Questions: Carbohydrate Structure, Classification, and Function

Question 1

A food has a glycemic index of 70 and a serving contains 30g of carbohydrate. A second food has a glycemic index of 90 but a serving contains only 5g of carbohydrate. Which food will cause a larger blood glucose response?

• A. The first food, because glycemic index alone determines blood glucose response
• B. The second food, because it has a higher glycemic index
• C. The first food, because glycemic load (GI × carb grams / 100) is higher for the first food
• D. They will cause identical responses because portion size does not matter

Answer: (C)

Glycemic load = (GI × grams of carbohydrate) / 100. Food 1: (70 × 30) / 100 = 21. Food 2: (90 × 5) / 100 = 4.5. Food 1 has a much higher glycemic load, meaning it will raise blood glucose more despite its lower GI. This illustrates why glycemic index alone is insufficient — portion size matters.

Question 2

Fructose from fruit juice is metabolically equivalent to fructose from added sugar, so consuming large amounts of fruit juice is just as likely to contribute to metabolic dysfunction as consuming the same amount of soda.

• T. True
• F. False

Answer: True

The body metabolizes fructose by its chemical structure, not its source. Excess fructose from any source — honey, fruit juice, or high-fructose corn syrup — is processed primarily in the liver and can contribute to de novo lipogenesis and metabolic dysfunction when consumed in large amounts. Whole fruit is different because fiber slows absorption, but fruit juice removes that buffer.

Question 3

Why does dietary fiber reach the colon largely intact while starch is absorbed in the small intestine?

Model answer: Fiber consists of polysaccharides with glycosidic bonds (typically beta-1,4 linkages) that human digestive enzymes cannot hydrolyze, so it passes undigested to the colon. Starch has alpha-1,4 and alpha-1,6 linkages that amylase and other enzymes readily break down into glucose, which is absorbed in the small intestine.

The key is bond geometry. Human enzymes evolved to cleave alpha glycosidic bonds (as in starch and glycogen). Cellulose and many other fibers use beta bonds that our enzymes cannot recognize. Gut microbiota in the colon do possess enzymes for beta bonds, which is why fiber gets fermented there — producing short-chain fatty acids rather than glucose.