The World Turned Upside Down: The Second Low-Carbohydrate Revolution (14 page)

When you
wake up in the morning..

There
are two goals in human
metabolism. You have to provide energy and you have to maintain blood
glucose
at a relatively constant level. In the
fed state
,
which nutritionists call the
post-prandial state,
the eight hours or so after a meal, diet can provide a greater or
lesser amount
of the material needed to meet these two needs. The period at which
ingested
food no longer provides material for metabolism directly is referred to
as
fasting, or post-absorptive state
.
When
you wake up in the morning (
Figure
5-8
), after an overnight fast: insulin is low
and glucagon is
high, fat is broken down (lipolysis) to fatty acids and glycerol –
lipolysis is
inhibited by insulin and stimulated by glucagon. The fatty acids are
oxidized
for energy. Blood glucose is maintained by the processing of liver
glycogen.
Muscle also stores glycogen which can be broken down but this glucose
is used
by the muscle itself and is not exported. Again, we think of muscle as
a
consumer of glucose and the liver as a supplier, or more generally, as
a
command center for metabolism.

Figure
5-7.
After an overnight fast or
low-carbohydrate diet. Insulin is
low and glucagon is high, fat is broken down (lipolysis) to fatty acids
and
glycerol. Gluconeogenesis (GNG) from protein is another source of
glucose.

Gluconeogenesis

Frequently described as a kind of
last ditch source of
glucose or energy when no food is coming in and glycogen is depleted
but, in
fact, gluconeogenesis (GNG) goes on all the time. When you wake up in
the
morning, more than half of the free glucose in the blood or produced
from
previously stored glycogen comes from GNG. Although there is no form of
protein
formally defined as a storage site as there is for stored fat or
glycogen,
muscle can be thought of as providing an internal source of protein, a
source
that must be replenished from the diet, a dynamic store of amino acids
for
metabolism. (More information in reference
[51]
).

Starvation - Good way
to lose weight?

In the fasting state, then,
adipocytes (fat cells) supply
fatty acids from the breakdown of fat. Most tissues, including the
heart,
oxidize the fatty acids for energy. Some tissues, primarily brain and
central nervous
system require glucose and that is provided by the combination of
glycogen
breakdown and replenishment from gluconeogenesis. The two goals of
metabolism
are taken care of. Is starvation a good way to lose weight? Of course
it is
not. The requirement for amino acids from protein for the maintenance
of blood
glucose is the problem. In the absence of dietary protein, your body
will turn
to its own sources.

Are carbohydrates
fattening?

Harper's
Review of Biochemistry
is one of the standard
texts in medical and
graduate schools. Now in it's 28th edition, it is a multi-authored
comprehensive view of the field. I am grateful to Adele Hite of
University of
North Carolina for pointing out that in the
8th
edition
(1961), when it was called
Harper's
Review of Physiological Chemistry
and Harper
himself was the sole
author, the close connection between carbohydrate and fat was evident.
The
Chapter on Metabolism of Carbohydrates begins as follows.

In
the average diet carbohydrate
compromises more than half of the total caloric intake. However, only a
limited
amount of this dietary carbohydrate can be stored as such. It is now
known that
the un-stored portion of the ingested carbohydrate is converted to fat
by the
metabolic processes of lipogenesis
[52]
.

In other words, the third sentence of
the carbohydrate
section of a biochemistry text emphasized the closeness of carbohydrate
and
fat. In the twentieth edition (1985), the Chapter, now written by Peter
Mayes,
begins similarly and continues,

It
is possible that in humans
the frequency of taking meals and the extent to which carbohydrates are
converted to fat could have a bearing on disease states such as
atherosclerosis, obesity and diabetes mellitus
[53]
.

In the current edition, the process
of conversion of
carbohydrate to fat now has a chapter of its own. The process is known
as de
novo lipogenesis (DNL), new synthesis of fat, or more precisely de novo
fatty
acid synthesis since the immediate product is a fatty acid, the
saturated fatty
acid, palmitic acid (C16:0). DNL appears to be the explanation of the
counter-intuitive result, demonstrated in several studies that dietary
carbohydrate diet leads to increases in saturated fatty acids in the
blood.
Chapter 9
describes
an experiment from Jeff Volek at the University of Connecticut where
such an
increase in saturated fatty acids was greater in the blood of people on
a high
carbohydrate diet compared to those on a low-carbohydrate diet even
though the
latter had three times the amount of dietary saturated fat.

