Simple sugars, complex sugars, and a fuel of four kilocalories

A Korean nutrition essay sets out to answer two assignment questions at once: how carbohydrates are digested, and what sugars do in the body. It begins with a division. Simple carbohydrates include the monosaccharides — ribose, glucose, fructose, galactose — and the disaccharides: maltose (two glucose units, found in sprouting grain), sucrose (glucose and fructose, in beet, cane, and fruit), and lactose (glucose and galactose). Complex carbohydrates are longer chains of those simple units: starch, and dietary fiber.

Glucose appears in fruit and sweeteners; fructose in fruit, honey, and vegetables; galactose mainly as part of lactose. The first job the essay gives to carbohydrate is fuel. One gram yields about 4 kcal. Brain cells, nerve cells, and red blood cells rely on glucose as their main energy source. The second job is to spare protein. If carbohydrate is short, the body starts to burn protein for energy, and over a long stretch that protein is lost. The third is to keep fat metabolism on a complete path. When carbohydrate is scarce, fat is also used as fuel, but without oxaloacetate made from glucose, fatty acids cannot enter the TCA cycle cleanly. In the liver the oxidation is incomplete, ketone bodies form, and ketosis can follow. The essay also ties carbohydrate to the steadying of blood sugar.

From the mouth to the brush border

Digestion, in this account, starts in the mouth. Salivary amylase begins to split starch into dextrin and maltose. The stomach does not continue that work. In the small intestine, pancreatic amylase from pancreatic juice takes the starch on, to disaccharides. Then the enzymes on the villi of the intestinal wall finish the job. Maltase splits maltose into two glucose units; sucrase splits sucrose into glucose and fructose; lactase splits lactose into glucose and galactose. Those monosaccharides can be absorbed.

What remains as fiber is treated differently in the large intestine. Insoluble fiber, after bacterial fermentation, is largely excreted; soluble fiber can become acids and gas. Sugars that have been absorbed may be used as energy, stored in the liver as glycogen, or, if still in surplus, stored as subcutaneous fat.

What fiber does, and what excess can cost

Insoluble fiber, the essay says, helps prevent constipation. It holds water, adds bulk, softens the stool, and encourages the bowel to move. Soluble fiber slows the emptying of the stomach and intestine, so fullness lasts longer. Pectin in fruit is mentioned as a help in diarrhea. Both kinds of fiber, in this account, slow the release and absorption of glucose, and both are described as having a cholesterol-lowering effect that can support the care of diabetes. The essay also notes recent work on soluble fiber in high-cholesterol blood.

Excess is given a shorter, more cautious list. Streptococcus mutans can ferment carbohydrate into acid that attacks the teeth. Too much fructose and sugar, the essay warns, can raise insulin resistance and the chance of diabetes; surplus that never becomes useful energy can turn into triglyceride, and fructose can form fatty acids quickly. Those are the claims the essay is willing to make from a nutrition assignment. The path it wants a reader to keep is simpler than a full textbook: know the sugars, know the 4 kcal, and know that starch is taken apart in the mouth and finished at the small-intestinal wall. CampusDown holds the longer Korean original.

Sources

1. A student nutrition account of how carbohydrates are classified and digested, and of the work sugars do as fuel, as a spare for protein, and in fat metabolism.

Last updated 15 November 2026