Good question and one I have thought about. These are different systems for sure. I wish there was more quantitative information available on the actual amounts of FODMAPs in various foods that would make comparisons easier. Instead, the approach restricts foods with FODMAPs regardless of the actual amount. I would also like more information on the glycemic index of FODMAP-containing foods as well as more careful evaluation of dietary fiber (DF) in FODMAP foods. Most FODMAPs should increase the dietary fiber result (since digestible carbs are removed prior to measuring DF). Yet, many FODMAP-containing foods are reported to have relatively low levels of DF. Another possibility is that (at least some) FODMAPs decrease the glycemic index.
Here is one example there is some data for. A 2.8 ounce (80 gram) serving of Jerusalem artichokes (which I recently added to the FP tables) has one gram of DF and 13 grams net carbs. Following the assumption that FODMAPs (which are not showing up in the DF) might decrease the glycemic index, I did the calculation with a theoretical (and conservative) estimate of GI = 35. The grams of FP come out to 9 grams vs 4 grams for the same serving size of regular artichoke. According to this reference (Muir JG1, Shepherd SJ, Rosella O, Rose R, Barrett JS, Gibson PR. Fructan and free fructose content of common Australian vegetables and fruit. J Agric Food Chem. 2007 Aug 8;55(16):6619-27.), a 2.8 ounce serving of Jerusalem artichoke has 9.6 grams of fructan (one of the FODMAPs). So in this particular instance, the answer is similar. If the glycemic index was 50 instead of 35, the FP would be 8 grams – not much different.
If you look at the table in the reference above, you will notice that most vegetables have zero or trace amounts of fructans (perhaps some have more of the other FODMAPs, but we need the data), but a couple stand out including onions, garlic and leeks. In these instances, I still think the FP calculation will likely give a relatively similar read out. Amount does matter though. If you only use a few cloves of garlic (total weight 9 grams, net carbs 3 grams, fiber 0 grams), for example, in a stir-fry, the FP is only 2 grams. It doesn’t even matter if you use a glycemic index of 50 or 35, you still get an FP of two grams. According to the above reference, garlic has 17.4 grams of fructans per 100 gram serving. So a 9 gram serving would have 1.6 grams of fructans. Pretty close to the FP. The same answer if you round up to the nearest gram.