The interval diet is a way to lose weight that promotes flexibility over counting calories. This approach limits meal times to reduce calorie intake. At least there is such a theory. A popular version of the interval diet is the 5:2 diet, which involves eating a low-calorie meal (about a quarter of your usual calorie intake) for two days a week. In the remaining 5 days, food is taken indefinitely.
This approach works well for some people, but not for everyone. In one study, we found that people change their interval diet without realizing it. For an interval diet to lead to weight loss, the number of calories burned must exceed the number of calories consumed.
What the study actually showed
The three-day study aimed to find out how food and physical activity changed during the period of calorie restriction. A group of male participants completed two trials. On the first trial day, they were told that they would be on a very low-calorie diet (about 700 calories) the next day.
For the rest of the day, we tracked how much participants ate and assessed their hunger before and after each meal. The physical activity of the subjects was also monitored during the day.
The next day, the participants ate a very low-calorie meal, and we monitored their physical activity. On the morning after completing a low-calorie diet day, we measured food intake at an unlimited breakfast and assessed their hunger before and after the meal.
Each participant also completed a control test using the same method. During the control trial, participants ate a regular diet (about 2,800 calories) instead of a low-calorie diet.
What research has shown
We found that participants ate 6% more on the first day of the study. At an unlimited breakfast with a low-calorie diet, 14% more. This suggests that the participants ate more because they knew food intake would be limited the next day, not because they felt hungry.
Physical activity was also 11% lower the day before the low-calorie diet and 18% lower on the low-calorie diet.
Interestingly, low-intensity physical activity, such as washing dishes, which is often a spontaneous behavior rather than a consciously planned activity, was the most affected component of physical activity. We found that changes in diet and physical activity occur before, during, and after a day of low-calorie dieting. These behavioral changes reduce the likelihood of interval dieting resulting in weight loss.
For an interval diet to lead to weight loss, the number of calories burned must exceed the number of calories consumed. This should cause a calorie deficit. The fasting interval diet assumes that a large calorie deficit caused by fasting or a very low calorie diet is not reversed indefinitely. So there is a calorie deficit.
But our research shows that a little more food and reduced spontaneous physical activity may be enough to restore nearly half of that calorie deficit. The calorie deficit can also be reduced with subsequent meals after a very low calorie diet.
Is It Worth the Sacrifices for Interval Dieting?
Previous studies support our findings. Skipping breakfast for six weeks has been shown to reduce physical activity and increase calorie intake at later meals. This was enough to fully compensate for the calories skipped at breakfast. The question arises:
Is it worth sacrificing hunger or strict calorie restriction?
Weight loss from any diet can always be lower than expected. Compensatory mechanisms protect against a calorie deficit much more than an excess of calories. In scientific studies of interval dieting, participants are often guided by how many calories they should eat on unlimited days. They still lose less weight than would be expected if the calorie deficit was fully maintained.
Our study highlights how and when compensatory behavior occurs. This information can be used to improve the effectiveness of the interval diet. Paying more attention to food before and after a period of calorie restriction and including exercise in diet plans can increase the results of interval dieting leading to weight loss.
The interval diet is not a miracle diet. However, some people may benefit from its flexibility. With a few modifications, it can be an effective tool.