The human body has many, many biological processes going on at once.
Very often there are several factors involved in any biological
process (like "production of serotonin").

In the PNP book, they are looking at one specific factor - transport
of the amino acid tryptophan across the blood/brain barrier so it is
available in the brain to be used in making serotonin.

The basic idea is this: there are a limited number of transport cells
for getting amino acids into the brain, and all the amino acids
compete for them.  Under normal conditions, then, not a whole lot of
tryptophan gets through because of the competition from all the other
amino acid molecules that are around.  When insulin is produced,
however, it not only forces glucose into cells, it also forces all the
other amino acids into cells - but not tryptophan.  With the
competition from other amino acids reduced, more typtophan is
transported to the brain.

A great number of people looking at lowcarb diets, however, are not
producing insufficient insulin.  If you look at Dr. Atkins book, he
spends time going into the mechanics of how many people are producing
TOO MUCH insulin.  His diet is aimed at reducing this down to normal
levels.  His main thrust is that EXCESS insulin produces
quickly-falling blood sugar numbers, which causes hunger.  

Even here, the idea is that the insulin actually produced is WORKING.
A great number of overweight people have a condition known as "Insulin
Resistance" or "IR".  This means that the insulin does not work as it
should to get glucose to enter the cells, so in general people with IR
need to produce MORE insulin to reduce their blood sugar.  This is the
first step towards type 2 diabetes.  Now, while I have seen lots of
articles that mention how IR affects =glucose= transport into cells, I
have not seen a study that applied this term to =amino acid= transport
into cells. =IF= (and I want to emphasize that this is an IF, I have
seen no data either way) amino acid transport into cells goes on at a
regular rate when glucose transport is reduced, then every person with
IR is going to have oodles and oodles of tryptophan available in the
brain to make serotonin.  

The idea that "low carb means low serotonin means depression" may well
be true for SOME people.  If their insulin levels are normal (or
perhaps low, because there are built-in margins for error) to begin
with, a low carb diet might reduce their insulin levels and create a
typtophan (and thus a serotonin) deficiency.

In =general=, however, most of the people who have posted on this
newsgroup report that their mood was either unchanged or improved by
going to lowcarb.  That does not, however, give a =random= sample.
There may well be others who tried lowcarb, got depressed and stopped.
Such people would be unlikely to continue to contribute to a lowcarb
support newsgroup. So while we have a lot of anecdotal evidence that
many people reported better moods on lowcarb, we really don't have
hard evidence of what lowcarbing does to the moods of average people,
nor do we have evidence of what lowcarbing does to the moods of people
with high insulin levels (from IR or other causes).

HEHE.. that's pretty much what freud used cocain for, as an anti-depressant  :)

-i