CARBOHYDRATE; WHAT'S IN THE NAME?

The classical macronutrients are protein carbohydrate and fat. While it is easy to define a protein and not much more difficult to define a fat, carbohydrates defy a simple classification and the situation is not helped by proliferation of terms such as "carbs" and "net carbs", and confusion between "sugar" and "sugars". Hopefully the following expose will help you view this in shades of grey and not in the stark black and white which many would have you see! The bottom line which you need to know is that "carbs" are carbohydrates period, not necessarily "sugar" or "sugars", and that "net carbs" or "net impact carbs" are meaningless phrases coined by those who really have no ideas about carbohydrate bioavailability or metabolism and are relying solely on the legal definition of "sugars" under the Nutrition Labelling and Education Act (NLEA)! Which as you will shortly see sweeps a lot of simple carbohydrates with varying utilization by the body into one heap. The NLEA is a valiant attempt to clarify a complicated situation for the average consumer but not without pitfalls and loopholes which can be exploited to misinform or disinform rather than inform.

The low-level definition of carbohydrate can be deduced easily from the name, which implies that carbohydrates are compounds of carbon, hydrogen and oxygen in which the hydrogen and oxygen atoms are present in the same proportions as in water, namely two hydrogen for one oxygen. In fact, in the 19th Century it was even thought that carbohydrates were in fact "hydrates" of carbon, a belief that was reinforced by the observation that heating sugar to a high enough temperature releases steam and leaves a black residue of carbon.

This definition is unduly simplistic, but when no reference is made to nutritional properties is probably as useful as any of the others that are of more recent origin. For example, students of food chemistry may be taught that carbohydrates are aliphatic polyhydroxy aldehydes and ketones and their derivatives, where aliphatic means open chain organic compounds and those cyclic compounds that resemble them (excluding cyclic aromatic compounds), such as the two representations of glucose shown here (bearing in mind, of course, that glucose exists mainly in the cyclic form):

The food chemistry student would then go on further to learn that definitions of carbohydrate are relatively vague and ambigous, but that the simple monosaccharide molecule in ring form could be combined into chains to give disaccharides, oligosaccharides and polysaccharides, thus resulting in the major groups of sugars, mainly mono and disaccharides (fructose, galactose, glucose, lactose, maltose and sucrose, which contain 1 or 2 units as shown), oligosaccharides (containing 3 - 10 units as shown above), and polysaccharides, which include both starches, and non-starch polysaccharides (cellulose and non-cellulosic polysaccharides - "fibre"). Other carbohydrates, present in smaller quantities, include sugar alcohols.

Defining carbohydrate is not as simple as the terms simple (mono or disaccharides) and complex (polysaccharides and oligosaccharides) would suggest. This terminology fails to recognize the nutritional differences between various carbohydrates in each group (i.e. starch vs. non-starch polysaccharide) and their specific health effects. The emphasis placed on "complex" carbohydrates in nutrition recommendations, for example, may suggest that "simple" carbohydrates are not important, although there is no scientific evidence to support this inference.

There is a little light at the end of this tortuous tunnel, and for a general understanding of what carbohydrate means on a food label it suffices to follow the official definitions used in both the United States and Canada. The U.S. Code of Federal Regulations (21CFR101.9) states that ``Total carbohydrate`` content shall be calculated by subtraction of the sum of the crude protein, total fat, moisture, and ash from the total weight of the food. The Regulations further prescribe that "Dietary Fiber" and "Sugars" must be declared, and that "Soluble Fiber", "Insoluble Fiber", "Sugar alcohols" and "Other Carbohydrates" may be declared, all these being the components of the "Total Carbohydrate" that is declared on the label. For nutrition labelling purposes, sugars are defined as the sum of all free mono- and disaccharides (such as glucose, fructose, lactose, and sucrose), and sugar alcohols are defined as the sum of saccharide derivatives in which a hydroxyl group replaces a ketone or aldehyde group and whose use in the food is listed by FDA (e.g., mannitol or xylitol) or is generally recognized as safe (e.g., sorbitol). ``Other carbohydrate`` is defined as the difference between total carbohydrate and the sum of dietary fiber, sugars, and sugar alcohol.

