The World Turned Upside Down: The Second Low-Carbohydrate Revolution (11 page)

Chapter
5

Basic
Nutrition:
Macronutrients

Carbohydrate, fats and protein. These
are
the
macronutrients
because of the quantities in which they are consumed.
Micronutrients
include
vitamins and minerals, obviously to be taken in small amounts. It has
become
common to refer to foods as "nutrient-rich" or "nutrient-dense," or
not,
according to whether somebody thinks that they have high amounts of
micro
nutrients.
This
annoying imprecision is taken by some people – I'm certainly one of
them – as
an indication of how unscientific, that is, how lacking in attention to
detail,
the field of nutrition is.

During the period from 1970 to 2000,
roughly the time in which
observers began to notice an "obesity epidemic," there was an excess of
consumption of calories, almost all of which was due to carbohydrates.
Protein
is usually the most stable part of the diet. In the absence of
financial
considerations, the generally higher satiety of protein probably makes
intake
self-limiting although there may be unique metabolic regulation of
nitrogen
sources. During this period, the total amount of protein in the
American diet
did not change.
Figure 2-2
shows, that fat, if anything, went down.

Basics of carbohydrate
chemistry

Chemically, the class of compounds
called
carbohydrates includes
the
simple sugars
(
monosaccharides
)
like glucose and fructose, and combinations of the simple sugars,
disaccharides
like sucrose (one glucose, one fructose) and polymers (polysaccharides)
like
starch and glycogen, as well as derivatives of the sugars, for example,
the
so-called "sugar alcohols."

"Alcohol," that is, ethanol, is not a
carbohydrate despite what
you may hear on YouTube. Whatever the extent to which sugar can make
you as
loopy as alcohol, chemically, ethanol is not a carbohydrate. A horse is
not a
dog. One of the reasons that we make pre-meds study organic chemistry
is that
the precision in naming organic compounds is presumed to carry over
into
pharmacology. It is likely that manufacturers started spelling klonopin
(antidepressant) with a 'k' because they didn't want physicians to
accidentally
prescribe clonidine (antihypertensive).

Figure 5-1
Structure of
simple sugars.

The formal chemical description of
sugars
is that they are
polyhydroxy aldehydes and ketones:
Figure 5-1
shows the chemical structures
of glucose and fructose. To simplify where they fit in in biochemistry,
the
common sugars, and related polymers, can be represented cartoon-style
as in
Figure 5-2
.

The most common sugar is glucose and
it
almost always cyclizes
(folds up) in the form of a hexagon, at least in aqueous (water-based)
solutions. Fructose can also form a six-membered ring but is more
likely to
cyclize in a pentagonal shape, as in the cartoon representation in the
figure.

Figure
5-2
reveals that starch
is a polymer of glucose (polysaccharide), and that the breakdown to
simple
sugars is brought about in digestion. If you never did the experiment
in grade
school, you can try chewing a piece of bread for several minutes. The
sweetness
that develops is an indicator of the digestive enzymes in saliva which
are catalyzing
the conversion of the starch in the bread into sugar (glucose). Most
starch has
a somewhat more complicated structure than that shown in the figure:
while some
starch molecules, called
amylose,
do have a linear
structure, other types,
amylopectin
,
have many
branch points.

Figure 5-2
.
Structure and transformations of the common carbohydrates.
Starch is a
polymer of glucose. In digestion, the glucose units are released and
absorbed
as such. Sucrose is a dimer of fructose and glucose and digestion
produces the
two monosaccharides.

Glycogen – glucose
savings account.

Glycogen
is a polymer of glucose,
very highly branched, and it is the storage and supply depot for body
glucose
flux. Liver, the main command center of metabolism and muscle, the main
consumer
of glucose are the sites of glycogen metabolism. Liver has the highest
concentration but there is overall much more in muscle where most
glycogen is
stored.

Branching
means that there are a
lot of ends so that glucose units can be chopped off as needed.
Glycogen is a
very dynamic storage site. The extensive branching and 3-D structure
means that
glycogen occupies a lot of space (
Figure
5-3
).
Children with one of the inborn errors of metabolism known as glycogen storage diseases will have
visibly distended abdomens due to the increased storage of glycogen.

