Fructose, along with glucose and galactose, is one of the three most basic types of sugars. The technical term for it is monosaccharide, which means single sugar in Greek. When these monosaccharide units bond together, they form more complex carbohydrates such as sucrose, which is a combination of glucose and fructose, or starch, which is made from hundreds of glucose units chained together. The digestive system must break down the carbohydrates and absorb the sugars before the body can utilize them.
Fructose Intake
The largest natural source of fructose in the human diet is fruit. The USDA recommends eating about 2 cups of fruit a day as an important source of potassium, fiber, vitamin C and antioxidants. Fructose also is present in honey and root vegetables such as sweet potatoes and sugar cane, but also, somewhat less obviously, beets, pepper, corn and carrots. According to P.J. Skerrett, editor of the "Harvard Heart Letter," the average American throughout the 1800s and early 1900s consumed 15 g of fructose per day, mostly from fruits and vegetables. Today Americans average 55 g of fructose per day, much of it from artificial sugars such as high fructose corn syrup. This increase in the amount of fructose consumption also has paralleled increases in obesity, diabetes and non-alcoholic fatty liver disease.
Digestion
The precise nature of fructose digestion depends on whether you consume fructose as a monosaccharide, meaning a free unit of fructose, or as part of sucrose. The small intestines can quickly absorb free fructose without requiring much digestion, but sucrose requires an extra step. The enzyme sucrase — a special protein that facilitates chemical reactions — must break the bonds of sucrose to form free glucose and fructose. This occurs entirely in the upper intestines. Fruits contain both free fructose and sucrose, so both types of digestion will occur.
Absorption
Fructose enters the bloodstream with the assistance of specialized fructose transporters and ends up in the liver, the only organ in the human body that can metabolize fructose. In rat models, the greatest absorption rates occurred when the animals consumed glucose and fructose in equal quantities, but the intake of sucrose with the fructose and glucose appeared to diminish the absorption of fructose.
Health Problems
People with the condition fructose malabsorption have trouble absorbing the sugar due to a deficiency of fructose transporters. This results in an increased concentration of fructose in the small intestine. Symptoms include abdominal pain, gas, bloating and diarrhea. The solution usually is to limit or avoid fruits and other fructose-containing foods. Fructose malabsorption is a ubiquitous problem — it might affect up to 30 percent of the population in developed countries — but you should not confuse it with the more serious genetic disorder of hereditary fructose intolerance, in which the liver lacks an enzyme to properly break down fructose. This rare condition can cause kidney and liver damage.
References
- Harvard Medical School; Is Fructose Bad for You?; P.J. Skerrett; April 2011
- Colorado State University; Absorption of Monosaccharides; R. Bowen; April 2006
- "Gastroenterology"; Intestinal Absorption of Fructose in the Rat; T. Fujisawa, et al.; August 1991
- MayoClinic.com; Fructose Intolerance: Which Foods Should I Avoid?; Katherine Zeratsky; March 2011
