or fat. Further studies showed there were no key differences in cardiometabolic health, metabolic molecules, or gene expression in fat cells between the 3 groups.

Templeman et al. noted that the alternate-day fasters tended to be less active than before starting the diet, hinting at one factor that may have impacted fat loss. The authors speculate that individuals considering alternate-day fasting should make sure to include opportunities for physical activity to maintain their energy expenditure.