What's up with the fructose:glucose ratio? If I experience dietary fructose intolerance, why can I tolerate fructose better if there is an equal amount of fructose in the meal?
First, let's recall the context here: the goal is to manage gastrointestinal symptoms, not to eliminate fructose, per se, from the diet. That's just not necessary (unless one has hereditary fructose intolerance, another subject entirely). Next, recall that high fructose foods include many fruits, honey, and sweetening ingredients such as high fructose corn syrup, crystalline fructose, and fruit juice concentrates.
The ability of the small intestine to absorb fructose is limited, at best. This was less of a problem years ago, when our intake of fructose was also limited! As recently as 30 years ago, the average U.S. intake of fructose was only 2/3 of what is is today, due to changes in the food supply. Imagine the distant past, when honey would have been a rare sweet treat, requiring feats of courage and strength to obtain. Cane or beet sugar and a wide variety of fruits would not have been available to most people in the world, except on rare occasions. Now, our diets contain lots of fructose, although over half of us can't fully absorb a 50 gram serving of fructose. Any time you are eating fructose faster than your personal ability to absorb it, the unabsorbed fructose will be carried on into the large intestine, where it is fermented by the resident bacteria, causing gas, bloating and altered bowel habits.
It appears there are probably two way fructose is absorbed through the cell walls of the small intestine. The first way is a low-capacity type of diffusion, in which a fructose molecule can pass through the cell wall on its own. The second mechanism is transport system that only works when there is one glucose molecule available for every fructose molecule. If there is no glucose-fructose pair, there is no absorption.
This explains why granulated sugar, which contains an equal amount of glucose and fructose is well tolerated in moderation, even by fructose intolerant individuals. Also, fruits such as berries, certain melons, and citrus fruits, which do not have an excess of fructose compared to glucose, can be tolerated in small servings. The fructose intolerant individual could still experience symptoms if very large portions of these "allowed" foods or beverages are consumed.
If you learn that you have dietary fructose intolerance, but would still like to consume high fructose foods, you might be able to do so if you take a glucose/dextrose tablet along with the food, to improve the fructose:glucose ratio. These inexpensive tablets are sold at your local pharmacy along with diabetic supplies.
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