Fructose and sucrose digestion

Fructose exists in foods either as a monosaccharide (free fructose) or as a unit of a disaccharide (sucrose).  After we have eaten a meal containing these sugars, the food passes through the stomach where the food is broken down to form ‘acid chyme’ and then passed on to the intestines.  For more details see Metabolic process.

Digestion in the Intestines

Nutrients are extracted in the intestines:

• Sucrose - As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed.
• Free fructose -  is absorbed directly by the intestine. The mechanism of fructose absorption in the small intestine is not completely understood

In effect, whether the intake is as sugar [sucrose] or fructose, the end result once the fructose enters the intestine is fructose anyway.  When fructose is ingested as part of sucrose, absorption capacity is much higher because fructose exists in a 1:1 ratio with glucose.

Fructose from sucrose or free fructose may not be completely absorbed in the small intestine. When fructose is not absorbed in the small intestine, it is transported into the large intestine, where it is fermented by the colonic flora. Too much fructose can completely upset the balance of the intestinal flora and since fungi thrive on sugars, overdoing on fructose can result in candidiosis and IBS.

One way of knowing whether you are overdosing on sugars is via the ‘hydrogen breath test’.  Hydrogen is produced during the fermentation process and dissolves into the blood of the portal vein. This hydrogen is transported to the lungs, where it is exchanged across the lungs and can be measured.

Another indicator is the presence of bloating and wind.  The colonic flora also produces carbon dioxide, short-chain fatty acids, organic acids, and trace gases in the presence of un-absorbed fructose. The presence of gases and organic acids in the large intestine causes gastrointestinal symptoms such as bloating, diarrhoea, flatulence, and gastro-intestinal pain. Exercise immediately after consumption can exacerbate these symptoms by decreasing transit time in the small intestine, resulting in a greater amount of fructose emptied into the large intestine.

After absorption, fructose enters the hepatic portal vein and is directed toward the liver.

Fructose and the liver

When glucose enters the liver it can enter the blood stream directly for use by cells as fuel without requiring further processing.  A process called glycolysis, however, is used to regulate glucose supply so that too much glucose does not enter the blood stream.  Excess glucose might be stored as glycagon in the liver for use later. 

Fructose, on the other hand, has to be further processed to make it usable by cells, but after the process is complete, the resulting outputs are identical to those obtained in glycolysis and can enter the gluconeogenic pathway for glucose or glycogen synthesis.

The process whereby fructose is converted is called fructolysis.  Fructose [and galactose another sugar] are phosphorylated in the liver by fructokinase, triokinase and galactokinase – three enzymes.  Phosphorylation is the addition of a phosphate (PO43−) group to a protein or other organic molecule.  So for this to happen you need to have an adequate intake of phosphorus as an essential mineral.  Any drugs, toxins or pathogens that destroy or attack these enzymes is also going to produce problems.

Excessive fructose consumption may contribute to the development of non-alcoholic fatty liver disease.