Fructose is a monosaccharide found most abundantly in fruits. Unlike galactose, it cannot be converted into glucose by our body, and it also cannot fuel our brain. While our muscles can utilize fructose for energy in situations where it is the only fuel available, they vastly prefer to use glucose. As a result, the most common fate for fructose is processing by the liver. If the liver is not at full glycogen capacity, fructose will rapidly be converted into glycogen. If the amount of fructose consumed is too high for the liver to process into glycogen (if it overloads the enzymes, for example), or if the liver has enough glycogen, then fructose will be converted to fat for either immediate use or, more likely, storage.
Fructose, as found in whole fruits, is not bad. The absorption is slowed down by the digestive process and fiber, and even a large piece of fruit only has limited fructose in it, not nearly enough to overwhelm your liver. In addition, fructose increases the oxidation rate of glucose, meaning that your body uses more glucose for energy (as opposed to storage) when fructose is present as well. This makes fructose a great sugar to consume alongside glucose when exercising.
Times have changed significantly since fructose, and sugar in general, were found primarily in fruits and sweet vegetables, and the addition of excess sugar in our diet has contributed to many afflictions of lifestyle, from obesity to tooth decay. Though glucose, fructose, and galactose are metabolized differently, the truth is that sugar is sugar, and excess sugar in the form of syrups and crystals are not and will never be “healthy.”
That being said, the different ways we metabolize sugars do make some better choices than others, if still less than healthy choices. Table sugar (sucrose), high fructose corn syrup (HFCS), agave nectar, honey, and brown rice syrup are the five most common caloric sweeteners, and they each have different percentages of glucose and fructose.
Table sugar is formed of sucrose, which comes from sugar canes or sugar beets primarily. It is a disaccharide (a double sugar) formed of glucose and fructose, and is thus 50% glucose, 50% fructose. Our body splits it rapidly in the small intestine into its constituent sugars, which are then absorbed into the hepatic portal vein, which travels from the small intestine to the liver. The glucose is released near instantly from the liver into our bloodstream, the fructose gets metabolized by the liver into either glycogen or fat.
Sucrose is very rapidly absorbed, requiring almost no digestive effort. The only limiting factor is how long it takes the food or drink you’ve consumed to reach your small intestine, as no sugar is absorbed through the stomach. Once in the small intestine, the sugars will be absorbed at a rate of around 30 calories per minute. If your sugar is in the form of a soda, you will have completely absorbed 100% of the sugar in about five minutes. Food forms may take slightly longer to digest, solely due to transit time from stomach to small intestine. These absorption times hold true for the rest of the sugars listed below as well.
High-Fructose Corn Syrup
High-fructose corn syrup is very similar to fructose, but 55% fructose, 45% glucose. Because of its higher fructose content, there is the possibility more will be stored as fat (remember that fructose is normally only used for glycogen storage or fat storage). While our muscles may use the glucose as energy, fructose will never be used instantly as energy. This makes high-fructose corn syrup worse than sucrose, but only by the smallest amount. A better reason to avoid it is its relationship to foods which contain it–it is only found in highly-processed products which likely will contain other ingredients you should avoid, like trans fats, excess omega-6 fatty acids, and artificial flavors and colorings.