Everything we consume can be broken down into four macronutrient categories: carbohydrates, proteins, fats, and alcohols. For optimal health, it is important to make carbohydrates the bulk of our diets. Researchers who looked at eating patterns in the US found that Americans who ate the least amount of carbohydrates, those in the bottom 25%, had a 32% higher risk of death from any cause than those who had the highest carbohydrate intakes. (Banach)
The beneficial effects of consuming fiber-rich, whole plant foods include improving gut health; decreasing systemic inflammation; lowering elevated LDL-cholesterol; reducing the risk of metabolic syndrome, cardiovascular disease, some cancers, stroke, and type 2 diabetes; and improving the odds of healthy aging. (Banach)(Ho)(Serafini)(Veronese)(Miller)(Rowland)
Carbohydrates are found in a wide range of foods that bring a variety of important micronutrients to the diet including vitamins, minerals, phytonutrients, antioxidants, and dietary fiber. Fruits, vegetables, whole grains, and legumes supply substances crucial to our health, but only some are currently categorized as ‘essential nutrients.’ This will likely change as the anti-inflammatory, anti-oxidant, and anti-cancer properties of these substances are fully realized. Many diseases and cancers may well be diseases of malnutrition.
Yet carbohydrates have gotten a bad rap. All carbohydrates are not equivalent. The health benefits of carbohydrates are found in whole plant foods, not processed foods. Understanding the different types of carbohydrates can help us make better nutritional choices.
For those of us who suffer from irritable bowel syndrome (IBS), understanding a subset of carbohydrates known as FODMAPs can be particularly helpful.
Classification of Carbohydrates
The term “carbohydrate” is based on its components: carbon (“carbo”) and water (“hydrate”). Carbohydrates contain combinations of carbon, hydrogen, and oxygen with a ratio of 1:2:1. Carbohydrates have traditionally been classified by their molecular structure, often into the oversimplified categories of simple and complex.
Simple refers to molecules of one or two simple sugars — monosaccharides and disaccharides.
Complex refers to polysaccharides — chains of 3 or more simple sugars bonded together. However, for this discussion, we will refer to complex carbohydrates as those with chains containing more than 10 simple sugars.
A third middle category is known as oligosaccharides — which have between 3 and 10 monosaccharides. Oligosaccharides tend to be high in micronutrients and have many health-promoting properties. All known prebiotics are carbohydrate compounds — primarily oligosaccharides. Oligosaccharides are abundant in many vegetables and they are also in human milk. Some of the ingested oligosaccharides reach the neonate’s colon intact where their prebiotic effects promote healthy gut colonization.
Polyols, also known as sugar alcohols, are neither sugars nor alcohols; however, they are carbohydrates — carbohydrates that are hydrogenated. Fruits belonging to the Rosaceae family, such as apples, pears, cherries, plums, peaches, and apricots, contain appreciable amounts of sorbitol. Xylitol is also a naturally occurring sugar alcohol that can be found in many fruits and berries. However, polyols are best known as sweeteners in dietic foods.
Most foods contain multiple types of saccharides, or carbohydrates.
Classification: Simple Carbohydrates
Simple carbohydrates are sugars; they are made up of monosaccharides, which are one sugar molecule, or disaccharides, which are two sugar molecules.
Simple Carbohydrates: Monosaccharides
Fructose is the sweetest of the simple sugars and is naturally concentrated in honey and fruits. Other foods containing fructose include dates, agave, raisins, and figs. However, fructose is mostly consumed through sucrose and the man-made sweetener high-fructose corn syrup (HFCS). Both sucrose and HFCS are 55% fructose and 45% glucose. Consuming large amounts of fructose (more than 50-60g/day) has been reported to increase serum uric acid and/or triglycerides in some individuals.
Research shows that high uric acid producing foods, such as fructose, trigger a pro-inflammatory response. (Lu) As the body breaks down fructose, chemical compounds called purines are released. The breakdown of purines produces uric acid, which is the substance that forms painful crystals in the joints and kidneys, causing gout or uric acid kidney stones. Within minutes after drinking high-fructose corn syrup-sweetened soda, uric acid levels rise.
Studies show that high dietary fructose also causes significant increases in postprandial (post-meal) triglycerides and LDL cholesterol concentrations. Fructose can be metabolized only by the liver, where limited amounts are converted into glucose and the rest is converted into triglycerides. Triglycerides in the presence of inflammation are associated with atherosclerosis and cardiovascular disease.
Foods naturally high in glucose include honey, agave, molasses, dates, dried fruit, fruits, and fruit juices. Unlike fructose, dietary glucose does increase postprandial serum glucose and insulin.
