Posted 12 March 2015 - 05:51 AM
I've gathered a lot of information on carbohydrates over the years and I thought it might help to put all the notes together in an easy to read form.
Low carb VS low starch
It can be confusing when people refer to diets which are low-carb. This term is often used interchangeably with low-starch, but the two are very different diets. A low-carb diet generally means restricting all carbohydrates - starch and fibre - and eating a mainly high fat and protein diet such as the Ketogenic diet. A low starch diet on the other hand isn't necessarily low carb. This diet allows vegetable and fruit fibre as well as proteins and just limits starches.
The low carbohydrate high fat (LCHF) diet for instance is actually low starch. Fibre carbohydrates are allowed.
Basic information on carbohydrates
A carbohydrate is a molecule that consists of carbon, hydrogen and oxygen, Another name for carbohydrate is saccharide, and this terms includes sugars, starch and cellulose.
Carbohydrates are polymers. Polymers are large molecules consisting of smaller molecules.
The saccharides are divided into 4 groups:
Monosaccharides (1 monosaccharide)
Disaccharides (2 monosaccharides)
Oligosaccharides (complex carbohydrates. Include trisaccharides. Typically 3 to 10 monosaccharides)
Polysaccharides (complex carbohydrates. Includes starches and non-starch fibre, e.g.cellulose).
Some polysaccharides are digestible by enzymes, such as starch, some such as fibre, aren't.
In general, the smaller saccharides are called sugars. The names of monosaccharides and disaccharides often end in -ose - glucose, sucrose, lactose.
Glucose is the main monosaccharide in the body.
Common monosaccharides are glucose, fructose and galactose.
Common foods that contain the monosaccharide fructose are apples, pears, mangoes, grapes and watermelon.
Table sugar (sucrose) is a common diasaccharide made from glucose and fructose.
Common disaccharides are sucrose, lactose and maltose.
Oligosaccharides and Polysaccharides are made up of the monosaccharides, glucose, fructose and galactose. Oligosaccharides aren't digested by enzymes and are fermented by gas producing bacteria that do have the enzyme. This fermentation creates carbon dioxide, methane and or hydrogen.
Two common Oligosaccharides are raffinose (3 monosaccharides) and stachyose (5 monosaccharides) found in vegetables, beans and legumes. Gut microbes digest these and produce gasses that help friendly bacteria.
Raffinose can be found in beans, brussel sprouts, broccoli, cabbage and asparagus.
Polysaccharides are complex carbohydrates.
Starch is a polymer of glucose. Starch has 2 types of polysaccharide, amylose - a linear polysaccharide and amylopectin - a highly branched saccharide. Starch contains roughly 2/3 amylopectin and 1/3 amylose. Glucose is bound in different ways in amylose and amylopectin. Most amylose and amylopectin are formed between the 1st and 4th glucose carbon atoms. Some amylose bonds form between the first and last carbon atoms (1-6 bonds). Only gamma amlylase can break down these bonds, which humans don’t have.
Starch begins digesting in the mouth due to an enzyme called Amylase. Amylase is a catylyst that degrades starch into sugar. Amylase comes in 3 different types: alpha, beta and gamma amylase. Humans only have one of these enzymes, Alpha amylase, which can only break down the 1-4 type bonds. The small portion in amylose that has 1-6 bonds will pass through undigested. This is resistant starch.
Resistant starch passes through intact to the colon, where it feeds bacteria. The end products of this fermentation create short chain fatty acids that nourish the cells lining the colon as well as inhibiting pathogenic bacteria. Short chain fatty acids also trigger the release of hormones that reduce appetite (letprin andpeptide YY).
Foods high in resistant starch:
Grains, seeds and legumes. The starch is resistant to digestion because it is bound up in tough, cell walls. Potatoes, bananas and cashews also have resistant starch. Roasted potato has much higher resistant starch than boiled potato.
Storage polysaccharides (starches) are broken down to Monosaccharides and Oligosaccharides.
Dietary fibre is made up of non-starch polysaccharides that are mostly indigestible. Fibre includes cellulose, resistant starch, inulin, and oligosaccharides.
Dietary fibre has 2 basic types: soluble fibre and insoluble fibre. Soluble fibre dissolves in water and is fermented in the colon into gases and short chain fatty acids. Insoluble fibre doesn't dissolve in water and is metabolically inert. It provides bulk. Both fibres are prebiotic.
Cellulose VS starch
Cellulose (fibre) and starch are both glucose chains. Starch is digested whereas cellulose isn't. Cellulose is more resistant to digestion than starch because of the strength of its bonds. Starch has alpha bonds and fibre has beta bonds. Enzymes that break down alpha bonds are amylases and those that break beta bonds are cellulases. Humans lack cellulases and therefore can’t digest fibre.
Maltose and sucrose contain the alpha bond and are easily digested. Lactose has the beta bond and is general not digested well.
Fibre is made up of cellulose, hemicellulose, and pectin. Fibre isn't digested in the small intestine, however bacteria can use proteins that capture and break down these complex polysaccharides into their monosaccharide sugars. These proteins are called polysaccharide utilization loci (PULs). Bacteroides can break down hemicelluloses. The surprising aspect of this is that some of these the bacteria don’t grow as well on simple sugars as they do on polysaccharides. Because of this we do digest some fibre.
FODMAPs are short-chain carbohydrates - oligosaccharides, disaccharidies, monosaccharides and polyols that are poorly digested in the small intestine. These foods ferment in the colon causing gas. A low FODMAP diet doesn’t restrict starch. Some starches, especially resistant starch, can also cause fermentation. Grains aren’t excluded from the FODMAP diet with the exception of wheat and rye. FODMAP friendly grains are listed as corn and rice products, e.g. Rice cakes. Most high FODMAPs are fibrous vegetables and fruits which would be allowed on a low starch diet.