There are literally hundreds of different carbohydrates present in the food we eat, but the labelling requirements do make it simpler to discuss the various groups. The rest of this article will therefore focus mainly on "sugars", "fibre", "sugar alcohols" and "other carbohdrates".
Since "Sugars" are in the forefront of most people's minds, we will take them out of the label order. For labelling purposes, sugars include all mono- and disaccharides, even when they are the rare pentose sugars. The most common "sugars" in food are:

Structure Category
Examples
Common Food Sources
Monosaccharide:
single sugar molecule

• glucose (glu)
..(or dextrose)
• fructose (fru)
• galactose (gal)

• most foods

fruit, honey
• milk

Disaccharide: molecule with
2 monosaccharide units

• sucrose (glu + fru)

• lactose (glu + gal)
• maltose (glu + glu)

• fruit, vegetables,
...table sugar
• milk
• beer

Glucose is, of course, the single most significant "energy substrate" in the body, and in the absence of dietary sources, other nutrients are converted to glucose. It is also the most readily absorbed carbohydrate, and costs the body the least "work" to absorb and utilize. This is an important point when designing products for weight loss; the digestion, absorption and conversion to glucose of other carbohydrates all require a certain amount of "work", or energy, which has to come from the metabolism, so a judicious choice of the carbohydrates in a product can not only cause the body to expend more energy (thus increasing the energy deficit required for weight loss), it can also flatten the blood sugar curve and the insulin secretion curve, with beneficial effects on metabolism and perception of hunger.

Since simple sugars in the ingested food can provide glucose to be utilized as an energy substrate directly in the intestinal wall, it has been speculated that they can improve the digestion and absorption of other nutrients by directly or indirectly fueling the "work" that needs to be done by the intestinal wall, for example the enzymatic cleavage of proteins and the absorption of the resultant amino acids and peptides. This is a logical assumption in terms of digestive physiology and indicates that the presence of small amounts of sugars in specially formulated protein supplements may be beneficial. However, there are many simple sugars known, and though they all qualify to be included in the "sugars" section of the label, some of the lesser known are absorbed relatively slowly, and some are scarcely metabolized, but mostly excreted unchanged in urine.

"Dietary fibre" is a very heterogenous group, since basically any carbohydrate that is not completely digested and absorbed in the small intestine may have some or all of the properties of dietary fibre. This would also include those sugar alcohols which are incompletely absorbed, polydextrose, oligofructose and the so-called "resistant starches", as well as conventional starches in those who are taking alpha-amylase inhibitors ("Carb-Blockers"). Labelling does not always correspond to function, and there are also differences in labelling rules from country to country. Thus sometimes polydextrose and oligofructose will be found under the "other carbohydrate" section of a label (for example, in Canada). Classically, dietary fibre is defined (LSRO, 1987) as "the endogenous components of plant materials in the diet which are resistant to digestion by enzymes produced by humans":

Structure Category
Examples
Common Food Sources
Non-starch polysaccharide (fibre) • cellulose
• pectin
• gums
• beta-glucans
• fructans

grains
• fruits
• vegetables
artichokes, chicory,
...grains

The key phrase in the classical definition is "resistant to digestion", which means that the material passes through the small intestine without being broken down (but not necessarily without effects), and this functional definition, which is physiological rather than chemical, is more practical than the classical definition (De Vries, 2003).

However, once in the large intestine, some of the material can be broken down by enzymes secreted by intestinal microflora. Since the breakdown products can be absorbed from the large intestine, dietary fiber does have some caloric content, which is usually estimated at about 1 kilocalorie per gram (or about 25% of the caloric content of digestible carbohydrates).
Classical dietary fibre may be soluble (also known as viscous) or insoluble (also known as particulate). Guar gum or pectin are good examples of soluble fibre, while wheat bran is the archetypical insoluble fibre. Currently, various carbohydrates that appear on labels under the headings of "other carbohydrates" or even "sugar alcohols", though outside the chemical definition of dietary fiber, do fall under the physiological definition (Cherbut, 2002; Flamm et al, 2001).

