Many early ketogenic diet studies simply subtracted muscle loss from total weight loss and
assumed the difference represented fat loss. The problem with this method was that assessment
of muscle loss can be seriously affected by the glycogen loss seen on a ketogenic diet (8).
Although newer methods are becoming available, most of the studies examined estimate
body protein losses by performing a nitrogen balance study. Nitrogen is brought into the body in
the form of dietary protein and is excreted from the body through a number of pathways including
urine, feces and sweat (9). Generally estimations are made for fecal and sweat nitrogen losses
and only urinary nitrogen excretion is measured.
Since it is easier to measure nitrogen than it is to measure protein directly, a nitrogen
balance study compares the amount of nitrogen being lost in the urine to the amount of nitrogen
being consumed in the diet. If less nitrogen is being excreted than consumed, the body is said to
be in positive nitrogen balance meaning that protein is being stored. Since the body has a
minimal store of non-tissue protein (9), it is assumed that this stored protein is being
incorporated into muscle or other tissues. If more nitrogen is being excreted than is being
consumed, the body is in negative nitrogen balance meaning that body protein is being broken
down and excreted. A negative nitrogen balance generally indicates a loss of lean body mass and
we will assume it to indicate a loss of muscle.
Problems with nitrogen balance
Although nitrogen balance is one of the best methods for determining muscle loss while
dieting, there are still problems. First and foremost, there tends to be a great degree of variability
in total nitrogen losses among subjects in diet studies. For example, one study found that daily
nitrogen losses on a ketogenic diet varied among the subjects between 1 gram of nitrogen/day and
6 grams of nitrogen/day. This represents a difference in bodily protein breakdown of 6-36 grams
of protein/day (10). Unfortunately most diet studies report nitrogen balances as average values
for differing diet groups. This tends to overstate either how well or how poorly a diet works to
spare protein. If one individual loses a significant amount of nitrogen while another loses very
little, reporting an average does not provide accurate information.
An additional problem is that there is no easy way to tell where the protein is coming from
or going to. In the case of a negative nitrogen balance, it is possible that liver proteins are being
broken down, while muscle is spared and vice versa. To simplify matters, we will equate a
negative nitrogen balance with a loss of muscle.
Why the variability?
The reason for such variability in nitrogen sparing is likely related to several factors.
Obese individuals appear to better spare protein losses due to having a greater store of bodyfat to
use as an alternate fuel (10). In contrast, higher levels of lean body mass appear to increase
nitrogen losses (10). This may be part of the reason that heavily muscled individuals tend to lose
muscle more easily. The ability to decrease insulin levels and establish ketosis may also play a
role (11,12). Finally, the ability to rapidly down regulate the levels of thyroid hormones may play
a role in nitrogen sparing (10).