Physical activity levels can be expressed in multiples of resting metabolic rates.
Even when one does nothing lying still in his bed,
there is still metabolic activity ongoing to keep the organism alive.
This is called basal metabolic rate and
amounts to about one kilogram per hour per kilogram of body mass.
Energy expenditure above this level is due to physical activity.
One can express the total energy expenditure as the sum
of the basic metabolic rate, physical activity related energy expenditure, and
some energy expenditure, about 10%, for the digestion of food.
Normally, this varies, this total energy expenditure,
between 1.2 times basic metabolic rate all the way up to several multiples.
It can reach about five for athletes who do the Tour de France.
Normally, for non athletes, it varies between 1.2 and, say, 2.2.
Now, for these ratios of total energy expenditure,
basal metabolic rates, between 1.8 and 2.5, the physiology works very well,
and energy the intake is matched by energy expenditure.
A certain increase or a certain decrease in activity
immediately will be matched with a change to intake in energy.
But below 1.8, this doesn't work anymore.
This is probably because evolution has never been confronted
with this type of low energy expenditure.
Our ancestors were always much more active than we are.
We are less physically active compared to ancestors, and
we also eat actually less than our ancestors.