What about protein?

In some classes that I teach, I ask
the
students for the
definition of life. I get different answers but I usually say "no, a
one word
definition." I try not to drag it out too long or to overact, but the
answer
that I am looking for is "protein." Everything that goes on in life is
controlled by proteins either as the actual component or as the source
for
other things. Because of its multiple roles in biology and the far more
complicated chemistry, I will present here only broad outlines in the
context
of an answer to an email.

I received the following question:

If one is on a
very-low-carbohydrate/high-fat diet,
what happens to excess protein that is not needed for muscle repair and
growth,
and gluconeogenesis? I see two alternatives:

1. It's excreted

2. More glucose is created.

#2 seems so unreasonable to
me...would your
metabolism actually make more than the little bit it needs? I'm open to
a #3
that I might be too unimaginative to think of. I'm interested in the
theory,
this isn't a request for diet advice. I like to understand things at
the
cellular level.

The answer is that it depends on what
else is going on, but
protein,
per
se
,
is not excreted (in the absence of some disease).  Protein is
a polymer.
Unlike glycogen which is a homopolymer of identical glucose units, the
individual units, amino acids, are picked from about twenty different
choices.
In digestion, protein is broken down to individual amino acids and are
absorbed
and re-assembled into body protein. The sequence of amino acids defines
the
biologic function and this sequence is encoded in the genetic material.
The
genetic code is largely the code of amino acid sequences).

After digestion and absorption, some
of the amino acids that
are not used for protein synthesis may be trashed. The nitrogen is
converted to
ammonia which is converted to the compound urea and excreted. The
remaining
carbon skeleton can be used for energy either directly in the TCA cycle
directly, or converted to ketone bodies, especially on a very
low-carbohydrate
diet. Some amino acids can be converted to glucose. Much more than a
little bit
is needed. The carbon skeleton from amino acids, directly or indirectly
can be
converted to fat. So, a practical answer is that, "excess" protein is
re-cycled, used for energy or for synthesis of glucose.

Protein, as such, is not normally
excreted.  Proteinuria is
an indication of some abnormality, kidney malfunction or other
disease-related
nephropathy. Amino acids are excreted at some low level. High excretion
of
particular amino acids is usually an indication of some metabolic
disturbance
or inborn error of metabolism. It is important to understand that
everything
that goes on in the body is mediated by proteins which turn over all
the time
and whereas muscle "repair and growth" is important, it is not the only
thing.  Body proteins, unlike glycogen or other homopolymers,
have
specific amino acids sequences so require a particular
make-up.  Some
amino-acids are interconverted and some (essential amino acids) are
required
from diet).  

Current tendencies are to try to
encourage
vegetarianism or, at
least, reduce meat consumption. Whatever the moral or practical
arguments are,
the scientific case is highly questionable. The proliferation of
studies trying
to demonstrate an associations between meat consumption and one disease
or
another are largely bogus and a couple of these studies are
deconstructed in
Chapters 18
and 19.

What we eat.

Protein is a stable part of the diet.
Total consumption of
protein has not changed during the obesity epidemic. Sources of protein
have
changed. Consumption of red meat has gone down and chicken consumption
has gone
up (
Figure 5-9
and
5-10
).
Other things have increased to a greater extent.
Figure
5-10
is from a paper by George Bray
[54]
.

Figure 5-9
.
Meat
Consumption in the indicated period.

Figure 5-10.
Food
Consumption in the indicated period. Figure redrawn from Bray, et al.
[54]
.

The original caption attached to
Figure 5-10
included the rather odd comment that "none of
the foods in
this figure show changes that would help to pinpoint this epidemic."
Something
about the increase in wheat and corn would seem to be relevant.

Figure
5-10
also suggests that,
contrary to almost everybody's recommendations, we are not going to
make great
headway simply by increasing "healthy"
fruits
and vegetables.