Glucose is the main source of fuel for our cells. Unlike fructose, almost all of the body’s cells, including the cells of the liver, muscles, brain, and fat, can absorb glucose circulating in the blood. Glucose can be converted to fat by fat cells, or it can be converted to glycogen by the liver or muscles. (Glycogen is the bodies principal storage form for glucose.)
Galactose is about 30% as sweet as sucrose (table sugar). Most of the ingested galactose is converted to glucose. Galactose is found in low concentrations in honey (again), beets, celery, and cherries, and it is a component of milk sugar. Galactose is a common ingredient in vaccines and non-prescription products and D-galactose, a form found naturally in the body, is used experimentally to induce inflammation.
Simple Carbohydrates: Disaccharides
Sucrose is also known as table sugar. It has approximately the same proportions of fructose and glucose as HFCS: 55% fructose and 45% glucose. No metabolic or endocrine response differences between HFCS and sucrose related to obesity or any other adverse health outcomes have been identified.
Lactose is also known as milk sugar. It is made of two monosaccharides: glucose and galactose. It is digested by the lactase enzyme.
Maltose is found in sweet potatoes, spelt (a grain), and Grape Nuts.
Polyols, also known as sugar alcohols, are neither sugars nor alcohols; however, they are carbohydrates — carbohydrates that are hydrogenated. They are common sweeteners in dietetic “sugar-free” candies, gums, jams, and other food products. Sugar-free does not mean calorie-free — there is some degree of digestion and absorption of polyols. Sorbitol is about 60% as sweet as sucrose, with one-third fewer calories. Polyols are incompletely digested and are referred to as a “low-digestible carbohydrate.” This makes their available caloric energy substantially less than sugar and they, therefore, create a lower glycemic response.
A handful of sugar-free gummy bears or a few or more pieces of sugar-free chewing gum can have consequences aside from a lower glycemic response than sugar ladened sweets. A piece of sugar-free chewing gum has approximately 1.25 grams of sorbitol. Excessive use, meaning as little as 5 grams/day, could cause chronic diarrhea, nutrient malabsorption, and over the long run could potentially lead to unintended weight loss.
Although polyols are found naturally in some stone fruits, berries, and mushrooms, the processed food industry uses man-made hydrogenated saccharides for their “sugar-free” products.
Hydrogenated monosaccharides include sorbitol, xylitol, erythritol, and mannitol. Sorbitol and mannitol occur naturally in a wide range of fruit and vegetables.
Hydrogenated disaccharides include lactitol, isomalt, and maltitol.
Oligosaccharides are the intermediate category and contain between 3 and 10 monosaccharides, or simple sugars.
There are many types of oligosaccharides; some are naturally occurring, some result from the microbial breakdown of polysaccharides, and others are manufactured.
They are found in Jerusalem artichoke, bananas, burdock, chicory, leeks, onions, asparagus, wheat, rye, legumes, and brewer’s yeast.
Oligosaccharide soluble fiber is highly fermentable and an excellent prebiotic carbohydrate.
Prebiotics are nondigestible components of our diet that pass through our digestive tract and become food for our gut bacteria. Our gut bacteria are called probiotics. In order for the probiotics colony to thrive, it needs prebiotics. The best food for our gut bacteria is the dietary fibers and resistant starches that we cannot digest. The bacteria break down some of these materials for their own nourishment and create acetate, propionate, and butyrate (short-chain fatty acids) as waste products, which in turn are used by the cell lining of the colon for nourishment.
Oligosaccharide fibers that escape degradation in the small intestine travel to the large intestine where they become fuel for friendly intestinal flora. Galacto-oligosaccharides have been shown to be an excellent substrate for health-promoting bacteria such as Bifidobacteria and Lactobacilli.
Research on rats suggests that the probiotic Bifidobacteria lactis and prebiotics such as oligosaccharides, resistant starch, and dietary fiber act synergistically to stimulate the elimination of DNA-damaged cells that might otherwise progress to malignancy. (Le Leu)
Note that cooking these prebiotic foods tends to destroy the carbohydrate chains; heat breaks down the medium chains to simple sugars.
Classification: Complex Carbohydrates
Complex carbohydrates come in two forms: Starch and Fiber. Starch and fiber are very similar from a molecular standpoint; however, the bonds between glucose molecules in starches are much weaker than the bonds between glucose molecules in fiber. This makes most starches easily digestible so they can be absorbed in the small intestine. Fiber moves through the small intestine undigested.
Many high carbohydrate foods, such as grains, potatoes, and beans, are high in both starch and fiber.
Complex Carbohydrates: Starches
The most common form of carbohydrate consumed is starches. When a plant synthesizes glucose, it stores the extra glucose in the form of starch — starches are made of glucose molecules linked together. Starches are comprised of two similar yet distinct polysaccharides: amylose (10-40%) and amylopectin (60-90%).
