A theory of weight management based on tracking calories was proposed. The theory is: consume more calories than you burn and you'll gain weight. Vice versa is true. Also, if you maintain energy balance, then you'll maintain your weight.
The National Academy of Sciences published the 2005 Dietary Reference Intake manual which presents this theory by showing from observations that a certain amount of calories are required for a person to maintain a certain weight.
Harvard's School of Public Health confirmed this relationship in Feb 2009 by showing that reducing calories leads to weight loss.
"Einstein's equation E = mc2 applied to biology moves this idea from theory to natural law," says Mr. Dow. "Dividing both sides by mass produces E/m = c2. In words, this means Energy divided by mass equals the speed of light squared. The speed of light is a constant so this equation turns into E/m = a constant. Increase Energy and mass has to increase in order for the constant to remain a constant. Vice versa is true. Also, no change in energy means there will be no change in mass. This natural law usually applied to nuclear reactions shows why adjusting calorie consumption changes one's weight."
"The reason why exercising helps people lose weight is that a person burns more calories which result in a decrease in overall energy and a decrease in mass. Some people exercise a lot and don't lose weight, but this is because they are consuming more calories and maintaining an energy balance which causes a weight balance. Exercising is important in increasing one's health, but weight is determined by calorie consumption."
In full disclosure, Michael Stephen Dow wrote a book regarding this subject, titled The Pen and Paper Diet published in May 2008.