All carbohydrates, such as fruit, vegetables and baked goods, are made up of simple sugars called monosaccharides. Fructose is a type of monosaccharide found in most carbohydrates, although it’s naturally most abundant in fruit. It's also artificially added to many commercially produced foods and beverages in the form of high fructose corn syrup. Fructose, also called fruit sugar, is metabolized in your liver and has a different effect on your body than other sugars.
Fructose is one of three simple sugars, along with glucose and galactose, which are absorbed directly into your bloodstream during the process of digestion in your small intestine. Fructose exists freely in most plant-based foods, although it’s also a component of a more complex sugar called sucrose, which is used for common table sugar. Sucrose must be broken down into fructose and glucose before the simple sugars are absorbed. Once absorbed through the intestine, fructose is transported to the liver via the hepatic portal vein for metabolism. In contrast, glucose typically passes right through the liver and is delivered directly to all the cells of your body for metabolism and energy production. That’s why glucose is called blood sugar.
Once in the liver, fructose is chemically changed by the enzymes fructokinase, aldolase B and triokinase. The end-product of fructose metabolism in the liver is a substance that can either become glucose, glycogen, fat or pyruvate, which is the end-product of glucose metabolism. In essence, fructose is a fairly versatile type of fuel that your body can use right away in the liver, store for later use as glycogen and fat, or send it on to other tissues as glucose. Glycogen is stored in the liver and muscles and serves as the secondary long-term energy source for your body. The primary long-term energy source is fat stored in adipose tissue. Too much fructose consumption is converted by the liver and quickly stored as fat.
Effects on Insulin Levels
Because fructose metabolism in the liver takes time, it has the lowest glycemic index of all the natural sugars. The glycemic index is a relative measure of how quickly a substance impacts blood glucose levels and insulin release. Fructose has a low value of about 17, whereas processed sucrose or table sugar has a value of 80. A naturally sweet substance such as raw honey, which is about 50 percent fructose, has a glycemic index of about 30. On the other hand, artificial sweeteners made from fructose, such as high fructose corn syrup, dramatically affect blood sugar and cause large secretions of insulin, which is harmful to the pancreas over time. For comparison, the glycemic index of high fructose corn syrup is 87.
Almost everyone likes to have something sweet now and then, but the types of sugar you eat have different impacts on your health. For example, fruits higher in fructose, such as most berries, pears and apples, have less impact on insulin levels than fruits with more glucose or sucrose such as apricots and peaches. Diabetics, in particular, need to watch their blood glucose levels because most can’t secrete adequate amounts of insulin. Sweetening your tea or coffee with raw honey instead of processed sugar, or drinking fresh apple juice instead of soda, are examples of healthier choices.
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
- Contemporary Nutrition: Functional Approach; Gordon M. Wardlaw et al.
- Encyclopedia of Human Nutrition; Benjamin Caballero et al.
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.