We
think of glycogen as desirable
because of the association with endurance in sporting events. This is
the basis
for carbohydrate loading the day before a marathon but, even in the
area of
athletics, things are not clear cut – marathons are mostly run on fat,
even if
you are not adapted to a low-carbohydrate diet. A sprinter may depend
more on
glycogen. And marathon runners train, that is, they use and store
carbohydrate.
For the rest of us, analogous to fat, glycogen is a storage site for
calories
so any benefit from carbohydrate loading depends on what you've been
doing.
Storing calories that you don't use is not a good thing.

Figure.
5-3
.
Structure of glycogen. Each repeating
unit is a glucose molecule. There are rarely more than nine units
before the
structure branches. The colored object in the center is the protein
glycogenin
on which the glycogen structure is assembled.

On a low-carbohydrate diet, glycogen
storage tends to be
reduced, typically, around 60 % on a very low-carbohydrate intake
(<100
g)/day. However, metabolism does not run on mass action, (the chemical
principle that the amount of a compound will drive its chemical
reactivity) but
rather on hormones and enzymes. So under conditions of low dietary
intake,
glycogen will be replenished by the glucose produced from
gluconeogenesis and
storage will be maintained. It is important to understand that
gluconeogenesis
and glycogen metabolism are really one process: glucose synthesized
from
protein may be stored as glycogen and then only later appear in the
blood. An
important feature of a carbohydrate-restricted diet, however, is the
switch to
a metabolic state that runs on fat rather than carbohydrate.

Glucose is at the center of
metabolism.
Looking ahead, the main
theme in human biochemistry is that there are two major fuels – glucose
and
acetyl-CoA
(derived largely from fat, pronounced ass-a-teel-co-ay). Two fuels and
two
goals: provide energy and maintain blood glucose at a constant level.
Too
little blood glucose (hypoglycemia) is not good because some tissues,
particularly the brain and central nervous system require glucose but
too much
(hyperglycemia) is also not good. Glucose is chemically reactive and
interacts
with body proteins. Glycation (reaction with a sugar) may inhibit the
protein
or it may be cleared from the cell or circulation. You need glucose
but, again,
you don't have to ingest any. You can make it from protein and other sugars.

A major motif that reappears in this
book
is that carbohydrate
and protein can be turned to fat but, while glucose can be made from
protein,
with a few exceptions, you can't make glucose from fat. Looking at the
two
fuels, glucose can provide acetyl-CoA, but acetyl-CoA cannot be
converted to
glucose.

Glucose can be made
from protein, but, with a few exceptions, you can't make glucose from
fat.

lipid chemistry: Good
fats, bad fats

Most of the
fatty acids have common names
– many were
discovered before we had systematic chemistry and before we had the
professional panels to set the rules. Some of the names tell you how
they were
discovered – palmitic acid is found in palm oil, oleic in olives and
you can
guess that caproic and capryllic acids smell like goats.

There are a lot of disclaimers about
"good
fats, bad fats" but in
one way or another, the government and private agencies, and individual
researchers are still recommending that you reduce fat. The
recommendation for
reduced fat may be accompanied by an explanation that the type of fat
is more
important than the total amount but the when you get down to it, the
recommendation, usually run along the lines of the following
contradictory
statements: "fat is not bad. Only saturated fat is bad. Eat low-fat
foods." The
big targets are saturated fat and, of course,
trans
-fat.
The juxtaposition of saturated fat and
trans
-fat
is an indicator that it is the politics, not the science, that is at
work here.

The American Heart Association (AHA)
website provides a
truly maniacal cartoon video on the hideous Sat and Trans brothers (
Figure 5-4
)
:

"They're a charming pair, Sat
and Trans.  But
that doesn't mean they make good friends.  Read on to learn
how they clog
arteries and break hearts -- and how to limit your time with them by
avoiding
the foods they're in."

Figure
5-4
. The evil Sat and Trans brothers from the
American Heart
Association Website
(http://bit.ly/OhiHNC):
.

What's missing from the website is
the
story behind
trans
-fat.
The crusade
against dietary saturated fat, in which the AHA and other health
agencies
fought so vigorously, led to a search for alternatives to butter and
lard. It
is important to understand that this mission was led by physicians, not
by
physiologists, not by biochemists. The movement was accused of trying
to carry
out a grand experiment with the American people as guinea pigs but
there's no
stopping zealots. Butter was seen as the quintessential high saturated
fat food
and it was clear that no progress could be made without deposing it and
installing a substitute.