Amylopectin is more easily digested and absorbed than amylose. High amylopectin foods are high glycemic foods. (The glycemic index of foods is a measure of how much blood glucose levels increase after consumption.) Eating foods rich in amylopectin can lead to spikes in blood sugar, insulin, and cholesterol. However, the fiber present in many high amylopectin foods reduces insulin and blood sugar spikes and helps chaperone cholesterol out of our bodies. High amylopectin foods include short-grain sticky rice, jasmine rice, and red bliss potatoes.
Amylose is insoluble in water while amylopectin is soluble is. Amylose isn’t well digested and absorbed in the small intestine. It is resistant starch; in other words, it resists digestion. Foods with higher amounts of amylose have been found to delay carbohydrate digestion and absorption and decreased blood sugar and insulin levels. Resistant starch has also been shown to reduce fat storage, increase satiety, lower cholesterol, and blood sugar, and improve insulin sensitivity. Beans and other legumes typically contain 30 to 40% amylose.
Food starches have semi-crystalline granule structures, which differ in size and shape, amylose and amylopectin content, chain lengths, degree of branching, and X-ray diffraction patterns. Starch structure is associated with digestibility and it by their rate of enzymatic digestion to glucose in the small intestine that we classify starches.
Rapidly digestible starches (RDS) are digested with 20 minutes. RDS starches are low in amylose. RDS foods include fresh white bread and freshly cooked potatoes.
Slowly digestible starches (SDS) are digested between 20 and 120 minutes. SDS foods have a moderate amount of amylose. SDS foods include tapioca flour and corn starch.
Similar to soluble fiber, resistant starch (RS) foods are not digested or absorbed by the small intestine but are fermented in the large intestine and have prebiotic properties.
RS foods are relatively high in amylose and include green bananas, cold cooked russet potatoes, and cold cooked rice. (Sonia)
Starch digestion begins in the mouth where amylase enzymes start the digestive process. Starch must be completely broken down into glucose before it can be absorbed in the small intestine.
Slowly digestible starch (SDS) delivers a slow and sustained release of blood glucose from the lumen of the small intestine into the bloodstream. SDS provides both low glycemic and low insulinemic response.
SDS is found in raw whole grains and pulses (dried beans, peas, and lentils). However, exposure to heat, pressure and/or moisture converts longer starch chains to shorter starch chains; in other words, converts it into rapidly digestible starch, which makes it more accessible to digestive enzymes — in other words, “easier to digest.”
Resistant starch does not release glucose within the stomach or small intestine. It remains undigested until it reaches the large intestine where it is consumed or fermented by colonic bacteria (gut microbiota).
The fermentation of resistant starch produces short-chain fatty acids. Short chain fatty acids feed the colonocytes (epithelial cells of the colon) which in turn produce mucus. SCFAs are thought to reduce the risk of inflammatory diseases, type 2 diabetes, obesity, heart disease, and other conditions.
A meta-analysis on the effects of resistant starch in pigs concluded that RS decreases the pH of food undergoing digestion in the large intestine and increases lactic acid-producing bacteria in feces. It is postulated that this limits the growth of opportunistic pathogens in the colon. (Metzler-Zebeli)
One of the best sources of resistant starch is legumes — beans, peas, and lentils. Amylose comprises up to 40% of the starch found in legumes. Resistant starch is also found in green bananas, corn, barley, rice, raw potatoes, and potatoes that have been cooked and then cooled.
Resistant Starch Subtypes
Resistant starch has been categorized into four subtypes:
RS1 is physically inaccessible or undigestible resistant starch, such as that found in seeds or legumes and unprocessed whole grains.
RS2 is inaccessible to enzymes due to starch conformation, as in green bananas and high amylose corn starch.
RS3 is formed when starch-containing foods are cooked and cooled, such as pasta, rice, and potatoes. A process called retrogradation converts short glucose chains into larger, less digestible chains.
RS4 are starches that are chemically modified to resist digestion. (Sajilata)
Complex Carbohydrates: Fiber
Dietary fiber is non-digestible by humans but has many health benefits: stabilizes blood sugar, removes cholesterol, decreases circulating bile acids, improves bowel motility, increases satiety per calorie intake, and nourishes healthy gut microbiota.
The majority of dietary fiber comes from plant cell walls. There are many different plant cell wall polymers and they have many different characteristics, but historically for nutritional purposes, fiber has been categorized into two basic varieties: soluble and insoluble. Although, there is no standardized method for separating soluble and insoluble fibers and it is largely a matter of degree. Most foods contain both soluble and insoluble fiber.
