Updated January 2024.
For people with irritable bowel syndrome on the low FODMAP diet (LFD), adhering to the low FODMAP serving sizes during the elimination phase (and challenge phase) is key for relieving IBS symptoms.
Unfortunately, the subject of serving sizes stokes a lot of confusion and generates heaps of questions among IBS sufferers.
Since serving sizes can make or break the diet, it’s important you have a good grasp on what they’re all about.
So let’s deep dive on how much you can eat (ie. serving sizes) and explore commonly asked questions…
Portion size vs serving size – what’s the difference?
Serving size: the amount of food that nutrition information is based on.
In the Monash University FODMAP app, a serving size that is accompanied by a ‘green light’ is considered low FODMAP and thus a safe serving size. A serving size that is accompanied by a yellow light is moderate FODMAP and a serving size accompanied by a red light is high FODMAP.
On nutrition labels, the serving size is not a recommended amount to eat. It is merely a reference amount.
In the Monash app however, the serving size corresponding to a green light is a recommended amount to eat in one sitting when you’re on the LFD.
Portion size: the amount of food you eat in one sitting.
Your portion may contain several serving sizes. For example, 1 medium carrot is a low FODMAP serving size. If you eat 2 medium carrots, then your portion size consists of 2 serving sizes.
Where to get the most accurate serving size information?
As you’ve probably surmised already, the Monash FODMAP app is currently the best source of info on the FODMAP content of food. This is because Monash scientists rigorously test foods and do so regularly to expand the list of foods and update old info.
Consequently, I suggest that you avoid using low FODMAP food lists you find online or in books as they may be outdated or incorrect.
Understanding the Monash FODMAP app traffic light system
Monash’s traffic light system has two sets of traffic lights:
1) the large traffic light, which you find in the list of foods, and
2) the small traffic lights, which you find under the name of each item. Note: not all foods have small traffic lights.
Oftentimes, the serving size for the large traffic light is based on a “typical” amount the average Australian (Monash is in Australia) would eat in one sitting according to databases of peoples’ usual intake. Other times the serving size is simply arbitrary. The large traffic light is the FODMAP rating for the ‘typical’ amount eaten.
When a food has small traffic lights under its name, it means there are different serving sizes that are lower or higher in FODMAPs.
If there is a small yellow light under the food name, then there is a serving size that is moderate FODMAP. If there is a small green light under the food name, then there is a serving size that is low FODMAP – which you can eat!
Weights and volumes are the best measurements to use
The Monash app often gives you 2 different measurements for the serving size:
1) weight in grams (and millilitres for liquids)
2) common measures like 1/3 bell pepper, 1 cup chopped, 4 squares, etc
The trouble with the common measure is that foods can vary greatly in size. One third of a large avocado will have more FODMAPs than 1/3 of a small avocado.
That’s why it’s best to use the weight (g) or volume (mL) to measure out a food and ensure it’s the right amount. This is where a kitchen scale comes in very handy as it’s the easiest way to guarantee accurate serving sizes.
One drawback to weights and volumes is they’re hard to visualize, so let’s take a peek at what they look like…
Low FODMAP serving sizes
Chocolate
Chocolate is a great example of why you need to use the weight rather than the common measure. Milk chocolate is low FODMAP at a serving size of 4 squares. Problem is, chocolate squares come in various sizes and some chocolate bars don’t even have squares!
Peanuts
The common measure for peanuts is 32 nuts. Who is going to count out 32 peanuts? Puh-lease. I find it easier to put them in a little container and weigh out 28 grams, which is the green light serving size.
Over time you’ll be able to eye-ball the low FODMAP serving size and won’t need to use the scale all the time.
Grapes
Grapes are not a low FODMAP fruit. In fact, their FODMAP rating (ie large traffic light) is high (red). But grapes do have the 3 little traffic lights indicating that there are moderate and low FODMAP serving sizes.
The Monash app says red seedless grapes are low FODMAP at 6 grapes (common measure) or 28 grams. My grapes were massive, so 6 grapes weighed 59g, almost a high FODMAP serving size. At 28 grams (low FODMAP), I can have 3 grapes.
3 grapes. That’s pretty sad.
Red Pepper
I must have grabbed a red pepper the same size as the ones Monash used in testing because 43 grams cut up red pepper came to 1/3 cup (their common measure).
Avocado
The Monash app describes a low FODMAP serving size of avocado as 1/8th an avocado or 30 grams. When I managed to cut a weasley 1/8th sliver off my small avocado, it yielded 10 grams.
