Dietary fructose intolerance is the inability to efficiently absorb fructose, a simple sugar the body uses for energy. Thanks in part to research done at The University of Iowa, the condition is being increasingly diagnosed and treated. Satish Rao, MD, gastroenterologist at University of Iowa Hospitals and Clinics and primary investigator of the fructose study talks about dietary fructose intolerance:

Why is this study with fructose intolerance so important?

Things have changed over the last 10 to 20 years on how we take foods, what type of foods we eat, etc. For example, the amount of fructose we consumed in the 1960s was less than a ton a year. Today as a nation, we consume over 8.8 million tons of fructose. So as a nation, we are now consuming more fructose than we ever did. Now unlike other sugars--let's take for example sucrose, which is the common table sugar or lactose, the milk sugar--we have enzymes called sucrase or lactase that allows these sugars to be broken down and absorbed easily in the small intestine. Unlike these sugars and these enzymes, when it comes to fructose, we do not have an enzyme for digesting fructose. If we exceed the absorptive capacity that we have, then we will not be able to digest this sugar and this can lead to symptoms we can talk about a little later. So dietary fructose intolerance refers to the entity where an individual subject has inability to digest and absorb the fructose that they're consuming in their routine diet.

What foods naturally contain fructose?

Yes, there are several fruits, particularly fruit such as apples, peaches, prunes, etc., that have fructose in them. Likewise, honey has fructose in it; a few other things, chocolate for example has some fructose in it. So most fruits and vegetables and in that category have variable amounts of fructose naturally in it. Many times these natural foods also have glucose in it, and this glucose actually facilitates the absorption of fructose. What we now consume is a diet or artificial food products, that have highly concentrated fructose.

This is present in high fructose corn syrup. High fructose corn syrup has an excess amount of fructose and is quite sweet and very tasty of course/ But we consue a large amount of high fructose corn syrup in various forms in our diet in sodas, beverages, food products, and canned products, etc. Our natural ability to digest this amount of fructose is limited and if we consume in these large quantities, we can have intolerance and we can have symptoms from it.

Who will most likely develop intolerance to fructose ?

Anybody can develop intolerance. As I said, we have only limited capacity to digest it. Studies we have done over the last decade have given us some good ideas as to what the normal absorptive capacity is in healthy humans. It appears we can absorb, at any one time, up to 25 grams of fructose. Now what is 25 grams? A glass of orange juice has up to 14 grams of fructose, a can of Coke sometimes has 15 to 16 grams of fructose, so you have a rough idea of how much 25 grams is. If you exceed that amount at any one time, individuals with limited fructose absorptive capacity can not absorb this substance and can get symptoms.

What are the symptoms of fructose intolerance?

The usual symptoms are bloating, gas, cramps, diarrhea, constipation--indigestion and sometimes excessive belching. These are common symptoms that a number of patients with this condition describe. These are also symptoms that many patients with other gastrointestinal conditions also describe. It is important that we understand the symptoms and do a diagnostic test, such as the fructose breath test, to identify these patients and then embark on appropriate treatment strategies.

How were patients identified for your study and what things did those participants do?

We performed a systematic study in about 20 healthy individuals here in Iowa for the first time in the world. We gave them four different doses of fructose--15 grams, 25 grams, and two different solutions of 50 grams of fructose--to identify what is the critical threshold beyond which a normal, healthy individual cannot absorb fructose.

The test comprised of giving them a drink of the sugar solution, and then every half hour collected a sample of breath to find out how much of breath hydrogen or methane is present. Normally what happens is, if an individual can completely digest this sugar, there is no change in breath hydrogen or methane. If an individual cannot digest the sugar, the sugar is dumped into the colon, where the sugar gets fermented by colonic bacteria and the process of fermentation will produce gas, and some of this gas is absorbed into the blood stream, and then exhaled in breath. That is how we measure the hydrogen values in breath.

For individuals who digest this sugar, there is no change in breath hydrogen level. But for individuals who fail to absorb fructose, there is a rise in breath hydrogen. Using this test, we found that the critical threshold in normal, healthy individuals appears to be 25 grams of fructose. If they take more than 25 grams of fructose, they can develop dietary fructose intolerance. Normal people can tolerate 25 grams, but patients with dietary fructose intolerance cannot tolerate even 25 grams. They have less or even limited capacity for handling fructose, and these are the individuals with dietary fructose intolerance.

Is fructose intolerance something that can be self-diagnosed or controlled simply by diet?

I think ideally you want to do a fructose breath test in this individual. It's a simple test. It takes about three hours, it's non-invasive, it is painless, and is easily administered.

I think because change in diet is going to be life-long, it's not a one-time deal that you have this. Implications are life-long, so you have to be watchful, mindful what you eat, what you cook, how you shop, and so on and so forth. If you're making a life-altering change, it is better to have a proper diagnosis and only then, embark on the treatment program.

What I forgot to mention is that in our other study, we took a group of patients with unexplained IBS (or irritable bowel syndrome). We administered the fructose breath test and found that approximately 30 percent of patients presenting with IBS-like symptoms had dietary fructose intolerance. There is a large group of patients in the community who have this problem who are not recognized or treated appropriately. It is important, therefore, that we do this test to identify these patients so that they can become symptom-free and live a normal life.

What are the next steps in your research?

Our next step is to see if there is something we can do to help these people they so they can take an enzyme-like preparation to aid absorption of fructose. This would mean they can eat the things they like and lead a more normal life rather than having to shop for different products and change their dietary habits. Currently we are doing research with an enzyme-like preparation we believe, and our pilot studies showed, that actually facilitates fructose absorption and doesn't cause the symptoms in patients that already had dietary fructose intolerance. This is a double blind study where we compare individuals with this disorder. We give them the enzyme or a placebo to see whether we can change their absorptive capacity. If so, then we'll have a new treatment for treating patients with this disorder.

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Satish Rao, MD