The idea about a low-FODMAP diet is to decrease the intake of Fermentable Oligo-, Di- and Monosaccharides and Polyols and thus reduce the risk of diarrhea due to their laxative effect and the amount of gas produced by normal intestinal microbes.
A low-FODMAP diet was first introduced by researchers from Monash University; the search for low fodmap diet monash gives different pages related to them and you can consider that original information. You can see they insist in describing FODMAP content of foods as "low" and "high" and do not provide any numbers about total amount of FODMAPs; anyway, this may not help everyone, because:
- Someone is sensitive only to fructose from apples, but not to fructans from wheat.
- In different underlying disorders, such as fructose malabsorption, lactose intolerance or Crohn's disease, only certain FODMAPs are problematic.
- The effect of certain FODMAPS, for example, fructose + sorbitol, is additive.
- The effect of fructose is cancelled by glucose, so, a food that contains equal amounts of glucose and fructose (and no other FODMAPs), should theoretically cause no problems.
The approach to a low-FODMAP diet as suggested by Monash University:
- Starting with a strict low-FODMAP diet
- Reintroducing foods one by one, 3 days apart, to see if they cause problems. For this to work, it is good to learn which foods are typically high in particular FODMAPs; for example, Nutrients Review has an article that links to particular FODMAPs. Nutrients-foods relations to figure out which exact FODMAP is problematic by trying foods that are typically high in only one FODMAP:
- Lactose: milk
- Fructose: an apple
- Polyols (isomalt, lactitol, maltitol, mannitol, polyglycitol or hydrogenated starch hydrolisates, sorbitol, xylitol): sugar-free chewing gum or diet soda
- Fructo-oligosaccharides (FOS): Jerusalem artichokes
- Other oligosaccharides: beans
- Fructans: wheat bread
Such approach seems to be good enough to figure out which foods are problematic for a person, without the need to know exact amounts of particular FODMAPs in each food.
And now to answer the question about the amounts:
The amounts of fructose and "net fructose" (which actually causes problems) are presented in Chart 1 on Nutrients Review. This is how you can calculate the amount of net fructose:
Individuals with fructose malabsorption benefit from a low-fructose
diet, which means avoiding meals that contain more than 0.5 g of “net
fructose” (Chart 1), which is the amount of fructose higher than the
total amount of glucose from all glucose sources in a meal: glucose
(100% glucose), starch (100% glucose), maltose (100 % glucose),
sucrose (50% glucose) and lactose (50% glucose) [7,8]. For example, a
meal that contains 20 g fructose, 10 g starch (which is 10 g glucose)
and 10 g sucrose (which is 5 g fructose and 5 g glucose) contains 20 g
+ 5 g fructose – (10 g + 5 g glucose) = 25 – 15 = 10 grams of net fructose.
Examples of foods highest in net fructose are agave, apple, pear, mango, honey and beverages sweetened with high-fructose corn syrup (HFCS). NutritionData has a Nutrients in Food Tool, by which you can search for the amount of fructose in many foods.
Sorbitol inhibits the absorption of fructose, so a food low in fructose but high in sorbitol can be problematic. Sorbitol alone can be problematic for some people. There's a list of foods high in sorbitol (in grams) on Food Intolerance.
Lactose in significant amounts is in milk, ice cream and soft cheeses, but not in hard cheese and butter. Foods high in lactose (NutritionData).
Fructo-oligosaccharides (FOS) + fructans = soluble fiber
FOS and fructans are soluble fiber, all of which can be problematic for some people. Since data about their amounts in food is not readily available, it can help to have a list of foods high in soluble fiber.
To answer specific questions:
- Why are onion/garlic often considered the worst FODMAP foods? They are both spicy and high in fructans, onion also in fructose, so the later is usually considered worse. "Spices" are often problematic for some people from personal reasons, not necessary due to their FODMAP content.
- Which are fruits and vegetables with the lowest amount of FODMAPs?
- Fruits: the ones with the lowest amount of net fructose and sorbitol, for example, grapefruit and cantaloupe.
- Vegetables: the ones the lowest in fructose and FOS/fructans, for example, lettuce, spinach, carrots, okra, sweet corn and potatoes.
- Data about the amount of fructose (and net fructose), lactose and soluble fiber (to estimate the amount of FOS and fructans) are widely available and may help one to find a personal threshold for them.
- I'm not aware of any free comprehensive list of foods with exact amounts of particular FODMAPs, but Monash University has a mobile app, which seems to be a bit more quantitative...Such a list still can't categorize foods into "above" or "below" tolernce threshold, because such threshold usually applies to a person, not a food.