The balance of our body weight can be seen as an act of balancing energy input and energy expenditure. There are four subcomponents that contribute to energy expenditure: resting energy expenditure (the energy used to just stay alive), thermic effect of food (the energy needed to digest food), activity energy expenditure (energy used from doing activities) and total energy expenditure (the combination of the 3 above). Calories-in-calories out is the traditional model for weight gain and weight loss. Many professionals hold the belief that a calorie is a calorie, no matter what you eat. However, it has became more apparent that not all calories are created equal, some calories will make you burn more energy, through altering one or more of the 4 subcomponents of energy expenditure.
A study conducted by Ebbling et al and published in the prestigious The Journal of the American Medical Association in 2012 compared the effects of three common diets, low-fat diet, low-GI diet and low-carb diet on energy expenditure. In contrast to the conventional recommendations, the study showed that the low-fat diet tested was probably the worst diet for weight loss and maintenance compared to the low GI and low carb diets. The authors concluded that low fat diet "produces changes in energy expenditure and serum leptin that would predict weight regain".
In agreement with some available diet programs, the study showed that low-carb diet resulted in the highest resting energy expenditure and total energy expenditure in most test subjects compared to the low-fat and low-GI diets. Test subjects on a low-carb diet used on average 67kcal per day more resting energy than subjects on a low-fat diet and 29kcal per day more compared to those on a low-GI diet. The figures shown represented average data from all test subjects, there were of course exceptions, some people tested seemed to respond better and burn more energy on low-GI and low-fat diets. One has to choose what is more suitable for them based on their own experiences.
Although low-carb diet is the most beneficial in terms of energy expenditure and a number of metabolic syndrome components, prolonged enforcement of this diet can increase the secretion of the stress hormone cortisol in the body. High cortisol levels may in turn promote fat gain, insulin resistance and cardiovascular diseases. Therefor, low-carb diet may not be a long-term solution to weight loss and maintenance.
Low-GI diet on the other hand, appeared to be the most healthy and sustainable in the long run compared to the low-carb diet, even though the effect on energy expenditure was not as pronounced, it was comparable nevertheless and more effective than the low-fat diet.
Altering the components of your diet based on how your respond to different foods can make a significant impact on the body's energy expenditure and consequently affects weight loss/maintenance. Reducing fat from your diet doesn't necessarily translate into fat loss. A low-carb diet may be an effective and safe short-term boot camp solution for some but may also be harmful in the long run for others. A low-GI diet might not have the impact of the low-carb diet but it may be good for weight maintenance. Different people will respond to different types of food differently and you will have to find what's best for you. Remember, not all calories are created equal.