It is all down to the type of carbohydrates you consume. There are 2 main types of carbohydrate and they are simple carbs (sugars) and complex carbs (polysaccharides).
Because of their structures, sugars are metabolised more quickly in the body compared to complex carbohydrates. Therefore, sugars get turned into glucose more quickly for energy use, hence the term "sugar rush" from the build up of energy in the body. Any unused glucose will end up as fat and stored in the body's fat reserves.
Sugars are found in a variety of natural food sources including fruit, vegetables and milk, and give food a sweet taste.
Sugars can be categorised as single sugars (monosaccharides), which include glucose, fructose and galactose, or double sugars (disaccharides), which include sucrose (table sugar), lactose and maltose.
What makes complex carbs different is that they are starches formed by longer saccharide chains, which means they take longer to break down.
Chemically, they usually comprise of three or more linked sugars.
When dietitians and nutritionists advise having complex carbohydrates, however, they are usually referring to whole grain foods and starchy vegetables which are more slowly absorbed than refined carbohydrate.
Whole grain starches include the wheat grain and kernel which provide the majority of fibre and nutrients to be found in starchy foods.
When it comes to picking starchy foods, such as rice, bread and any other products made from flour, it’s best to opt for whole grain versions of these products.
We should not rely too much on carbohydrate though. Whilst whole grain foods impact upon blood glucose levels more slowly than other forms of carbohydrate, higher levels of carbohydrate can still raise blood sugar levels substantially. So whilst aiming for complex carbohydrates rather than simple carbs, you still need to keep within the recommended daily calorie intake and ratios of carbs to other nutrients such as vitamins and fibre.