- Group 1, which fasted on alternate days, with their fast day followed by a day of eating 50% more than usual.
- Group 2, which reduced calories across all meals every day by 25%.
- Group 3, which fasted on alternate days (in the same way as Group 1) but followed their fast day with one day eating 100% more than usual.
Participants in each group were consuming a typical diet of around 2,000-2,500 calories a day on average at the start of the study. Over the course of the three-week monitoring period, the two energy-restricted groups reduced this to be an average of between 1,500-2,000 calories. Whereas groups 1 and 2 reduced their calorie intake by the same amount in different ways, group 3′s diet saw them fast without reducing overall calories.
At the end of three weeks, group 2 had lost slightly more than 4 pounds, and body scans showed it was all fat loss.
Group 1, which reduced its calorie intake by fasting on alternate days and eating 50% more on nonfasting days, lost almost as much body weight (3.5 pounds), but only half was from reduced body fat. The other half was from muscle mass.
Group 3′s weight loss was negligible, the researchers wrote.
“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,” Betts said. “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.
“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.”
The study was published Wednesday in the journal Science Translational Medicine.
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