The Sugar Family
The principal sugars are glucose and fructose. These are the simplest
CHO molecules, known by their single ring structure as monosaccharides.
Glucose is the fuel of all living things, supplying energy
to all living cells, both plant and animal. Fructose is the first cousin of
glucose and occurs in fruit and corn syrup. Sucrose is the sugar that is commonly called "sugar" and is found in most
commercially prepared foods. Sucrose appears in refined form as white table
sugar. Sucrose is a disaccharide with glucose linked to fructose. It requires a
special enzyme to separate glucose from fructose. Brown sugars and molasses are cruder sugar products that contain the same
sucrose in the presence of many other substances not yet removed. The preference
for brown sugars, syrups, molasses, and honey, in place of refined white sugar
is not based on any important biological information. Honey may be preferable
only by taste and implication (visions of bees, flowers, and summer days); it
contains the same sugars, sucrose, glucose and fructose.
The basic sugars, glucose, fructose, galactose, mannose,
are the building blocks of larger more complex carbohydrates. These "monosaccharides"
contain 6 carbon atoms, and typically form a six-sided ring structure. The
energy density of sugars is 4 kcal/gm.Lactate or lactic acid is the 3-carbon fragment of these sugars, produced
as the sugars are oxidized as fuel.
The same sugar has two mirror-image forms, determined by
the rotation of a test light beam through its crystal in the laboratory.
Biologically active sugars are the "D" form, rotating light to the "dextro" or
right side. D-Glucose is referred to as "dextrose". The general formula for a
6-C ring sugar is: C6.H12.O6
Five carbon sugars, pentoses, are Arabinose, Ribose, and
Xylose. Ribose is important in genetic material, formed into ribonucleic acid,
which gives RNA and DNA their long names. Polymers of arabinose are gums such as
Sugar alcohols are first cousins of the basic
monosaccharides. Adding a hydrogen ion or proton to a sugar's double bonded
oxygen, C=O, produces a sugar alcohol with a C-OH group. Sorbitol is such a
beast, found as a food additive. Most ingested sorbitol is not absorbed, but
passes onto the colon, where bacteria turn it into hydrogen gas. Absorbed
sorbitol is metabolized to fructose.
There is enough evidence to warn against sorbitol use. One
of the routes of tissue damage when blood and tissue glucose levels rise is the
conversion of excess glucose to sorbitol and fructose. The safest course of
action is to avoid packaged and processed foods that contain sorbitol. Mannitol
is another sugar alcohol sweetener, added to commercial foods. Inositol is
another sugar alcohol; mice and rats seem to require inositol in their diets,
but we do not. Inositol may be considered an accessory nutrient. Inositol is
combined with 6 phosphate groups in cereal grains to form phytic acid, a
undesirable substance, which binds minerals and prevents their absorption.
The next group of common sugars consists of two basic
sugars, linked together, called "Disaccharides". Lactose is the sugar in breast
and cow's milk, and contains glucose and galactose. Sucrose, as discussed above,
is glucose and fructose. Maltose is glucose and glucose.
Large carbohydrate molecules form the structure of plants,
and to a lesser extent, animals. A carbohydrate polymer, or polysaccharide, is a
string of sugar molecules linked together. The cell walls of plants are
constructed of elaborate polysaccharides made from 12 basic sugars. Cellulose is
the main structural carbohydrate; it is a polymer of glucose units linked
together to form a tough fiber. Vegetarian ruminants utilize special stomachs
that host bacterial populations that break down cellulose.
Starch is the most valuable polysaccharide. The starch
molecule is tree-like, with branches of varying length. Starch digestion begins
in the mouth with salivary amylase, continuing in the small intestine with
pancreatic amylase. Short chains of glucoses are referred to as alpha-dextrin,
maltotriose (3GL), and maltose (2GL). Glucoamylase breaks these short chains
down to individual glucose molecules that are absorbed. Starch is the best fuel,
supplying sustained-release glucose. Incoming glucose is rapidly absorbed and
utilized by body tissues. Some tissues such as muscle require insulin to absorb
glucose. Other organs, such as the brain, do not require insulin and are prime
glucose consumers. The liver balances blood glucose levels by absorbing and
releasing glucose. The liver stores glucose as glycogen and is capable of
producing glucose from amino acids if food does not supply adequate glucose.
Slow absorption of glucoses is better tolerated than the rapid absorption of