FODMAP Portion Sizes - Why Do They Matter?

You’ve decided to start the low FODMAP Diet. So, you’ve downloaded a list off the internet/been given one by a clueless dietitian (there are lots of very good dietitians, but also lots who just know nothing about FODMAPs), which has ‘high FODMAP foods’ in one column and ‘low FODMAP foods’ in another. So, obviously you think it’s black and white. Each food is either high or low FODMAP. You either need to avoid it or you can safely eat as much as you like of it. THIS IS NOT TRUE!!! FODMAP portion sizes play such a huge role in this diet and I’m going to tell you why!


So, let’s start from the beginning..



A huge list of foods have been classified by Monash University (the founders of the diet) as high or low FODMAP. The criteria they use to classify foods as high or low FODMAP are:

  1. The actual FODMAP content of that food (i.e. grams of FODMAPs)

  2. Standard serving size usually eaten in a single sitting/meal

  3. The amount of that food that commonly triggered symptoms in people with IBS

From this, they chose a ‘threshold’ level, or a ‘cut off’ level for each FODMAP group. So, any standard serving size of food that contained an amount of FODMAPs more than that threshold, would be classed as high FODMAP. E.g. for lactose, the threshold level is 1g per standard serving size. This is the amount that will typically trigger symptoms and therefore is classed as high FODMAP. Therefore, if a food contains 1.1g lactose in a single serving, it’s classed as ‘high FODMAP.’ If it contains 0.8g lactose in a single serving, it’s classed as ‘low FODMAP.’

Monash then created the traffic light system based on these thresholds. They’ve classified each food as green, amber or red and these classifications are based on the standard serving sizes of those foods. This is the theory behind those ‘high FODMAP’ and ‘low FODMAP’ tables you’ve seen around the internet.

This system is really simple and makes its super easy for everyone to see which foods are low and high FODMAP. It lets you get started on the diet in the easiest way possible, without too much help. However, there’s so much more to it than that..



As I mentioned above, foods are classified high or low FODMAP depending on the standard serving size of the individual food, and whether that’s over or under the threshold. Therefore, whether that food is high or low FODMAP is completely dependent on the standard serving size of that food.

I found this really hard to understand at first, so let me give you an example, with some screenshots from the Monash app…we’ll use raisins. 

  • Raisins are classed as red = high FODMAP by Monash. This is based on a standard serving size of raisins, which is 2 tbsp (26g).


  • However, half a standard serving size, 1 tbsp (13g) is classed as green = low FODMAP by Monash.


What this means is that raisins will be in the ‘high FODMAP’ column of all those lists, but they’re not completely off limits. You can have 1 tbsp of them and it’s completely low FODMAP. Therefore, you may be depriving yourself when you really don’t need to!

This works the other way around too. Let’s use green beans as an example.

  • Green beans are classed as green = low FODMAP by Monash. This is based on a standard serving size of green beans, which is 15 beans.


  • However, 25 beans is classed as amber = moderate FODMAP content by Monash.


  • To show the subtleties even more, 35 beans is classed as red = high FODMAP by Monash.


What this means is that green beans will be in the ‘low FODMAP’ column of all those lists, but you can’t just eat as much of them as you like. If you eat more than 25 of them, they’re high in sorbitol and actually high FODMAP. Therefore, you might be eating platefuls of them, thinking you’re being really ‘good’ but still getting symptoms and wondering why.



The best way to do this is to download the Monash app (which help me SO much). You’ll see a food guide, containing all of the foods they’ve tested, with a traffic light rating based on the standard serving size of that food. E.g. for raisins, this is red, like in the screenshots above.

If you then click into the food from the overall food guide, you’ll be able to see the FODMAP content of each individual FODMAP group (e.g. fructose) as well as the overall FODMAP rating of the food at DIFFERENT serving sizes. This is super important, because you’ll be able to see whether you could eat a smaller FODMAP portion size and it be low FODMAP. E.g. for raisins, 13g is green. Lots of foods rated red in full serving sizes are actually green in smaller serving sizes, and vice versa.

This is amazing, because it opens up a whole new world of foods you can eat! I was so sad that avocado was a high FODMAP ‘off limits’ food. However, when I discovered the importance of FODMAP portion sizes, I used the app to work out that 1/8 avocado is still low FODMAP….so I started adding 1/8 avocado to everything!!

On the other hand, it can help give you answers to why you’re been so ‘good’ and eating only low FODMAP foods, but still getting symptoms. One evening I ate almost a whole plate of zucchini, as it was ‘green’ on the list I had, but my stomach was SO bloated afterwards. I just couldn’t work out why as I hadn’t eaten anything high FODMAP. When I discovered the FODMAP portion sizes matter, I realised I’d just had too much of it! I actually felt so relieved to have an answer.



The annoying thing is, it’s not just as simple as sticking to a low FODMAP portion of individual foods… (of course, I hear you say, because NOTHING about this diet is easy). You’ve also got to consider your the overall amount of FODMAPs you’re eating in a meal or a day. That’s what I’ll be talking about next week! Pop your email address in the box below if you want me to send it straight to your inbox.


  • The question of whether a food is high or low FODMAP isn’t black and white - FODMAP portion sizes matter.

  • Monash classified foods as low or high FODMAP to make it simple and easy to understand, but there is a lot more to it than that.

  • Foods classed as high FODMAP might actually be low FODMAP in smaller serving sizes.

  • Food classed as low FODMAP might actually be high FODMAP in larger serving sizes.

  • Take the time to consider FODMAP portion size to unlock foods that you thought you couldn’t eat and to get answers to why some foods you thought you could eat are still triggering your symptoms.

  • Throw away those high and low FODMAP lists you’ve got and get the Monash app.

Thanks so much for taking the time to read this. Finally understand the portion size thing? Let me know in the comments below.

Sophie x

Sophie BibbsComment