Summary
The number of calories needed to maintain a stable bodyweight is determined by three
factors: resting energy expenditure, thermic effect of activity, and the thermic effect of food.
While estimations can be made for all three components of metabolic rate, a simpler and fairly
accurate estimation of maintenance calorie needs can be made by multiplying bodyweight in
pounds by 15-16 calories. Women should generally use the lower number, men the higher.
Section 3:
Setting calorie levelsHaving determined maintenance calories levels, it is now time to discuss the concept of
energy balance, fat loss and muscle gain. Energy balance refers to the difference in caloric intake
via diet and caloric expenditure via metabolic rate and activity. It is given by:
Energy balance = calories in - calories out
When energy balance is positive (i.e. calories in exceeds calories out), energy is stored in
the body as glycogen, protein and fat. When energy balance is negative (i.e. calories out exceeds
calories in), energy is pulled from the body’s stores. In the case of fat loss, stored energy in
adipose tissue is converted to usable energy and burned by the body.
Thus to lose weight, one must burn more calories than they consume. Therefore any
attempt to lose fat must center around decreasing caloric intake or increasing energy
expenditure. By corollary, to gain weight one must consume more calories than they burn. Any
attempt to gain weight must center around increasing caloric intake or decreasing energy
expenditure.
A gain in muscle or fat tissue revolves around the creation of a caloric excess. For the
most part, it is impossible to lose fat (requiring a caloric deficit) and gain muscle (requiring a
caloric excess) at the same time. While it is attractive to think that the body will pull energy
from fat stores to synthesize muscle tissue, this does not appear to occur in most cases. The
only exception to this is beginning exercisers and those returning from a layoff from training,
although the reasons for this are not known.
Calories and the Atkins diet: a misconception
A misconception, and commonly heard criticism, surrounding the Atkins diet is the
(apparent) claim that fat can be lost with an ‘unrestricted caloric intake’, which contradicts
basic thermodynamics (8). Strictly speaking, Atkins claimed that one could lose weight eating as
much fat and protein ‘as they liked’ meaning they could eat until they were full without worrying
about counting calories.
Atkins based this claim on the established fact that individuals on a diet restricted in