3. Carbohydrates
Carbs, or carbohydrates, seem to always be discussed as “high” or
“low,” but never quite in between. The latest rage is “low carb.” When I
was fueling long-distance races in my cardio endurance days, it was high
carb with adequate protein, but keep fats low. The bodybuilders, no
matter what else was happening, seemed less fat-phobic, drinking their
eggs and guzzling their whole milk along with everything else they
consumed.
The carbs range from the simple (sucrose or table sugar) to the
complex (starch). The simplest building blocks of carbs are
monosaccharides, single-unit molecules such as
glucose, fructose, and galactose.
These molecules are rarely consumed in food as-is; they are the
products of digestion that are absorbed from the small intestine into
the bloodstream or liver (in the case of fructose). In other words,
glucose and the others are usually connected together into
disaccharides (two-unit sugars) or more complex
food molecules called oligosaccharides
(bigger-than-two-unit sugars resulting from the breakdown of complex
polysugars). Polysaccharides are the most complex
sugars, like starch or cellulose.
Figure 4 shows a
sucrose molecule, which is what the table sugar you might sprinkle into
your tea or coffee is made of. Sucrose is made up of a glucose molecule
attached to a fructose molecule (so sucrose is actually about 50 percent
fructose, which is worth keeping in mind if you want to reduce your
daily fructose intake). Every carbohydrate is made of a carbon
“backbone” with hydrogen and oxygen molecules attached to it. This
combination of molecules gives carbohydrates their name.
When my son was finished romping about a Vermont village on
Halloween, 2011, he returned with a bag full of sucrose (I’m imploring
him to give a lot of it away, and/or make it last months...). When you
eat candy like a 3 Musketeers bar, that mush pile of addictively tasty
sucrose makes it down to the small intestine, where the sucrose molecule
has to be “cleaved” into smaller parts before the sugar or
monosaccharides ever make it across the intestinal barrier to the
bloodstream.
Nutrients have transporters that ferry them to the great beyond
across the intestinal surfaces; no sugars beyond the simple sugars or
monosaccharides have these transporters.
The sucrose molecules are split apart into glucose and
fructose molecules with the aid of enzymes that are present in the small
intestine. The enzyme in question for the 3 Musketeers bar is sucrase
(the “-ase” suffix usually signals an enzyme, just like the “-ose”
suffix means a sugar). Similarly, lactose, a dissacharide milk sugar,
requires lactase to be digested, and maltose has its digestive companion
maltase (hey, a sensible biochemical naming strategy!).
The chocolate bar, the full-sized bar that I used to like to see
in my own Halloween bag, contains 40 grams of sugar.3 This is about 160 calories
that consists of 20 grams of glucose and 20 grams of fructose.
Glucose goes into the bloodstream, where it can be utilized by
the brain for energy, or taken up by the muscle cells or the liver to
be reassembled into glycogen, a special starch
that animals like us store for later use (more on starch coming right
up). Excess carbs can and often are stored as fat, via the sultriest
term ever invented for body-fat-making processes: de novo
lipogenesis (DNL). DNL takes place in the liver as well as
in peripheral storage depots for your fats, like around your
hips.
The body has to get rid of excess glucose—in a way, dispose of
it—because it’s toxic to cells in excess quantities. If your glycogen
is already topped off (because you’ve eaten a lot of carbs, or you
simply never use it up by moving around), so you don’t have room to
store more glucose in the form of starch in your body, the sugar can
be oxidized (burned as fuel) by skeletal muscle cells for energy. Or,
the body can store the glucose as fat in the liver or adipose tissue
(other fat depots in the body).
3.2 Glucose Is Energy Fuel, but Not by the Ton
If you cannot store or burn up the glucose you’ve consumed, you
may have the beginnings of high fasting glucose, or excess sugars
dissolved in your plasma or blood. This is why the typical doc’s
office annual visit involves a test for fasting glucose, to determine
if you’re metabolically handling sugar and associated hormones (e.g.,
insulin and leptin) okay.
Note
You can keep your blood-sugar metabolism out of the
prediabetic range (measured by some medical associations or
countries’ standards as a persistent fasting glucose level of 100 or
more) by keeping your calorie and carb consumption within
bounds—i.e., in line with your total energy expenditure.
Intermittent fasting helps lower your fasting glucose levels .
The hepatic portal vein, a conduit of blood and
nutrients to the liver, takes fructose, a simple sugar that is part of
sucrose, to the liver to be metabolized and detoxified .
