Carbohydrates are in all plant foods and all plant foods contain a form of sugar. The only carbohydrate not found in a plant is lactose from milk. Carbohydrates include all grains (glucose), nuts (sucrose), fruits (fructose), vegetables (glucose / fructose). If a food grows in the ground, on the ground or in a tree it’s a carb and it contains sugar.

Carbohydrates are either sugar, starch or fibre.

3 simple sugar molecules make up carbohydrates: glucose, galactose and fructose.

Glucose is a single sugar, a monosaccharide.

Sucrose is a disaccharide of glucose and fructose.

Galactose is a monosaccharide. When combined with glucose, it becomes lactose – a disaccharide.

Most single sugars are easily digested, with the exception of fructose, the digestibility of which can vary from person to person.

Starches are the digestible carbs, and the highest in calories. Because of this we find starches satisfying to eat. Fibre has no calories as it travels undigested through the small intestine (this isn’t strictly true as a few calories are absorbed in the colon during fermentation).

Starch is made from long chains of glucose, and is mostly easily digested (with the exception of resistant starch, a type of fibre). Because starch is made up of long chains of glucose, and because glucose is readily absorbed, all starch is eventually converted to glucose.

Foods very high in starch are: all grains and white potatoes.

Vegetables with no starch are fibrous vegetables.

Fibre is also made of glucose chains however we lack the enzymes to digest these and fibre travels through the colon undigested where it feeds bacteria.

Not all carbohydrates are equally digestible: some are easily digestible (starches), some are virtually indigestible (fibre) and some are in-between (FODMAPs) with sugars that are partly digestible or not digestible at all. Some of these sugars are actually a prebiotic soluble fibre because they escape digestion entirely and feed bacteria in the colon.

Starch also contains a prebiotic fibre called resistant starch.

Prebiotic fibre is generally very healthy for the colon, however extra fermentation for those with IBS often makes symptoms worse. An overabundance of the byproducts of bacterial fermentation, short chain fatty acids (SCFAs), can be inflammatory.

What exactly does high carb / low carb mean?

There is a lack of clear definition when referring to high carb and low carb. Does “carb” refer to the amount (grams) of carbs or to the calorie content? And is a low or high carb diet good or bad? My own definition is that carbs are plant food, so if I eat a lot of vegetables, my diet is high carb. Other people think carbs refer to starches only. I prefer to say a diet high in vegetables as everyone understands what this means.

Foods that seem like high carbs but they’re not:

Hard cheese, butter and yoghurts are very low in the sugar lactose, and they are therefore considered to be low-carb, high protein foods (the protein is the dairy protein Casein).



Starch is a polysaccharide of glucose sugars (many chains of sugars).

Starch is a gnarly, tightly coiled collection of glucose chains, and some starch is indigestible; this is the resistant starch Amylose, which is a type of fibre. The digestible part of starch, amlypectin, is more easily digested (by most people).

Starches which have a higher percentage of amylose have a lower Glycemic Index (GI) as they are more difficult to break down, while those with a higher percentage of amylopectin have a higher GI. This is because the lower the level of amylose the greater the chance of digestion by amylase, the starch digesting enzyme, and degradation of starch to the simple sugar glucose, which in turn raises the blood sugar.

Because amylase resists digestion, it is a resistant starch that acts as a prebiotic. It feeds bacteria in the colon. This is generally a good thing for healthy people, but less so for those with IBS.

Boiling reduces starch content in vegetables, more so than frying or baking as a certain amount of starch will escape into the water (which is usually discarded).

Foods highest in starch include rice, white potatoes, and the man-made foods from grains: breads, cakes, biscuits etc. Wheat grain is very high in starch and the flour made from the grain is concentrated starch in a very dry form. Grain flours are high in calories and refined flours, (with the fibre removed) can raise blood sugar levels higher than sugar. Rice and wheat are essentially glucose, as this is ultimately what these foods break down into. High fibre wheat might have a lower GI as the sugars are slowly released into the blood stream, but eventually it still becomes glucose.

Zero starch vegetables

The following vegetables have zero starch: celery, lettuce, tomatoes, pumpkin, mushrooms, broccoli, cabbage, onions peppers, spinach, zucchini, baby raw carrots. Zero or low starch vegetables are typically salads and fibrous vegetables.

High starch vegetables – white potato and sweet potato

Potatoes have the highest starch of all the vegetables, much higher than any other. Vegetables with some starch include corn, peas, green beans, beetroot but only in fairly small amounts.

Resistant starch

Legumes and unripe banana contain high levels of resistant starch.

Why is starch such an issue for IBS?

Sugars and starches are the preferred foods of pathogens. Good bacteria prefer the SCFAs of prebiotic fibre in the colon.

Not everyone has the same number of copies of the gene for Amylase (AMY1), which breaks down starch. People with low levels of this gene might have a problem digesting starch.

Starch digestibility

Starches are generally made up of about 2/3 amylopectin (easily digested) and 1/3 amlylose (resistant to digestion). We shouldn’t really have a problem digesting amlyopectin starch, as this is considered to be easily digested in the small intestine. It is believed that in cases of SIBO, bacteria in the small intestine predigest starch before we do, causing gas. Not everyone with gastrointestinal problems like IBS will have SIBO, but almost everyone who does have these issues feels better when they reduce their starches.

