I am supposed to know the answer to this, since I got my PhD doing carbohydrate chemistry. Unfortunately, someone has to actually do the experiment to see if cooking at high temp or extended time produces significant amounts of free fructose. It may produce some short chains of fructose, but those are just more soluble fiber, i.e. carbs that move to the colon and are metabolized into short chain fatty acids by bacteria.

Response to the inulin and inulin fragments will differ from person to person, and as is the case with all food intolerances (and most diagnosed "food allergies"), the ability to handle it depends on the condition of your gut flora. One hundred + bacterial species are required in gut flora for health on a varied diet and long term avoidance of any food molecules that require bacterial enzymes (all foods except protein, fats, a few sugars, lactose, sucrose and starch), will lead to temporary intolerance until new bacteria can be acquired clinging to food. For example, lactose intolerance (even with a genetic deficiency) can usually be cured by merely eating live yogurt (contains lactose metabolizing bacteria) for a few weeks.

I suspect that intense cooking of inulin may free tiny amounts of fructose and make it sweet, but that amount of fructose is irrelevant. So enjoy your onions and leeks. They will also make your gut flora happy.

Have you ever cooked anything for 12-36 hours? If not then I would not worry.

This article is interesting on the subject of inulin consumption: High dietary intake of prebiotic inulin-type fructans from prehistoric Chihuahuan Desert.

The Native American groups living in the Chihuahuan Desert relied heavily on dessert plants full of inulin. The long cooking times at low temperatures, upto 40 hours, only seem to partially degrade the inulin into fructose.

They also estimated these people had an intake of 108-135 grams of inulin per day. These desert groups apparently lived on this diet for thousands of years.

Do not worry about the inulin or fructose in your onions or in any vegetable, however you cook them. Don't let a fear of fructose put you off eating real food.

I have wondered about this issue and did my online research. I also get valuable input from Dr Ayers at his site. I have read abstract of several articles that states that dry heating changes inluin to a different kind of fructose. One of this article claims that these heated inulin are better prebiotic than regular inulin. I think wet cooking effect will be different since inulin is water soluble. "Effect of dry heated inulin on intestinal bacteria" by A Hohm , B. Klessen T Henle "Abstract Degradation of a sample of high-molecular (degree of polymerisation, DP, between 13 and 30) and low-molecular (DP below 12) inulin from Jerusalem artichoke during dry heating for 30 min at 165 and 195 °C was analysed using high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and thin layer chromatography. Dry heating at 195 °C induced complete degradation of the fructan chains and the concomitant formation of low-molecular degradation products, most likely di-D-fructose dianhydrides. In vitro fermentation studies using mixed faecal samples of eight human volunteers for 24 h at 37 °C showed significant stimulation of the growth of bifidobacteria and Enterobacteriaceae and a significant decrease of possibly pathogenic bacteria of the Clostridium histolyticum and C. lituseburense group by inulin samples heated at 195 °C compared to unheated samples and samples heated at 165 °C. This preliminary data may point to the hypothesis that heat-treated inulin or its degradation products may cause improvements of the gut microflora superior to native inulin. "

Rashed Hasan

Lustig is right that excess fructose isn't good and his 50 gram / day limit seems reasonable. I might shoot for lower actually... Given that, I think you might have answered your own question; it seems that slow cooking inulin is a bad idea...

Whether eating lots of inultin is bad, I don't know. I know it is at least partially conveted into short chain fatty acids (good) and gas (mabe not so good) and increases bifido bacteria concetrations (probably good). Unless the gas causes you discomfort (bloating, pain, or GERD/reflux), the only downside I see is if a significant amount of the fructose gets absorbed (and from what I can tell, that's not the case).

Interesting from a scientific viewpoint, but I don't think it's worth worrying about. Most of us do not cook large amounts of inulin for a long time often. You would have to do this a LOT to really make it an issue. A few onions in the broth once in a while are not going to amount to huge tons of fructose per day. If you are already eating paleo, then your fructose input via other means would mainly come from fruit (assuming you eat them) which would be your main source and the easiest way to cut back on fructose. If you are not eating fruit, then your fructose intake would be quite low.

Since nearly all southwestern desert dwelling natives have what is called a Thrifty Genotype allowing them to thrive at very low calorie intakes, they are not comparable to Anglo American individuals. So their inulin intake is problematic. In Traditional Foods of the Indigenous People of Canada, a great reference, many foods are noted having quantitys of inulin that are said to be converted to fructose by slow cooking

Fructans (polymers of fructose) may have a minor role in the obesity epidemic. It has a much greater role in obesity in horses. While mammals cannot break down fructans to fructose, bacteria in the gut are able to , as well as processes such as slow cooking. For a more detailed review of the story, you might want to read a book I wrote, The Fat Switch (available at mercola.com or on kindle via Amazon) that discusses this whole story.

Richard J Johnson