Article # 436. How Do Carbohydrates Convert to Fat?

The energy nutrients are carbohydrates, fat and protein. The body breaks down carbohydrates to glucose, also called blood sugar, to burn for energy. The body prefers glucose as its main energy source. The body can also break down fat, in a process called beta-oxidation, and burn the breakdown products for energy. Proteins are generally spared to build all the structures and chemicals our body makes from protein, such as muscle tissue, enzymes, some hormones and chemicals in the immune system. When immediate energy needs are met the body will store excess calories, regardless of the source.

Simple Carbohydrates

Carbohydrates are classified as simple or complex. Simple carbohydrates are the monosaccharides and disaccharides. The main dietary disaccharides are sucrose, lactose and maltose. Each of these sugars consists of two smaller units, the monosaccharides, hooked together. Sucrose, or table sugar, is made of glucose and fructose. Lactose, or milk sugar, is made of glucose and galactose. Maltose, found in honey, cereals, legumes and other grain products, is two glucose units hooked together. Added sugars are often found in processed foods. The body breaks the disaccharides up into the single sugar units, which get absorbed into the bloodstream. Fructose and galactose can feed into the same metabolic cycle that break down glucose.

Complex Carbohydrates

Complex carbohydrates consist of many glucose units hooked together. They are the starch found in grain products like bread, cereal or rice and in starchy vegetables like potatoes or sweet potatoes. Enzymes in the small intestine break up the longer chains into single glucose units, which get absorbed into the bloodstream. Fiber is also considered a complex carbohydrate, but those molecules do not get broken down the same way so they do not raise blood sugar.

Storage as Glycogen

Glucose has three fates in the body: it gets burned for energy, stored as glycogen or stored as fat. Glycogen, a large structure made of glucose units, is found primarily in the liver and muscle tissue. When blood glucose gets low, the body can clip off glucose units from the glycogen in the liver to send it into the bloodstream to get burned for energy. The body is only capable of making a finite amount of glycogen, which can increase depending on a person's level of fitness.

Storage as Fat

After immediate energy needs are met and the body has stored all the glycogen it can, the excess glucose gets partially broken down into smaller units. The liver hooks these smaller breakdown products back together to form a different structure, fatty acids. The fatty acids get sent out into the bloodstream and dropped off at the fat tissue stores, also called adipose tissue. The fatty acids then form a larger molecule called triglycerides for storage. The body can store fat in quantities that sometimes seems limitless, unlike glycogen.