Resistance training alone, combined with a slight calorie restriction causes greater bodyfat
loss and a maintenance/increase in lean body mass than just restricting calories alone (24,26).
Essentially, the caloric deficit causes the fat loss and the weight training signals the body to keep
the muscle so that only fat is lost. This is an important consideration. From a calorie burning
perspective, aerobic exercise and caloric restriction are essentially identical.
Aerobic training alone, while increasing fat loss in some studies, does not generally increase
muscle except in very inactive individuals (24). Remember that adding muscle raises metabolic
rate in the long term. Any caloric restriction should be accompanied by resistance training to
prevent the loss of LBM and possibly to even increase it.
Aerobic exercise can increase fat loss and may be added if desired and if time allows.
However, too much aerobic exercise can have the same effect as too few calories: lowering
metabolic rate and slowing fat loss. A total caloric deficit of more than 1000 cal/day seems to be
the threshold for slowing the metabolism (14).
Exercise with very-low-calorie diets: less than 800 calories per day
In a very low-calorie diet situation (VLCD, 800 calories per day or less), there are
significant changes compared to higher calorie levels. VLCD without exercise causes a large drop
in LBM and metabolic rate. The addition of aerobic exercise alone does not improve fat loss or
prevent the drop in LBM and metabolic rate (27-30).
In severe dieting situations, aerobic exercise may actually be worse than just dieting
(health benefits excepted). In one study, the addition of aerobic exercise (27 hours total over 5
weeks) to a very-low-calorie ketogenic diet (500 cal/day) caused a greater drop in metabolic rate
than dieting alone and caused no additional weight or fat loss (29). It appeared that the body
compensated for the aerobic exercise by slowing metabolic rate at other times of the day.
When resistance training only is added to an 800 calorie diet, muscle size increases despite
a similar loss in LBM in both the diet only and diet plus exercise groups (31). This implies that
the loss in LBM is due to loss of water, glycogen and other non-muscle tissues (32). Metabolic
rate still goes down.
The conclusion from this data is this: on a VLCD, weight training but not aerobic exercise
will slow the drop in metabolic rate but not stop it. The inclusion of aerobic exercise may do more
harm than good at this calorie level.
Summary
There is a caloric threshold for exercise to improve the rate of fat loss. A calorie deficit
more than 1000 cal/day will slow metabolism. Further increases in energy expenditure past that
level does not increase fat loss (14). In some cases, excess exercise will increase the drop in
metabolic rate seen with very large calorie deficits.
This value of 1000 calories per day includes any caloric deficit AND exercise. Meaning
that if 500 calories per day are removed from the diet, no more than 500 calories per day of
exercise should be performed. If someone chose to remove 1000 calories per day from their diet,