Dietary carbohydrates are digested into glucose, fructose and galactose through the actions of a-amylase and brush border digestive enzymes in the upper gastrointestinal tract. The ability to digest carbohydrates is modified by:
- The relative availability of the starch to enzyme action;
- The activity of digestive enzymes;
The presence of other dietary factors such as fat that slow stomach emptying, viscous dietary fibers: pectins, gums. A diet, rich in fruits, vegetables, legumes, nuts and minimally processed grains tends to slow down the pace of glucose absorption.
Once digested, glucose is actively absorbed across the intestinal cell and transferred to the portal blood for transport to the liver. The liver removes about 50% of absorbed glucose for oxidation and storage as glycogen. Galactose and fructose are also taken up by the liver and incorporated into glucose metabolic pathways. Glucose exits the liver and enters the systemic circulation.
In 1981 Jenkins defined a glycemic index to rank different dietary carbohydrates on their ability to raise blood glucose levels as compared with reference food. Studies suggest that the glycemic index of a diet has a predictable effect on blood glucose levels and may have use in the dietary management of diabetes and hyperlipidemia.
The glycemic index is a numerical index that ranks carbohydrates based on their rate of glycemic response . Glycemic Index uses a scale of 0 to 100, with higher values given to foods that cause the most rapid rise in blood sugar. Pure glucose serves as a reference point, and is given a glycemic index (GI) of 100.
The brain uses the major portion of the approximately 200g of glucose required per day. If the blood glucose levels falls below 40 mg/dl, counter-regulatory hormones release macronutrients from the stores. If the blood glucose level rises above 180 mg/dl, glucose is spilled into urine. High intakes of carbohydrate can trigger large releases of insulin.
When you eat foods that cause a large and rapid glycemic response, you may feel an initial elevation in energy and mood as your blood sugar rises, but this is followed by a cycle of increased fat storage, lethargy, and more hunger.
Glycemic index (GI) in common foods:
- Peanuts (GI 14) 4 oz (113g)
- Bean sprouts (GI 25) 1 cup (104g)
- Grapefruit (GI 25) 1/2 large (166g)
- Pizza (GI 30) 2 slices (260g)
- Lowfat yogurt (GI 33) 1 cup (245g)
- Apples (GI 38) 1 medium (138g)
- Spaghetti (GI 42) 1 cup (140g)
- Carrots (GI 47) 1 large (72g)
- Oranges (GI 48) 1 medium (131g)
- Bananas (GI 52) 1 large (136g)
- Potato chips (GI 54) 4 oz (114g)
- Snickers Bar (GI 55) 1 bar (113g)
- Brown rice (GI 55) 1 cup (195g)
- Honey (GI 55) 1 tbsp (21g)
- Oatmeal (GI 58) 1 cup (234g)
- Ice cream (GI 61) 1 cup (72g)
- Macaroni and cheese (GI 64) 1 serving (166g)
- Raisins (GI 64) 1 small box (43g)
- White rice (GI 64) 1 cup (186g)
- Sugar (sucrose) (GI 68) 1 tbsp (12g)
- White bread (GI 70) 1 slice (30g)
- Watermelon (GI 72) 1 cup (154g)
- Popcorn (GI 72) 2 cups (16g)
- Baked potato (GI 85) 1 medium (173g)
- Glucose (GI 100) (50g)