Metabolism slows during kilojoule restriction
Restricting kilojoules during weight loss lowers metabolism because the body becomes more efficient, requiring fewer kilojoules to perform the necessary daily functions for survival. Consequently, this can slow (but not stop) the anticipated rate of weight loss.
For example, if an individual needs 8,400 kilojoules per day to maintain weight, reducing intake to 6,300 kilojoules, assuming exercise stays the same, should provide a 500g per week weight loss (note: 500g of weight is equivalent to about 14,700 kilojoules). Furthermore, reducing to 4,200 kilojoules should result in a weight loss of 1kg per week and going down to 2,100 kilojoules a day should result in a weight loss of 1.5kg per week. However, if an individual actually reduces their intake to 2,100 kilojoules, the weight loss would not likely be a steady 1.5kg per week because of the reduced metabolic rate. It would likely be around 1kg. This 'lower than expected' rate of weight loss is a lot different to 'no' weight loss as the 'starvation mode' notion proposes.
It is unclear as to whether the relationship between reduced kilojoule intake and a lower metabolism follows a straight path or becomes more pronounced the greater the kilojoule reduction. Some studies have found no significant reduction in metabolism until the kilojoule restriction is quite large (e.g. 3,360 kilojoules or less per day). Others suggest a linear relationship with small reductions in metabolism accompanying small reductions in kilojoule restriction, with the gap increasing as the kilojoule deficit is enlarged.
While there is no biologic evidence to support the 'starvation mode' myth, there may be behavioural reasons why weight loss stops when kilojoules are severely reduced. Over-restriction of kilojoule intake, known as high dietary restraint is linked to periods of overeating, hindering successful weight loss.
Metabolism after weight loss
The good news is that after the weight loss goal is achieved and weight has stabilised, it does not appear that the dip in metabolism is permanent. Several rigorous studies done at the University of Alabama in Birmingham showed that metabolism goes back to expected levels with sustained weight loss, discounting the theory that a lowered metabolism helps to explain the common phenomenon of weight regain following weight loss.