Carbohydrates are the most abundant biomolecules on our planet and in our food supply. They exhibit some of the largest differences in their metabolism by different members of the animal kingdom. At one extreme, herbivores can almost completely break down dietary plant material with the help of beneficial bacteria that dwell within their gastrointestinal tract; at the other extreme, true carnivores can’t process most dietary carbohydrates. Humans fall somewhere in between; we derive a great deal of nutrition out of some dietary carbohydrates, but are unable to process others.

In our diets, digestible carbohydrates consist of sugars and starches, while the indigestible carbohydrates are the fibers and resistant starches1. Dietary sugars are predominantly monosaccharides (sugars consisting of a single unit, such as glucose and fructose) or disaccharides (sugars consisting of two monosaccharides linked together, such as sucrose and lactose). Starches are long chains (polymers) of many linked monosaccharide molecules, usually glucose.

Monosaccharides are the preferred form by which sugars are absorbed from the intestines, therefore, starches and disaccharide sugars (sucrose, lactose) must be broken down by digestive enzymes before assimilation. Starches are fairly easily digested by the action of pancreatic enzymes, while disaccharide sugars are degraded by enzymes that dwell on the surface of the small intestines. The familiar lactose maldigestion (“lactose intolerance”) experienced by many individuals actually results from the lack of one of these intestinal enzymes (lactase, the enzyme that breaks down lactose into glucose and galactose).

Fibers and resistant starches are carbohydrates as well. Like starches, fiber is composed of polymers of linked monosaccharide sugars. Unlike starches, however, fibers and resistant starches are not used as a source of calories; humans lack the necessary enzymes to break down resistant starches and fibers, therefore, they are not absorbed. Some soluble fiber and resistant starch is broken down by intestinal bacteria, the rest passes through the gastrointestinal tract intact.

The majority of dietary carbohydrates are obtained from plant sources (fruits, vegetables, grains). In contrast to animal tissues, which are held together by mostly proteins, plants cells are held together by cellulose and lignin, two types of dietary fiber. The edible portions of plants are usually those that contain large amounts of storage carbohydrates, such as the kernels of grains (which store starches) or fruits (which store sugars). Smaller amounts of carbohydrates are found in animal products; carbohydrates constitute only about one percent of the mammalian body2.

Although they do not have the diversity in human metabolism as do proteins, dietary carbohydrates and fibers still have a number of fates:
Fuel Source and Fuel Storage.

As versatile as humans are in obtaining energy from a variety of macronutrients, the preferred energy source in our metabolism is the carbohydrate glucose. Under normal conditions, the brain uses glucose as an energy source almost exclusively, and most other tissues rely heavily on it. To accommodate the body’s need for glucose, most sugars and starches can be converted into glucose as they are absorbed and distributed amongst various tissue following a meal. Additionally, some amino acids from digested protein can also be converted into glucose (in true carnivores like cats, this is where most glucose comes from).

Unlike other cellular energy sources (amino acids and fatty acids), glucose can be converted into energy in the absence of oxygen (anaerobic glycolysis). This makes glucose a critical source of quick energy during times when oxygen is scarce, such as during intense exercise.

Glucose can also be stored for later usage, in the form of glycogen (“animal starch”). Glycogen is abundant in the liver, which stores about a day’s worth of glucose in order to provide enough energy to fuel the brain during periods between meals. Glycogen is also used to store glucose for use in muscles, which rely on it for quickly generating energy. If the dietary intake of carbohydrates exceeds what is needed for immediate energy and glycogen reserves, then the excess is converted to fat for long-term storage.

Precursors to other biomolecules. Carbohydrates are used to make other important biomolecules. These include: glycosaminoglycans (such as chondroitin, keratin, and hyaluronic acid), important constituents of joints and connective tissues; nucleic acids (DNA and RNA are partially constructed from the sugar ribose); as well as other amino acids and fatty acids for making new cellular proteins and cell membranes.

Stimulation of digestion. Fiber, despite its non-nutritive value, still has evolved important roles in human physiology. The bulk of insoluble fibers helps digested food to move more easily through the intestines and be readily eliminated from the body. Soluble fibers and resistant starches can provide a source of energy for intestinal bacteria, which themselves provide a number of health benefits, including the stimulation of immunity, protection from pathogenic bacteria, and enhanced absorption of minerals from the diet. Prebiotics, a subset of soluble fiber, have gained attention in recent years in their ability to be selectively fermented by gut flora for a diversity of potential health-promoting benefits3.

Many of the health benefits realized by modifying carbohydrate intake involve altering patterns of consumption: reducing intake of sugars, and increasing intake of fiber. For example, recent emphasis on increased intake of whole grains (which contain significantly more fiber, phytonutrients, and protein than do refined cereal flours) has resulted from several studies which suggest that its consumption may reduce the risk of certain cancers, diabetes, and cardiovascular disease4. Fiber intake, in particular, has been the subject of thousands of studies in humans and animals, in part for its ability to successfully reduce the risk of several diseases by different mechanisms:

Reducing Chronic Low-level Inflammation. In contrast to the conspicuous inflammation that is characteristic of an injury or infection, chronic low-level inflammation can progress unnoticed. This potentially silent affliction has been associated with the progression of several diseases, including cancer, diabetes, cardiovascular, and kidney diseases. In an analysis of 7 studies on the relationship between weight loss and inflammation, increased fiber consumption correlated with significantly greater reductions in C-reactive protein (CRP), one indicator of low-level inflammation5. In these studies, daily fiber intakes ranging from 3.3 to 7.8 g/MJ (equivalent to about 27 to 64 g/day for a standard 2000 kcal diet) reduced CRP from 25–54 percent in a dose-dependent fashion. The Women’s Health Initiative Study also found significant inverse relationships with dietary soluble and insoluble fiber (over 24 g/day) and certain markers of chronic inflammation6.

