Depending on which side of the fence you're on, Carbohydrates are either the Dogs' Doodads or the Spawn of Satan. As I get older, I prefer to sit
on the fence. Let's start with the basics.
What are Carbohydrates?
Carbohydrates are so named because they have the generic formula (CH
2O)n. C is carbon and H
2O is water hence Carbo-Hydrate. There are several different types.
1) Sugars. There are
monosaccharides, the most common being
Glucose (a.k.a.
Dextrose),
Fructose and
Galactose. There are
disaccharides, the most common being
Sucrose,
Lactose and
Maltose. Disaccharides are 2 monosaccharides linked by a
glycosidic bond formed by a
condensation reaction (removal of a water molecule, usually by an enzyme). Disaccharides have to be
hydrolysed (have a water molecule added back in, usually by an enzyme) into monosaccharides before they can be absorbed in the gut.
Sugars are
simple carbohydrates.
2) Starches. These are chains of glucose molecules linked by glycosidic bonds. Starches have to be hydrolysed into glucose molecules before they can be absorbed. There are
unbranched chains like
amylose which is also known as
resistant starch. There are
branched chains like
amylopectin and
maltodextrin.
Glycogen is a branched chain "
animal starch" that is synthesised inside muscle and liver cells and which can be rapidly converted back into glucose inside cells.
3) Non-Starch Polysaccharides (NSPs). These are also known as
fibre/
fiber and there are 2 types:
soluble (e.g.
pectin,
beta-glucan &
cellulose) and
insoluble (e.g.
bran). These aren't absorbed, but gut bacteria can feed on soluble fibre/fiber. Starches and NSPs are
complex carbohydrates.
For more information, see
http://en.wikipedia.org/wiki/Carbohydrate
The amount of carbohydrate that someone needs
varies from person to person and
increases with the
intensity and
volume of
exercise done. See
Everyone is Different. What are the best carbs to eat? "Complex" ones from "
wholegrain" cereals? Not necessarily.
Simple vs Complex
TV ads for breakfast cereals bang on about the wholegrain goodness of
complex carbohydrates releasing energy
slowly. The terms "
Simple" and "
Complex" actually refer purely to the
chemical structure of a carbohydrate and have nothing to do with how quickly they turn into blood glucose in the body. The
Glycaemic Index (
GI) (or
Glycemic Index if you're
American) relates to how
quickly carbohydrates turn into
blood glucose in the body. See
http://www.mendosa.com/gilists.htm for a list of 750 foods and their GI & GL (
GL =
Glycaemic Load =
GI/100 x carb content per serving). Here are a few extracts. Note: a
GI of 55 is low; a
GL of 10 is low.
The last three items in the list are all simple carbohydrates. As you can see, some wholegrain complex carbohydrates turn into blood sugar faster than simple carbs. This is because the wholegrains have been ground into powder which is rapidly digested and absorbed, despite the presence of fibre/fiber.
As
fructose has such a
low GI, does this mean that we can eat as much of it as we like?
No! When we eat fructose, it passes from the
small intestine into the
portal vein and goes straight to the
liver. As
liver cells contain an
enzyme called
fructokinase (which has a high affinity for fructose), all dietary fructose is absorbed by the liver where it tops-up liver glycogen.
Liver glycogen is also
topped-up by
glucose (obtained from the
digestion of
starchy carbohydrates).
Once liver glycogen stores are full, any further fructose is converted into fats, which are stored as ectopic liver fat and also exported as triglycerides.
High serum triglycerides are
heart-unhealthy. See
Cholesterol And Coronary Heart Disease.
Why is GI important? When we eat carbohydrates, they raise blood glucose levels. Pancreatic beta cells secrete a hormone called insulin, which allows more glucose to pass into cells (by moving Glu-T4 transporters inside the cells). When more glucose enters cells, glucose levels in the blood fall. It's a negative feedback loop. For millions of years, we lived on a diet of natural, unrefined carbohydrates and so the secretion of insulin never had to change blood glucose levels very rapidly.
When unnatural, refined, high-GI carbs are eaten, blood glucose levels rise much faster. This results in over-secretion of insulin (hyperinsulinaemia). This shuttles too much glucose into cells and results in.......low blood glucose, followed by low blood insulin. Rapidly-falling and low blood insulin levels cause feelings of severe hunger and cravings to eat more carbs. It's a vicious circle. Hyperinsulinaemia also has other bad effects on the body. See http://www.mercola.com/2001/jul/14/insulin.htm to learn about Insulin and its Metabolic Effects.
GI has a weakness because adding fats and some proteins to high-GI foods lowers the GI but can increase the insulin response. Saturated fats, monounsaturates and omega-6 polyunsaturates raise the insulin response to carbs.
There is another index called the Insulin Index (II). See http://www.mendosa.com/insulin_index.htm.
The II contains a few surprises. Some proteins (e.g. the whey in milk & yoghurt) produce a large insulin response. Insulinogenic proteins are also glucogenic, so they don't cause low blood glucose.
On the other hand, refrigerating some foods lowers their GI & II by changing the starch in them into resistant starch, even if the food is subsequently re-heated. Rice & potatoes are two such foods.
As the terms "simple" and "complex" are meaningless in terms of carbohydrates' effects in the body, I prefer to use the terms "slow" and "fast". In a nutshell, slow carbs are good and fast carbs are bad. These terms can be applied to proteins, too. Egg is slow and whey is fast. Sticking to mostly slow foods keeps blood glucose and insulin levels stable, which results in better appetite control and better health, too.
It was soaring serum insulin levels that were sending me to sleep after carby meals years ago. Postprandial hyperinsulinaemia results in amino acids from digested foods being shuttled into cells. However, L-tryptophan isn't shuttled into cells, so the level of this amino acid in the blood rises relative to others. As L-tryptophan competes with other amino acids to cross the blood-brain barrier, now that the competition has been removed, more L-tryptophan enters the brain. It's converted into 5-hydroxytryptophan (5-HTP), then serotonin & melatonin. High melatonin levels in the brain cause sleepiness.
So remember, "Right carbs, right amounts, right times."