Until about 30 years ago, the need for fibre in the human diet was unknown, and it took pioneering work by Trowell and his colleagues before the "essentiality" of fibre was realized (Trowell, 1972; Burkitt and Trowell, 1975; Trowell et al., 1976). They suggested that consumption of unrefined plant foods protected against many Western ailments such as carcinoma of the colon, appendicitis, diverticulosis, constipation, hemorrhoids, hiatus hernia, varicose veins, diabetes, cardiovascular disease, obesity and gallstones. Substantial research performed since the original work by Trowell and Burkitt has proved that apart from its regulating effects on bowel movements, fibre does indeed have protective or beneficial effects in coronary heart disease, colon and rectal cancers, stomach cancer, female gynecological cancers, diabetes, diverticulitis, hemorrhoids, hypertension and gallstones. More recently, an immune-regulating or even immune-enhancing effect has been shown (Schley and Field, 2002), and beneficial effects in weight loss and energy regulation have also been shown (Burton-Freeman, 2000;Howarth et al., 2001). Overall, dietary fiber has very beneficial properties in the prevention and control of many chronic diseases (Kushi et al., 1999).

The properties of dietary fiber relate to their effects on the rate of absorption of nutrients, their ability to entrain or otherwise retain some molecules physiologically present in the intestine (such as bile acids and cholesterol) and retard their reabsorption, a modifying effect on the microbial flora of the large intestine, and their bulking effect. In the more primitive fiber-rich and energy-dilute foods typical of the undeveloped countries, ingested food travels the length of the small intestine, releasing the products of carbohydrate digestion slowly. Refined energy-dense foods typical of Western civilization, however, rapidly liberate digestion products in the upper part of the small intestine; on a Western diet, carbohydrate absorption occurs mainly in the first 60 cm of the small intestine (Silk and Dawson, 1979), and though the human intestine is about 6 m long, it is possible to remove all but 60 cm of it (providing part of the ileum is left) without creating a need for parenteral nutrition. Not surprisingly, therefore, on diets with added fiber in contrast to low-fiber energy-dense diets, postprandial hyperglycemia is much reduced, (Jenkins et al., 1977a; Jenkins et al., 1978), in turn reducing the insulin response (Jenkins et al., 1977b; Levitt et al., 1980). Consequently, insulin-dependent diabetic patients can be stabilized on lower insulin dosages, or may even cease requiring insulin, when placed on high fiber diets. In fact, the glycemic indices of foods may relate closely to their fiber content (Bjorck and Elmstahl, 2003).

From the foregoing, it is obvious that "dietary fiber" is a very heterogenous group of natural substances, the common factor being that they are unabsorbed in man and, therefore, nutritionally non-available. They have wide-reaching physiological actions and though they share many of these actions qualitatively, they show very pronounced quantitative differences in their effects. Not only do these differences manifest in comparisons of the purified substances, but differences can also be observed between the purified materials and materials given in admixture with food or in complex food systems.

The health benefits of fiber appear obvious, but are people getting enough? The answer is almost certainly no. The minimum recommended daily intake (Daily Value) is 25 g, but many experts believe this should be 30 g or more. However, as you will see below, some of the "sugar alcohols" and "other carbohydrates" show many properties of fiber, and contribute to health benefits in much the same way.

Classically, sugar alcohols are defined as:

Structure Category
Examples
Common Food Sources
Sugar alcohols: all oxygens present as hydroxyl groups (polyols)

• sorbitol
mannitol
• xylitol
• erythritol
• maltitol
• lactitol
• glycerol

fruit, seaweed,
exudate of plants
or trees (i.e. olive trees)
or made from glucose,
maltose, lactose,
starch or fat (glycerol)

There are wide variations in absorption between the various sugar alcohols, and once absorbed, large differences in metabolic fate. The sugar alcohol with the smallest molecular volume, glycerol, is completely absorbed and rapidly metabolized. Experimental studies with erythritol, a 4-carbon sugar alcohol of slightly larger molecular volume, show that 65% - 90% is absorbed, but that it is not significantly metabolized after absorption (Van Ommen, 1996). The fraction not absorbed from the small intestine passes through into the large intestine, where it is fermented and broken down exactly like dietary fiber.

The pentitols (such as xylitol) and hexitols (such as sorbitol), on the other hand, have still larger molecular volumes; their uptakes by passive diffusion through hydrophilic "pores" of the cell membranes (transcellular pathway) and through the "tight" junctions between the intestinal epithelial cells proceed therefore at a lower rates and are less complete. Where the various sugar alcohols differ most is the extent of metabolism, which may be rapid (glycerol, ribitol, xylitol, sorbitol), slow (l-arabitol), or not to any significant extent (erythritol, d-arabitol, mannitol). Because of their metabolic inertness, absorbed yet not metabolized sugar alcohols are readily excreted unchanged in the urine. The unabsorbed fractions of sugar alcohols are fermented in the large intestine by the gut microflora to short-chain fatty acids which are absorbed almost completely and are metabolized in the body by ordinary metabolic pathways (Opus cit.).

