The simple answer would be that FM is when the intestines are unable to properly digest fructose, leaving a person with various health problems.
Our intestines are only able to digest so much fructose per sitting. When one has fructose malabsorption that means that they are not able to digest as much as the average person can. If a person is not able to digest 25 grams of fructose per sitting they are considered to have FM. 25 grams of fructose is a lot of sugar in a sitting. There are varying degrees of FM sensitivity, some can only absorb about 1 gram of sugar per serving and others can absorb closer to 20-25g.
Fructose is a simple sugar called a monosaccharide (from the greek meaning single sugar) found in many plants and often called fruit sugar. The body uses monosaccharide for energy. Pure, dry fructose is a very sweet, white, odorless, crystalline solid and is the most water-soluble of all the sugars. Fructose occurs naturally in fruits, some vegetables, honey, sugar cane, sugar beets, wheat, and some other plants. Fructose is found commercially in sucrose (table/white sugar) and high fructose corn syrup (knows as fructose/glucose in Canada), as well as crystalline fructose.
A person with FM has an inability of the small intestines to break down fructose into smaller molecular fragments for digestion. The fructose ends up progressing down to the large intestines where bacteria breaks it down into methane gas, hydrogen gas, carbon dioxide and fatty acids. This results in bloating, diarrhea, flatulence, abdominal pain, nausea and lethargy. Symptoms generally appear two to 24 hours after ingesting fructose-containing food. Because this usually causes diarrhea this also results in important vitamins and minerals being lost and over time a person can develop nutritional deficiencies, commonly calcium and iron, but long term can lead to anemia and osteoprosis.
It should be mentioned that Fructose Malabsorption was previously called Dietary Fructose Intolerance and is still sometimes referred to as this. Fructose Malabsorption is different then Heriditary Fructose Intolerance (HFI). In HFI a person lacks the enzyme to break down fructose, thus it can cause liver and kidney problems, as well as be fatal. Fructose Malabsoprtion (FM) although not always pleasant it is not life-threatening and is easily manage by diet. In HFI you must adhere to a very strict fructose-free diet whereas with FM you can learn to adjust your diet according to what you can tolerate.
Links to further reading: