Carbohydrates are the main source of glucose, or blood sugar. All cells in the body can use glucose as a fuel. Muscle and liver cells store glucose that is not used as an immediate fuel as glycogen for future use. Excess glucose can also convert into fat. When most of the diet consists of carbohydrates, glucose may accumulate in the blood stream, which can lead to insulin resistance and diabetes.
Carbohydrates in the Digestive System
Carbohydrates break down much faster in the digestive system than fat and protein. When you consume mostly carbohydrates, you quickly become hungry because the foods don't stay in your system for long. In the digestive system, carbohydrates convert to glucose, which absorbs directly into the bloodstream. Once inside the bloodstream, cells use glucose as a fuel. Excess glucose converts to glycogen or fat and is stored in muscle, liver and fatty tissue.
Fast and Slow Carbohydrates
Though carbohydrates break down quickly in the digestive system, different carbohydrates have different glycemic indices. Carbohydrates with a high glycemic index break down almost immediately. These include carbs that can be found in sugar, candy, white bread, white rice and white pasta. Carbohydrates with a low glycemic index break down at a slower pace. This class of carbohydrates can be found in whole-grain bread and pasta, brown rice, starchy vegetables and legumes
Carbohydrate and the Brain
While most cells in the human body can use fat and amino acids, the components of protein, as a fuel, the brain can only use glucose or a byproduct from fat metabolism called ketone bodies as a source of energy. When carbohydrates are restricted because of starvation or dieting, the brain switches from glucose metabolism to ketone body metabolism. Because ketone bodies are a more compact energy source than glucose, their metabolism requires more cell engines, or mitochondria. The increase in the number of mitochondria per neuron has a stabilizing effect on the brain and can offset seizures.
Excess Carbohydrate Consumption
When a large part of your diet consists of carbohydrates, particularly carbohydrates with a high-glycemic index, glucose blood levels remain at an elevated level. The pancreas produces insulin in response to high levels of glucose. A constant demand for more insulin can lead to a resistance to insulin. This is a condition in which insulin cannot properly transfer its signal to the cells, so the cells don't get enough energy. Despite the excess glucose in the blood, the brain interprets this condition as a case of starvation. This, in turn, causes you to feel hungry and eat more carbohydrates, which can lead to obesity and type 2 diabetes.
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