This speaks to how the variation in food size affects the common measure serving size. That’s why it’s always best to use weights when you’re getting started on this diet.
How to follow the low FODMAP serving sizes
First, don’t fret if you don’t follow the low FODMAP serving sizes every single time. Progress, not perfection.
- Use the Monash FODMAP app traffic light system; don’t rely on food lists online.
- Use a food scale and stick with weights and millilitres.
- Don’t rely on Monash’s common serving size for fruits, vegetables, nuts.
- Avoid eating large meals.
- Plan your meals in advance.
Frequently asked questions
Do you have to weigh your food forever?
No. Let me explain…
Sometimes the common measure of a low FODMAP serving size is pretty straight forward – like 1 cup of almond milk (240g) – so you can use it no problem.
But during the elimination phase and challenge phase of the LFD, it’s a good idea to weigh foods for which the common measure (eg. 6 grapes, 4 squares chocolate) is difficult to standardize because of variation in food sizes.
With practice and over time you’ll be able to visually match a serving size of food with its weight and then be able to eye ball serving sizes.
Once you move into the personalization phase of the diet, you’ll only be restricting FODMAPs you don’t tolerate and by then you’ll be an ace at eye balling serving sizes.
Can you eat high FODMAP foods in small portions?
If the high FODMAP food in question has a small green light serving size, then you can eat that amount. This helps add variety to the LFD and reduces the restrictions this elimination diet imposes.
Are the serving sizes per meal or per day?
Per meal. This means that since 1 slice of whole wheat bread is a low FODMAP serving size, you can eat 1 slice per meal.
Can you have multiple green light foods per meal?
Yes. The Monash scientists recognize that a balanced meal is made up of a variety of foods. In order to accommodate this, they set the threshold level for ‘low FODMAP’ fairly low so that you can have multiple low FODMAP foods at a meal.
Are serving sizes based on COOKED or RAW foods?
The app should specify whether the food is cooked or raw in the food listing. But generally speaking:
Raw: fruits, vegetables, nuts, seeds, etc – foods we typically eat uncooked.
Cooked: foods we normally eat cooked like pasta, rice, etc.
If 1 cup of X food is low FODMAP and 2 1/2 cup is moderate FODMAP, is the serving size green up to 2 1/2 cup?
I asked the Monash FODMAP team about this one. According to them, you should stick with the green light serving size in phases 1 and 2 of the LFD. That being said, it’s unlikely that you’ll develop symptoms if you eat more than the green serve but less than the yellow serve.
Once you enter phase 3 (personalization) then you can start experimenting with quantities closer to the yellow light serve to help expand your diet.
What if the low FODMAP serving sizes are not enough food for you?
If you’re crafting meals with low FODMAP serving sizes and find that you’re still hungry, there are a few things you can do to fill up:
- Increase your portions of non-FODMAP foods like poultry, meats, fish.
- If you don’t use much fat when cooking, start using a bit more (eg. olive oil, garlic-infused oil, other oils, butter) but back off if this causes gas or abdominal pain.
- Increase portions of foods that are very low FODMAP, like gluten-free breads, pastas, brown rice, lactose-free yogurt, etc.
- Make sure your meals are balanced with veggies, whole grains, proteins and healthy fat.
- Space meals 5-6 hours apart and have a healthy snack in between as needed.
Of course, if you try some of these strategies but you’re still struggling with hunger and/or losing weight, it’s time to talk to a registered dietitian (preferably a FODMAP-trained dietitian, like myself) or your doctor.
How much should you eat if there’s an unknown amount of FODMAPs?
Restaurant foods, processed foods and pre-packaged foods may contain many high FODMAP ingredients you’re unaware of, like onion, high fructose corn syrup, and inulin.
You can assume the unknown food is OK and monitor your symptoms, or avoid it until the challenge phase of the LFD to see if it triggers symptoms.
Another (less ideal) option is to search for the product using the Spoonful app. This app will inform you whether a food is FODMAP friendly after you scan the package’s bar code.
One caveat with this app: it determines if a product is low FODMAP based on the ingredients list, not based on laboratory testing. This is why Monash University doesn’t endorse barcode scanning apps.
Bottom line
During the elimination and challenges phases of the low FODMAP diet, stick with the green serving sizes.
Weigh foods that don’t have standard serving sizes, like fruits, vegetables and nuts. And remember: you won’t be weighing your food forever. Over time and with experience you’ll be able to visually eye ball serving sizes.
Got any questions? Let me know in the comments section below!
xoAndrea, RD, MHSc, Monash FODMAP trained
