When we think of going on a diet it usually means taking in less calories than we burn. However, if you cut out too many calories and diet for too long your body's metabolism will start to adjust and you won't burn as many calories.
Researchers in Australia found that men who dieted for 16 weeks straight actually lost less weight than men who dieted for two weeks then took two weeks off. The men who stayed on a diet for 4 months lost 20 pounds. Men who took breaks from their diet lost 31 pounds over the same period.
The on again off again diet seems to work because the metabolism of the dieter doesn't change. One thing to bear in mind is during breaks dieters still did make healthy food choices. Which means you can't go back to eating cakes, cheeseburgers and french fries every two weeks and expect the same results.
Source: Food and Wine
Source: University of Tasmania