Do an internet search for the terms "fructose malabsorption" and "depression" and you will find a lot of buzz over the possibility that the two are linked. Google returned over 6,800 hits on this combination. The idea seems to be taking on a life of its own, and is treated as fact on a number of web sites and blogs. I wondered if there was any of that pesky old evidence on the subject.
I've spent several evenings poking around the internet, reading abstracts and peer-reviewed research reports, most of them published in Europe. In fact there are a number of interesting published research reports that suggest there may be a link worthy of further study. It is far from proven, however. The handful of studies that have been done, most by a particular workgroup at the University of Innsbruck, Austria, included a very small number of study subjects. The researchers have tried to discover whether there was an association between fructose or lactose malabsorption and depression. They did find such an association, but only in females. But the studies simply weren't designed to prove that fructose malabsorption causes depression. It is intriguing, though, isn't it?
The researchers did some more small scale studies to explore possible mechanisms for the fructose or lactose malabsorption-depression link. They have discovered that there may be an association between fructose malabsorption and lower levels of tryptophan, zinc and folate in the blood. It is the job of scientists to speculate about how these observations could all fit together, and speculate they do. Could these nutrients be poorly absorbed because the fructose malabsorption causes diarrhea and reduces contact time between the contents of the GI tract and the absorptive surfaces of the small intestine? Could unabsorbed sugars be interfering with the absorption of tryptophan? Could fructose malabsorption be encouraging small intestinal bacterial overgrowth (SIBO)? Could the lower serum levels of tryptophan, zince and folate cause depression?
There are far more questions than answers, here. Scientists are choosing their words with care, using the terms "may" and "suggest" and "associated with," and they point out that further studies are needed.
What is the take-away message here for people with fructose and/or lactose malabsorption? If you're a woman, removing excess amounts of these sugars from your diet might make you feel better mentally as well as physically! That's all, folks.
The report that started it all? Fructose malabsorption is associated with early signs of mental depression, Ledochowski M, Sperner-Unterweger B, Widner B, Fuchs D. Eur J Med Res. 1998 Jun 17;3(6):295-8.