Primary role of dietary Carbohydrates: provide energy

• 4 cal/g

Classification

• Simple sugars

  • monosaccharides

  • disaccharides

• Complex sugars

  • polysaccharides

• Fibers

Monosaccharides

Principal monosaccharides found in food:

• glucose: fruits, sweet corn, corn syrup, honey

• fructose: found with free glucose in honey and fruits (e.g. apples)

High-fructose corn syrup (HFCS)

• corn syrup that has undergone enzymatic processing to convert their glucose into fructose and have been mixed with pure corn syrup (100% glucose) to produce a desired sweetness

• HFCS 55 (55% fructose and 42% glucose): substitute for sucrose on beverages (e.g. soft drinks)

• HFCS 42 used in processed foods

• composition of HFCS and surcose are similar

  • major difference: HFCS is ingested as a mixture of monosaccharides

  • no significant difference in either postprandial glucose or insulin responses

Disaccharides

Most abundant:

• sucrose: table sugar

  • glucose + fructose

• lactose: principal sugar found in milk

  • glucose + galactose

• maltose: product of enzymatic digestion of polysaccharides, found in beer and malt liquors

  • glucose + glucose

Polysaccharides

• complex carbohydrates

• most often polymers of glucose

• do not have a sweet taste

• Sources: starch, wheat, potatoes, dried peas, beans (legumes) and vegetables

Fiber

Dietary fiber: non-digestible, non-starch carbohydrates and lignin present in plants

• soluble fiber is the edible part of plants

  • resistant to digestion and absorption in the human small intestine

• insoluble fiber passes through the digestive track largely unchanged

• provides little energy

• Recommended intake (AI) for total fiber:

  • 25 g/day for women

  • 38 g/day for men

Benefits

• adds bulk to diet

• increasing bowel mobility during exposure of intestines to carcinogens

• promoting bowel movements (laxation)

• softens stool: decrease risk for constipation, hemorrhoids, and diverticulosis

• soluble fiber:

  • delays gastric emptying

    • which can result in a sensation of fullness (satiety)

    • reduce spikes in blood glucose following a meal

  • lower plasma LDL-C levels by increasing fecal bile acid excretion and interfering with bile acid reabsorption

Dietary Carbohydrate and Blood Glucose

• carbohydrates differ in their glycemic response (GR)

  • carbohydrate-containing foods produce either

    • rapid rise followed by a steep fall in blood glucose

    • gradual rise followed by a slow decline in blood glucose

• Glycemic index (GI): the area under the blood glucose curve seen after ingestion of meal with carbohydrate-rich food, compared with the area under the blood glucose curve observed after a meal consisting of the same amount of carbohydrate in the form of glucose or white bread.

• low GI: <50

• high GI: ≥70

• low-GI diet improves glycemic control in diabetic individuals

• foods with low GI tend to create a sense of satiety over a long period of time

  • helpful in limiting caloric intake

Carbohydrate Requirements

• not essential nutrients

• absence of dietary carbohydrate leads to ketogenesis

• RD for for carbohydrate: 130 g/day for adults and children

• adults should consume 45% - 65% of their total calories from carbohydrates

• added sugars should represent no more than 10% of total energy intake