Why is this? If people simply eat high starch foods that are easily digested; for instance, a variety of short grain rice which is predominantly amlyopectin, high GI (glycemic index) and quickly digested, then why do IBS symptoms persist? If we take SIBO out of the equation, why is starch such a problem food for some of us?

Starch Persorption

One interesting theory suggests that starch granules can get directly into the blood stream through a process called persorption. This means that particles can travel directly through the intestinal lining and into the blood stream. This is how Leaky Gut is caused, by particles bypassing the normal route of digestion through the brush border villi and entering the gut through the epithelium, the gut lining.

Consuming a diet high in starches causes a rise in inflammatory lipopolysaccharides, which also contributes to Leaky Gut.

Grains contain antinutrients (lectins, phytates, saponins) that are toxic for genetically susceptible people.

Vegetables, with the exception of white potatoes, are all fairly low in starch and very low in antinutrients.


FODMAP is an acronym for:
Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols.

FODMAPs are complex carbohydrates that are poorly digested or not digested at all. A low FODMAPs diet is recommended for people with IBS, however it is considered to be a temporary diet and ideally higher FODMAP vegetables are reintroduced once the harmful bacteria has been eradicated.

The only monosaccharide that causes problems is fructose, which has a wide degree of variation in tolerance from person to person. Fructose in excess is a natural toxin. The only disaccharide that is troublesome is lactose and oligo saccharides include all other sugars, tri-saccharides and tetra-saccharides, which are sugars most people find difficult to digest, although healthy people don’t seem to be bothered by them.


FRUCTANS are a soluble fibre. Those with a shorter chain length are called fructo-oligosaccharides. Fructans with longer chain lengths are called inulin (polyfructose), a soluble fibre prebiotic.

Because inulin is a prebiotic and fermentable by bacteria, does this mean we shouldn’t eat any foods containing inulin?

Dosage is everything. Foods with a high proportion of inulin are FODMAPs, but there are also foods with a much lower percentage of inulin, and these are low FODMAP. Bananas and Kale are 2 examples. The difference between a high FODMAP and a low FODMAP is just dosage.

Here is a rough guide to the levels of inulin in certain fruits & vegetables:

Inulin content of some foods

Jerusalem artichoke – 20% inulin (high FODMAP)
Garlic – 15% inulin (high FODMAP)
Leeks – 10% inulin (high FODMAP)
Onions – 6% inulin (high FODMAP)
Banana – less than 1% (low FODMAP)

Prebiotics are generally very healthy for the colon as these are foods that good bacteria ferment. However for people with IBS who can’t digest carbs properly the excess gasses, including short chain fatty acids (SCFAs) caused by the fermentation can be inflammatory, especially in the small intestine where they don’t belong.

Short Chain Fatty Acids

Short Chain Fatty Acids are produced during fermentation of soluble fibre by bacteria in the colon. SCFAs are nutrients for the cells lining the gut. We can actually metabolise some of the calories from SCFAs so strictly speaking soluble fibre does have calories – around 1 to 3 calories per gram.

Short chain fatty acids lower the pH of the colon, making the environment more inhospitable for pathogens.

Types of soluble fibre that produce SFCAs:

Resistant starch
Pectin from fruit
Fructooligosaccharides and inulin

Are SCFA’s / fermentation byproducts of good bacteria OK for IBS?

Again, it’s a matter of dosage. A small amount of SCFAs are beneficial. But most of us  with IBS maldigest carbs, producing a lot of gas. Too much gas is toxic, especially in the small intestine.

We do want to feed the good bacteria though, and so a low FODMAP prebiotic would be a much better choice.

Are FODMAP sugars digestible?

The answer is some are. The FODMAP sugars seem to be a cross between digestible complex carbohydrates that we do digest and fibre, which we don’t. Some FODMAP sugars will be broken down to the simpler sugars by enzymes however some won’t. This is highly variable among people, with some being able to digest more FODMAP sugars than others. Galacto-oligosaccharides are the least digestible FODMAP for everyone (legumes).

The FODMAPs that aren’t digested pass intact through the colon, feeding bacteria. The more undigested sugar there is, the more bacterial byproducts will be produced from the fermentation. These are the short chain fatty acids.

The individual sugars in FODMAPs seem to have a wide variability in digestibility from person to person as well. Certain races are more able to digest lactose and some people can’t digest this sugar at all. Lactase, the enzyme that digests lactose, lives on the brush border of the gut and if the gut is damaged then lactase production will be compromised, making lactose intolerance worse.

Some people are unable to completely digest fructose, and these people are fructose intolerant. Fructans are oligosaccharides of fructose molecules and these are also difficult to digest FODMAPs.

Galactans are long chains of galactose and we lack enzymes necessary to digest these. These are possibly the least digestible sugars of the FODMAPs and most people can only digest a small percentage of these sugars, if any at all. Galactans are considered to be a type of indigestible fibre. Galactans are found predominantly in legumes (baked beans, etc).



Fibre comes in 2 types: soluble and insoluble. No fibre is digested in the small intestine. Soluble fibre is fermented by colonic bacteria; insoluble fibre isn’t.

Soluble fibre is soluble in water; insoluble fibre isn’t and retains water, helping to form larger, softer bowel movements.

Soluble fibre is prebiotic fibre, meaning it feeds bacteria. (Probiotics are the bacteria themselves).

Both types of fibre are very important for colonic health.

Most plant foods have a mix of both soluble and insoluble fibre. Vegetables that are considered insoluble have a higher ratio of insoluble fibre and these include green beans and dark leafy vegetables.