The physiological properties of fiber such as water holding capacity, solubility, viscosity, binding or absorption properties, and fermentability influence our physiological responses by affecting the process of digestion and absorption.
Soluble fiber absorbs water, turning into a gel-like mush (think of what happens when you add water to oatmeal) while insoluble fiber doesn’t (think apple skin).
Foods rich in soluble fiber include oatmeal, nuts, beans, apple flesh, and blueberries.
Insoluble fiber is found in the seeds and skins of fruit, flax seed, celery, as well as whole-wheat bread, and brown rice.
Soluble fiber increases the viscosity of digested materials, which makes them resistant to digestive enzymes and slows our glycemic response. And the ability of soluble fiber to form a gel that sweeps along the gut helps to move the bowels. Soluble fiber is generally highly fermentable and is food for our gut microbiota.
Insoluble fiber is more resistant to fermentation than soluble fiber. Its affect on the gut microbiota is currently not well understood. However, it does promotes the movement of material through our digestive systems and it increases stool bulk. Foods high in insoluble fiber also provide greater satiety per calorie than low fiber foods and for this reason, they provide a healthy and satisfying means of calorie restriction.
Researchers note the beneficial effects of both soluble and insoluble fibers in the diet for lowering cholesterol and reducing the risk of diseases such as atherosclerosis, colon cancer, diabetes, diverticulosis, and obesity.
The Carbohydrates Known as FODMAPs
High-carbohydrate and high-fiber foods have many health benefits, but they can be problematic for IBS sufferers. Fermentable carbohydrates are sugars that are easily fermented in our digestive systems. Undigested foods consist largely of indigestible carbohydrates, resistant starches, and undigested protein. These foods escape degradation in the upper digestive tract and reach the large intestine where they are broken down by acids, fermented by bacteria, or pass through the digestive tract with minimal changes.
For those with small intestine bacterial overgrowth, or SIBO, fermentation can take place in the small intestine rather than the colon. Bacterial fermentation produces gases, which can lead to bloating and cramping, especially when fermentation takes place in the small intestine.
Foods that contain fermentable carbohydrates are known as FODMAPs — foods touted as troublesome for IBS sufferers. Yet, in 2014 the American College of Gastroenterology released new guidelines that state that soluble fiber can be helpful for IBS. For people with constipation-predominant IBS, soluble fiber can help ease constipation. However, in people with diarrhea-predominant IBS, or IBS with alternating constipation and diarrhea, soluble fiber can exacerbate symptoms.
Relatively recent research has investigated the relationship between IBS symptoms and a large group of dietary carbohydrates known as FODMAPs — fermentable carbohydrates. FODMAP is an accronym that stands for fermentable oligosaccharides (fructo-oligosaccharides, also known as fructans, and galacto-oligosaccharides, also known as galactans), disaccharides (double sugars), monosaccharides (simple sugars), and polyols (such as polydextrose, isomalt, maltitol, mannitol, sorbitol, and xylitol).
High-FODMAP foods — foods with high-soluble fiber content — can wreak havoc on a sensitive gut. The “Top Vegan IBS Triggers” list includes apples, pears, cherries, globe artichokes, avocados, beans, bran, broccoli stalks, savoy cabbage, cauliflower, cashews, pistachios, garlic, onions, honey, dates, mushrooms, peas, sweeteners (sorbitol, mannitol, and xylitol) and wheat.
Monash University has a phone app that rates many foods for FODMAP content (oligosaccharides, fructose, polyols, and lactose) based on serving size.
Insoluble fiber undergoes minimal change in the digestive tract. Its effect on the gut microbiota is not well understood. Whether insoluble fiber is beneficial for IBS sufferers is also not well understood, as studies show mixed results. However, insoluble fiber is known to shorten colonic transit time — the time it takes food to travel through one’s digestive tract.
If you suffer from IBS symptoms, we recommend that you work with Dr. Harlin to determine the root cause of your symptoms. He will rule out other potential gastrointestinal disorders such as inflammatory bowel disease or celiac disease that could be causing your symptoms. He will also assess potential dysbiosis (an imbalance in your microbiota), SIBO, parasites, enzyme insufficiency, a permeable gut barrier, or other potential issues. While you and Dr. Harlin are determining the root cause of your symptoms, we recommend a low FODMAP diet. To help you maintain a balance of nutrients while on a low FODMAP diet, we recommend that you work with our nutritionist, Maureen Magner. Note that a low FODMAP diet is a temporary measure. There is strong evidence to suggest that the more complex and varied the diet, the more complex and varied the gut microbiota, which is key for multiple facets of health — not just gut health.
For more on IBS and IBD see our page Irritable Bowel Syndrome and Inflammatory Bowel Disease.
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