Lets look at what exactly sugar is from a chemical and scientific perspective. First of all every food we eat can be placed into three nutrient categories. These three general nutrient categories I'll refer to as Macro Nutrients. They are 1. Carbohydrates, 2. Proteins, and 3. Fats. They are all essential to our health and must be consumed in balance and proportion to one another to maintain our health. One of the most important reasons for eating food is to create energy. All three of the Macro Nutrients can provide us with a source of energy. Carbohydrates provide a fast quick source of energy. Proteins although mainly used as building blocks for the body can provide a more consistent inflow of fuel. Fats are a slow steady consistent source of fuel for the body.
Sugar falls into the carbohydrates category and so is a fast quick source of fuel for the body. Eating large amounts of carbohydrates will make you hungry more often and leave you feeling dissatisfied after a meal because of the speed at which the body consumes this rapid burning fuel. When you consume grains, beans, rice, legumes, bread, pasta, fruits, or vegetables you are consuming different forms of carbohydrates which are converted into sugars at different rates. The body breaks these foods down into their most simple forms so that we can use them for fuel which are sugars. Some foods such as fruit contain sugar in simple forms that the body can absorb into the bloodstream and turn into fuel very quickly. Other carbohydrates such as beans, winter squash and whole grains contain less simple forms of sugar called complex carbohydrates and lots of indigestible fiber which slows down the absorption of these sugars into the bloodstream.
This being the case logic would dictate that eating a piece of fruit would raise your blood sugar higher than if you ate a bowl of wholesome brown rice. However scientists have discovered that this is not always true. In recent years a system has been developed to measure the speed at which the body converts food into simple sugars and how fast it raises blood sugar levels called the glycemic index. More recently scientists have created what's called the "glycemic load" for different foods, which estimates how high blood sugar levels are raised after ingesting what would be considered a normal portion of that food. Foods with a higher glycemic load raise blood sugar levels and cause a large release of insulin and can have damaging effects on your health; foods low on the index less so. You can go to any number of websites (here's one http://www.glycemicindex.com/index.php) to look up the glycemic load on a food to get an idea of how it is going to effect your health. Generally starchy carbohydrate foods such as bread, crackers, pasta, rice, cereal, chips have a much higher glycemic load than whole fruit such as berries for example.
Food for thought- It doesn't need to taste sweet to raise your blood sugar to harmful levels.