The "other carbohydrate" from your Nutrition Facts panel
is a heterogenous but interesting group:

Structure Category
Examples
Common Food Sources
Oligosaccharide: chain of 3-10 mono-saccharide units
• raffinose
• stachyose
• verbascose
• fructooligosaccharides
(oligofructose)
Legumes; onions; leeks; garlic; bananas; grains or commercially made from sucrose, lactose, glucose or starch
Polysaccharide: chain of more than 10 monosaccharide units*
• starch
(amylose & amylopectin)
• dextrins
• polydextrose
• potatoes, grains, ...rice,legumes, bananas
• modified corn starch
• from fermentation
Animal polysaccharide

• glycogen

• animal muscle (meat)
... & liver

It contains the oligosaccharides, of relatively small molecular size, and the high molecular weight polysaccharides, such as starch. Starches, despite their large molecular sizes, are generally completely available, unless one is using an alpha-amylase inhibitor ("starch blocker"), though there are "resistant" starches which are less available for digestion and absorption. The oligosaccharides, however, include some interesting carbohydrates such as oligofructose, which are almost completely non-digestible. On the other hand, they can be fermented in the large intestine, and thus behave like dietary fiber.

So really, oligofructose is a dietary fibre. Like all dietary fibres, it is not digested in the stomach or small intestine. However, because it is completely fermented in the colon, it contributes to better gut function, improves regularity and reduces constipation. Oligofructose is selectively fermented by Bifidobacteria and boosts the total number of these micro-organisms present in the colon. Bifidobacteria are widely recognised as micro-organisms that have potential beneficial effects on their host. These health benefits include the stimulation of the immune system, prevention of the growth of pathogenic species, increased mineral absorption, cancer inhibition and improvements in blood lipid metabolism (Jenkins et al., 1999; Kaur and Gupta, 2002; Kolida et al., 2002; Murphy, 2000). Because oligofructose passes through the upper gastro-intestinal tract intact and is then selectively fermented in the intestine, it causes a positive change of the microfloral composition of the colon, which is now known as a prebiotic effect. Extensive research in in-vitro models has shown that oligofructose is selectively fermented by Bifidobacteria. Human studies have confirmed that the ingestion of moderate amounts of oligofructose (from 5 grams per day) or inulin (which has the same type of structure but is a larger molecule) results in a significant increase (up to 10 fold) of the Bifidobacteria in the colon. As the total number of bacteria in the colon does not change, the numbers of other, often less desirable bacteria, are significantly reduced. Once a new equilibrium is reached, no further increase in Bifidobacteria growth can be expected. If the ingestion of oligofructose is stopped, the composition of the colonic microflora will slowly return to its original state. It is therefore important to consume oligofructose regularly to maintain the microflora in an optimal condition.

The effect on mineral absorption is also indirectly part of the prebiotic action. Under normal circumstances about one third of dietary calcium is absorbed from the gastro-intestinal tract. Fermentation of oligofructose in the colon produces short chain fatty acids which cause the pH of the colon content to drop. More of the calcium, present as insoluble salts, becomes soluble and can therefore be better absorbed.

Polydextrose, though listed above as "other carbohydrate", also behaves as a fiber (Murphy, 2001) and is associated with the same health benefits.

There you have it. Nutrition Facts labelling is a brave attempt to categorize and teach the public, but where it comes to carbohydrate there are so many shades of grey that you need to be an expert to know what the product really contains, and what benefits or otherwise you might expect. The current obsession with "sugars" and their confusion with "carbs" or "net carbs" does not improve the situation. If the consumer wants to avoid too great an intake of readily available monosaccharides such as dextrose (glucose) from a food, particularly a complex food, then it may be wiser to look at the ingredients and work out firstly what the "sugars" present are, and secondly, how much of the "other carbohydrates" is really plain old starch that becomes available to the body as dextrose (glucose) almost as quickly as glucose itself!

 

- back to top-