Glucose is the basic source of energy for every cell of our body, but it can become the cause of many diseases if overeaten.
When we consume carbohydrates, they are broken down to their simplest molecules: glucose and fructose. Those two are digested in a totally different way. Glucose serves as an energy source and can be metabolized in many different cells, while fructose goes straight to the liver and is transferred to fat.
Here is a more detailed analysis of what happens to glucose in our body.
The priority metabolism of glucose is a direct glucose oxidation (which is using glucose as fuel without storing). When this need is met, glucose is transformed into glycogen and stored by the liver and skeletal muscles.
The direct oxidation uses 20-25 grams of glucose while we are at our resting state (the majority of the time). In other words, the oxidation daily need is low. By following a typical unhealthy diet that is rich in carbohydrates (bread, pizza, pasta, etc.), we supply more glucose than we can oxidize, so the rest which is not oxidized is transformed into glycogen in the liver and muscles. However, the glycogen storage limit is low—approximately 400 kcal (120g of glucose) in the liver and approximately 1,200 kcal (400g) in the muscles. Those numbers depend on the size of a person, food intake, sport activity, lean muscles, and body fat percentage.
What happens to the rest of the consumed glucose which is not immediately metabolized or turned into glycogen? It is transformed into fatty acid and stored, for example, as “belly tire” fat. This transformation is irreversible. The fat cannot be used as easily as glycogen; it has to be burnt during exercise or used up while dieting.
Eating too many carbohydrates and sugars contribute to the production of body fat. However, there is another important issue related to elevated sugar level: the more sugars you eat, the more insulin is produced, and elevated levels of insulin decrease the ability to burn fat and increase the capacity to store fat.
The green box in the plot below represents a low-carb diet, which demands low levels of insulin and therefore promotes fat breakdown to produce energy, while the red box represents a high-sugar diet which generates high levels of insulin and promotes fat storage and carbohydrate breakdown. Overall, a high-sugar diet promotes fat storage and contributes to many serious diseases in the long term.
Source: Jensen et al, Insulin regulation of lipolysis in nondiabetic and IDDM subjects. Diabetes 38: 1595–1601, 1989.
Jensen et al, Insulin regulation of lipolysis in nondiabetic and IDDM subjects. Diabetes 38: 1595–1601, 1989.
Keith N. Frayn Metabolic Regulation – A Human Perspective, 3rd edition, 2010, Wiley Blackwell