Fructose intolerance
The apple's revenge
After a fast food binge, you decide to adopt a healthier lifestyle. You buy six apples at your local supermarket—the reddest, the biggest, without the slightest blemish, and with their shiny, waxed surface.
You arrive at your house in high spirits, visualizing a life more in touch with nature, with the essence of the primitive human being, full of exercise, healthy habits, and meditation.
You wash one of the six apples and ingest it with a mystical air, rising above a world of social media junkies, addicted to videos of kittens and puppies, poor slaves of big business that dominate minds. Before long, you're in the bathroom with diarrhea, your dream of mystical communion with the prehistoric human being fading.
This story is about food, chemistry, herds of fierce flatulent hunters, and pissed-off humans.
Let's start with the food.
Fructose is a single sugar molecule, unlike milk lactose (which has two sugar molecules joined together, as we saw in the previous post). It is sweeter than other sugars, such as glucose or galactose. It is present in many fruits and foods (below this post, I share with you which foods have a lot of fructose). It is also used in industrial sweeteners such as high fructose corn syrup, a very sweet syrup used in sweetened beverages, desserts, yoghurts, breakfast cereals, cold cuts, soups and dressings. This syrup has a lot of concentrated fructose and glucose.
Fructose from food is absorbed directly into the blood through the cells in our gut, the so-called enterocytes (literally gut cells) that line our lower digestive tract.
Although it is not well understood how fructose is absorbed into the gut cells, two proteins called GLUT2 and GLUT5 transport fructose into these cells. As these are very boring names, we will call GLUT2 the double glutton and GLUT5 the lazy glutton.
Double glutton (GLUT2) can transport fructose from the gut to the inside of the cell, but only if glucose is present. It can transport both. Without glucose in the gut, it does not absorb fructose.
Lazy glutton (GLUT5), however, can specifically absorb intestinal fructose into the cell but gets tired quickly. If there is too much fructose, it cannot absorb it all. It is overwhelming for this limited and lazy transporter. And this happens in a society that uses fructose industrially for sweetening, which we are not used to.
What happens if the double glutton and lazy glutton transporters cannot handle all the fructose we have eaten? It stays in the gut, and this is where chemistry comes in.
If you remember your basic chemistry classes (although you've probably forgotten them after years of interacting with Facebook memes and watching kitten videos), a solution is a homogeneous mixture of a solvent (for example, the mineral water you drank at lunch and for which you were charged 80 euros for being the exclusive Svalbardi Water, harvested from an Iceberg) and the fructose that the glutton transporters couldn't absorb.
Well, a solution (a homogeneous mixture) of water and fructose has a property called osmotic pressure, common to all solutions. As this blog is not about chemistry but about the digestive system (and I confess that I know very little about osmosis), we will only say that if we have liquid in the intestine with a lot of dissolved fructose, it has a lot of osmotic pressure and this means that the water in the intestine tends not to be absorbed, to stay inside the intestine, attracted by the osmotic pressure of the fructose.
In other words, fructose inside the intestine means the liquid is absorbed poorly. This can lead to diarrhea.
As that foul liquid travels with fructose not absorbed by overwhelmed gluttonous transporters and liquid that we cannot absorb due to the influence of excess fructose, the packs of fierce, flatulent, voracious hunters come into play: the microbiota.
We are talking about an unimaginable number of microbes that our body tolerates, in order to take advantage of foods that we cannot digest, such as plant fibre and resistant starch (carbohydrates that our body cannot digest directly).
Microbiota are fabulously and unimaginably abundant in the large intestine (colon), the final part of the digestive tract, but relatively scarce in the small intestine. When there is abundant colonization of microorganisms in the small intestine, we speak of Small Intestine Bacterial Overgrowth (SIBO). If SIBO is present, the symptoms due to the presence of unabsorbed fructose will be more significant.
Bacteria use fructose for energy through fermentation, breaking it down into simpler compounds such as fatty acids, gases (such as hydrogen, methane, and carbon dioxide, hence flatulent hunters), and other organic compounds. This excess gas can cause discomfort to humans, especially if they have irritable bowel syndrome, a very common condition.
In summary, fructose may not be fully absorbed in some people (due to factors that are not fully known). Unabsorbed fructose predisposes to diarrhea due to its osmotic pressure, and its subsequent fermentation by intestinal bacteria, especially if there is bacterial colonization of the small intestine, leads to excess gas, which, in particularly sensitive people, can cause unpleasant symptoms.
It is the revenge of the apple, which grew peacefully on its branch, and was seized to be sold in your neighborhood supermarket. All in all, citizens who do not fully absorb fructose may experience bloating, flatulence, diarrhea, and abdominal discomfort if they consume fructose-rich foods.
What are they?
Fructose-rich and fructose-poor foods
I have obtained the following list of foods from the article ‘Dietary fructose intolerance, fructan intolerance and FODMAPs’, by Amy Fedewa and Satish S. C. Rao, available at this link.
Fruits: all fruits are high in fructose, especially juices, dried fruits (such as prunes, sultanas or dates) and canned fruits in juice or syrup, EXCEPT the following low fructose fruits: avocado, blueberries, lime, melon, pineapple, strawberries, tangerines, and bananas.
Vegetables: Artichoke, asparagus, broccoli, leeks, mushrooms, onions, peas, red peppers, shallots, tomato paste, tomato products (canned tomatoes, ketchup) are high in fructose. Low fructose content: bamboo shoots, beetroot, carrots, celery, chives, green pepper, kale, parsnips, radish, rhubarb, spinach, sweet potato, turnip greens, white potato, Brussels sprouts, cabbage, cauliflower, lettuce (although these last four produce a lot of gas by other mechanisms).
Cereals: foods with wheat as the main ingredient (wheat bread, pasta, couscous), cereals with added nuts, cereals with added high fructose corn syrup are high in fructose. Low fructose: buckwheat flour, corn chips, corn flour, corn tortillas, gluten-free breads, biscuits and pasta without added high fructose corn syrup, corn grits, oat flour, popcorn without added high fructose corn syrup, quinoa, rice, rye breads without added high fructose corn syrup, and all other flours made from permitted cereals.
Meats and proteins: marinated or processed meats containing restricted ingredients. Unprocessed meats of any kind (beef, chicken, fish, eggs, etc.) are low fructose. Also, legumes, tofu (note that they tend to form more gas and may need to be avoided), and nut butter that do not contain high fructose corn syrup.
Dairy: Any product with high fructose corn syrup. Be especially careful with yogurts and flavored milk. Low fructose milk, cheese, unflavoured yogurt, soy milk, rice milk, and almond milk with no added high fructose corn syrup.
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Bibliography:
Amy Fedewa and Satish S. C. Rao. Dietary fructose intolerance, fructan intolerance and FODMAPs. Curr Gastroenterol Rep. Author manuscript; available in PMC 2015 Jan 1.
Miles Benardout , Adam Le Gresley , Amr ElShaer, and Stephen P. Wren. Fructose malabsorption: causes, diagnosis and treatment. Published online by Cambridge University Press: 05 April 2021.
Wikipedia: High-fructose corn syrup
I have not used generative artificial intelligence software (AI) to write this post. Only the picture has been generated with generative AI.