Carbohydrates (sugers and starches)
provide energy to body cells, particularly the brain, which is the
only carbohydrate-dependent organ in the body. Carbohydrates
are classified according to the number of suger units they contain.
The simplest carbohydrate is glucose.
Glucose, also called blood sugar and
dextrose, which contains one suger unit, flows in the bloodstream
so that it is available to every cell in your body. Your cells absorb
glucose and convert it into energy to drive the cell. Glucose is
made up of carbon and water with a chemical formula of C6H12O6
(6 carbon, 12 hydrogen, and 6 oxygen atoms), hence the name
Maltose is made from two glucose atoms
bonded together. Fructose and galactose
have the same chemical formula as glucose but the atoms are arranged
differently, the liver converts them to glucose. Sucrose
is made of one glucose and one fructose molecule bonded together.
Lactose is made of one glucose and
one galactose molecule bonded together. Glucose, fructose and galactose
are monosaccharides and are the only
carbohydrates that can be absorbed into the bloodstream through
the intestinal lining. Lactose, sucrose and maltose are disaccharides
(they contain two monosaccharides) and are converted to monosaccharides
by enzymes in the digestive tract. Monosaccharides and disaccharides
are called simple carbohydrates. Suger
alcohols, such as sorbitol and mannitol, are alcohol forms of glucose
and fructose respectively. The term suger
is usually used to describe monosaccharides and disaccharides.
Oligosaccharides contain 3 to 10 suger
units are often breakdown product of polysaccharides
which contain more than 10 suger units. Monosaccharide units linked
together by glycosidic bonds are called polysaccharides,
glycans or complex carbohydrates; there are three major polysaccharides
that occur in food: starch, glycogen, and cellulose.
Starch is the principal food reserve
in plants and the main source of carbohydrates in the human diet.
It is a mixture of polyglucans; that is, homopolysaccharides made
from glucose. There are two types of polyglucans in starch: amylose
and amylopectin; amolose is digested slowly and amylopectin is digested
quickly. Glycogen, the storage polysaccharide
of animals, is present in all cells but is most prevalent in skeletal
muscle and liver cells; its major components are triglycerides of
fat. Cellulose, a non-starch polysaccharide,
is the primary structural component of plant cell walls and is the
major part of insoluble dietary fiber.
Dietary fiber is nondigestable carbohydrate and lignin that are
part of plants. Soluble, or functional, fiber is beneficial and
can create feelings of satiety, lower blood cholesterol levels,
reduce postpandial blood glucose levels, and possibly improve cell
insulin sensitivity. For more information on dietary fiber, see
effects of dietary fiber. Lisa Brown, Bernard Rosner, Walter
W. Willett and Frank M. Sacks. American Journal of Clinical Nutrition,
Vol. 69, No. 1, pages 30-42, January 1999.
Most grains (wheat, corn, oats, rice) and vegetables are high in
starch. Your digestive system breaks a complex carbohydrate back
down into its component glucose molecules so that the glucose can
enter your bloodstream. Simple carbohydrates enter the bloodstream
at the rate of about 30 calories per minute, complex carbohydrates
are digested more slowly and most enter the bloodstream at around
2 calories per minute. The rate of carbohydrate food digestion and
glucose absorption into the bloodstream is indicated by its glycemic
index (GI) and glycemic load (GL); low GI or GL indicates slow
Simple carbohydrates are converted quickly to blood glucose which
causes the pancreas to secrete a large amount of insulin to lower
blood glucose levels; this large amount of insulin tends to lower
the amount of blood glucose to very low levels within 3 to 5 hours,
which often leads to feelings of nervousness and irritability.
Insulin, a simple protein, controls how the body uses blood glucose.
It enables glucose to enter cells, which use it for energy; it helps
convert glucose into glycogen for storage in the liver and muscles;
it facilitates the conversion of excess glucose into fat; it slows
down the breakdown of fats and protein into glucose; it promotes
the uptake of amino acids for making proteins. The action of insulin
is opposed by glucagon and epinephrine.
Carbohydrate deficiency signs?
- Decreased secretions from the mouth, nose, and eyes
- Muscle weakness
- Inability to concentrate
- Easily startled
- Difficulty swallowing
- Voice affected by stress
If you suspect you have a carbohydrate deficiency, contact your
doctor or healthcare professional immediately.
How much carbohydrate is needed?
The World Health Organization
(WHO) recommends a minimum carbohydrate intake of 50g/day
to avoid ketosis with the bulk coming from non-starch polysaccharides
from a wide variety of appropriately processed and cooked cereals,
vegetables, legumes, and fruits. I'm confused by this recommendation,
non-starch polysaccharides are not digestable so they can't provide
energy; the main source of energy for the body are carbohydrates
that are digestable! I presume that the recommended intake of 50g/day
are digestable carbohydrates. I'm led to conclude that they mean
that we should eat carbohydrates that are high in dietary fibre
with 50g/day available as digestable carbohydrates.
The recommended dietary allowance
for carbohydrates is 130g/day based
on the minimum amount of glucose needed by the brain. This minimum
amount of glucose is usually execeeded because additional glucose
is gained from fat and protein.
The WHO also recommend that
between 55% to 75% of our daily calories
come from carbohydrates with a low glycemic
index. I personally find this recommendation at odds with the
experiences of many type 2 diabetics, including myself, who have
discovered that limiting their carbohydrate intake to below 55%
is necessary to keep their blood glucose levels within an acceptable
range. Higher levels of cabohydrates in the diet are also associated
with higher triglyceride levels in the blood. For Type 2 Diabetics,
I think that 55% should be the upper, not the lower, limit! I personally
aim for around 40% of total calories from carbohydrates. The WHO
also recommends that we should include 25
to 35 grams of dietary fibre/day (5-15g
soluble) into our diet.
For more information on carbohydrates,
Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids,
Cholesterol, Protein, and Amino Acids (Macronutrients), 2002,
Chapter 6, pp207-264, Food and Nutrition Board (FNB), Institute
of Medicine (IOM); and Carbohydrates
in Human Nutrition, FAO/WHO
Joint Report, April 1997. Also see my Protein
and Fats pages.