Understanding Sweet Carbohydrates

By Christina Fitzgerald, M.S., R.D., L.D.N., Contributing Editor

Its easy to be confused by all the news about carbohydrates in food: good carbohydrates versus bad carbohydrates, complex carbohydrates versus simple carbohydrates, andof coursefad diets that promote drastically cutting or eliminating carbohydrates altogether. In reality, carbohydrates come in many different forms and can be both healthy and unhealthy.

However, an arsenal of evidence shows consumers intake of sweet carbohydrates has drastically increased over the past several decades. Research has shown repeatedly that if we consume any one nutrient or total calories in excess of what the body needs, it can lead to poor weight management and chronic diseases. Today, 75% of total sugar consumed is in our commercial food supply. A 1998 USDA Economic Research Service report showed 45% of added sweeteners consumed go into beverages, 18% into cereal and baked goods, and 11% into confectionery goods (Agricultural Economic Report No. 772).

By definition, a carbohydrate is any group of organic compounds that includes sugars, starches, celluloses and gums, and also serves as a major energy source in the diet of animals. However, carbohydrates vary tremendously in sweetness, texture and rate of digestion.

Saccharide science

Not only does the sweetness differ greatly among these saccharides, but they are further differentiated by their chemical structure and divided into four categories: monosaccharides, disaccharides, oligosaccharides and polysaccharides. The sweetest carbohydrates include fructose, sucrose, glucose and lactose (galactose and glucose).

Glucose, or dextrose, (4 kcal per gram) is the most-abundant sugar found in nature, but it is seldom found in its monosaccharide form. It has approximately 75% the sweetness of sucrose. Glucose is typically found in nature as starch or cellulose, but also links with fructose to form sucrose. Fructose is the sweetest of all monosaccharides, about 1.4 times the sweetness of sucrose, providing 4 kcal per gram. It is known as natural fruit sugar; most fruits contain 1% to 7% fructose. The increase in sweetness as a fruit ripens is due to the separation of sucrose into fructose and glucose.

Sucrose, a dissacharide known more commonly as table sugar, cane sugar, beet sugar or even grape sugaras well as being the main component in natural sugars, like turbinadoprovides 4 kcal per gram. Sucrose occurs naturally in many foods, but is used abundantly in commercially processed foods.

Invert sugar, also used in many commercial foods, typically in liquid form, is sucrose is inverted by hydrolysis to half glucose and half fructose, although some forms are not completely inverted. Honey is considered a type of invert sugar; it has approximately 38% fructose, 31% glucose, 7% maltose, and a small amount of sucrose and other sugars.

Out of the sweet carbohydrates, high-fructose corn syrup (HFCS) seems to receive the greatest amount of bad publicity. HFCS, created by changing some of the glucose in corn starch to fructose, is intensely sweet, yet inexpensive. The predominant forms of HFCS are HFCS-55 (55% fructose, 41% glucose, and 4% glucose polymers) and HFCS-42 (42% fructose, 53% glucose, and 5% glucose polymers), the type typically used in beverages.

Metabolism of carbohydrates

Much of the current controversy in carbohydrates pertains to how they are metabolized in the body and, subsequently, affect health and weight management. Carbohydrates are ultimately digested into glucose, fructose and galactose by their enzymatic counterparts. Glucose and galactose are actively absorbed in the gut via the adenosine triphosphate (ATP) sodium-potassium pump, whereas fructose can be absorbed via active transport or facilitated diffusion, leaving unabsorbed fructose free to travel down the intestine. Fructose is an intermediary in the digestion of glucose and, when ingested alone, is poorly absorbed by the GI tract and almost completely eliminated by the liver. This metabolism of fructose favors lipogenesis. Several studies have found significant changes in circulating lipids among those consuming diets high in fructose. Once glucose is digested, it is transferred to the blood for transport to the liver for oxidation and glycogen storage.

That said, existing evidence does not support the claim that diets high in any particular nutritive sweeteners have caused an increase in obesity rates or other chronic conditions. Science has determined that human metabolism does not differentiate between sugars found naturally in food versus those added to food. For example, fructose found naturally in fruit is not metabolized differently than fructose in a fruit drink. Fructose is absorbed and metabolized in a similar manner despite the manner of ingestion (Journal of the American Dietetic Association, 2004; 104:255-275). Several other factors affect metabolism, such as: the carbohydrates availability or resistance to their enzymatic counterpart; the availability of the enzyme itself; or other dietary factors, such as fat content, which slows stomach emptying, or viscous fiber, which dilutes enzyme concentration.

Christina Fitzgerald, M.S., R.D., L.D.N., specializes in pre- and postpartum, as well as pediatric, nutrition. She is the owner of the private nutrition practice, Nourished Nutrition and Wellness, based in Chicago, and can be reached at [email protected].