Promoting Healthy Blood Pressure. It is not clear how dietary fiber reduces blood pressure, but many studies have observed this trend. Fiber, when taken with a meal, may by reducing the glycemic index of foods and lowering the response of insulin following a meal (insulin may play a role in blood pressure regulation). Soluble fibers may also increase mineral absorption (such as calcium, magnesium, and potassium; all important for healthy blood pressure) by feeding intestinal flora, which lowers intestinal pH and establish a favorable acidic environment for mineral absorption7. Whatever the cause, at least thirty randomized, controlled clinical trials examined the effects of fiber in both hypertensive and normotensive patients. Across all participants, increased fiber intake demonstrated modest average reductions in systolic (1.13–1.15 mm Hg), and diastolic (1.26–1.65 mm Hg) blood pressure89. Amongst hypertensive patients, the average blood pressure reductions were much larger: A significant average reduction in both systolic (-5.95 mm Hg) and diastolic (-4.20 mm Hg) blood pressure was observed over 8 weeks in trials where hypertensive participants increased their daily fiber intake9.

Promoting Healthy Levels of Blood Lipids. High-fiber diets have been associated with lower prevalence of cardiovascular disease (10). When included as part of a low-saturated fat/low cholesterol diet, dietary fiber can lower low-density lipoprotein cholesterol (LDL-C) by 5–10 percent in persons with high cholesterol, and may reduce LDL-C in healthy individuals as well10. Dozens of controlled clinical trials have shown the cholesterol-lowering potential of dietary fibers including soluble oat fiber, psyllium, pectin, guar gum, b-glucans from barley, and chitosan3,12,13.

Soluble fibers lower cholesterol by several potential mechanisms (3). They may directly bind cholesterol in the gut, preventing its absorption. The high viscosity of soluble fiber and its ability to slow intestinal motility may help to limit cholesterol and fat uptake as well. Fiber can also increase satiety, which can limit overall energy intake14,15. Lowering Uric Acid. Elevated blood uric acid (hyperuricemia) is a risk factor for kidney disease, cardiovascular diseases, and diabetes; it is also a primary cause for gout16. Fiber intake may lower blood uric acid levels. A significant inverse relationship between fiber intake and hyperuricemia risk was established by analyzing dietary fiber intake data from over 9000 otherwise healthy adults participating in the National Health and Nutrition Examination Survey (NHANES) from 1999–2004. Based on these data, participants with high fiber diets (over about 19 grams fiber/day for the average 2000 kcal diet) had a 55 percent reduction in hyperuricemia risk compared to those on lower fiber diets (<9.2 g fiber/day)17. While these mechanisms for this reduction is unknown, dietary fiber may reduce the absorption of purines from the diet, one of the inciting factors for hyperuricemia.

The amount and composition of carbohydrates in the “ideal” diet is amongst the most heavily debated topics in nutrition. There are scientifically-substantiated merits to both the “low-carb” and “low-fat, high-carb” diets in terms of reducing disease risk and maintaining a healthy body mass index (these will be discussed in greater detail in a future article). The common ground between the two schools of thought is that the average Western diet probably contains too little fiber, and too much refined grains and added sugar. A low-fiber/high-sugar diet, when coupled with excessive caloric intake, has been associated with significant increases in the risk for a number of ailments, including obesity, insulin resistance/type II diabetes, and cardiovascular disease.

As mentioned previously, the benefits of dietary fiber are numerous. The average daily fiber intake in the American diet, based on data from 2007–2008 NHANEs survey, is about half of the 28 grams/day recommendation by the Institute of Medicine (IOM). Significant numbers of people consume even less than the national average. The highest intakes of dietary fiber are associated with the lowest disease risks; for several observational studies, the greatest risk reductions required intakes exceeding the IOM recommendations.

In contrast, the American diet contains no shortage of refined grains or sugars. The U.S. Department of Agriculture estimates average grain consumption at about 33 percent more than 6 oz./day recommended in its Dietary Guidelines for Americans. Most of this grain is refined; the same group estimates Americans consume only one-third of the recommended 3 oz./day of whole grains18,19.

Analysis of data from the last NHANEs survey (2007–2008) determined that Americans consume an average of 120 grams/day of total sugars (about 30 teaspoons), most of which are added sugars. This amounts to approximately 480 kilocalories of energy per day. Most of these sugars come from sweetened carbonated beverages (~37 percent); other top sources include desserts and fruit drinks (fruitades and fruit punches). While arguments can be made that it is the added fructose or corn syrup are particularly dangerous to health (there is evidence that supports and refutes this hypothesis), or that sugar is additive and contributes to overeating (animal models may support this claim), added sugar clearly contributes a significant amount of calories to the average diet, and in many cases displaces essential nutrients20,21.

To read the series on Macronutrients:


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Kevin M. Connolly, PhD

Kevin M. Connolly, PhD received his bachelor’s degree in anthropology from Brown University, and doctorate in biochemistry and molecular biology from UCLA. Before consulting for the dietary supplement industry, he spent 15 years in basic biochemistry research elucidating such diverse mechanisms as bacterial antibiotic resistance and collagen synthesis. He contributes to several online and print publications, and is a frequent guest on radio health programs throughout the country. When not writing, he teaches undergraduate biochemistry.