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Lactose guidelines for the FODMAP diet by Dr Megan Rossi

Lactose guidelines for the FODMAP diet by Dr Megan Rossi

by FoodMaestro | 26 July 2016

Lactose guidelines for the FODMAP diet by Dr Megan Rossi

As we covered in my previous blog the low FODMAP diet involves the restriction of a group of five different fermentable carbohydrates, one of which is the Disaccharide, lactose. Although few people realise lactose is a carbohydrate, many people have heard of lactose outside the FODMAP context because of the high prevalence of lactose intolerance (a condition independent of irritable bowel syndrome (IBS), although the two often co-exist).

So you’ve heard of lactose intolerance, but what is it exactly?

Time to go back to basic chemistry and physiology (sorry!)- lactose is a type of carbohydrate found in cow’s milk. It’s made up of two sugar units, glucose and galactose, which is why it is called a DI-saccharide (saccharide is the “sciency” name for carbohydrate). In order for lactose to be absorbed from the gastrointestinal (GI) tract into our blood stream it needs to be broken down into the single sugar units, glucose and galactose. This is the job of an enzyme called lactASE (when you see -ASE tacked on the end of a word think enzymes), which lines the GI tract. However, 70% of the adult population, ranging from 5% British to 90% Asian1, don’t make enough lactASE and therefore lactOSE isn’t absorbed efficiently. Despite this occurring in 70% of people only a subset of these malabsorbers experience GI upset and it’s only this subset of people who are called lactose intolerant (according to the official diagnosis).

How do I know if I am lactose intolerance?

To confirm a diagnosis your doctor can order a special breath test called a lactose hydrogen breath test- this is one of the FEW occasions where a breath test is validated diagnostic tool. (Beware: breath tests can NOT diagnosis IBS so please don’t waste your time or money).

If a lactose hydrogen breath test is not available or appropriate, national guidelines recommend a trial period of a low lactose diet.

What?! A LOW lactose diet? Do this mean I can still include small amounts of lactose? 

It sure does! Unlike food allergies and coeliac disease where strict avoidance of the allergen is important, in food intolerances (including lactose intolerance) consuming small amounts is okay. But don’t just take my word for it, check out consultant gastro dietitian (and newly awarded MBE), Dr Miranda Lomer’s tweet: 

FODMAP lactose

Enough about lactose intolerance I hear you cry, I’ve got IBS, how does it affect me?

The prevalence of lactose intolerance in people with IBS is highly variable across populations with reports ranging from ≈20-80%.2 For this reason lactose restriction is included in the low FODMAP diet, HOWEVER, as Dr Lomer shared at the King’s College London low FODMAP education course, “if you have ruled out lactose intolerance already, there is no need to restrict lactose when on the low FODMAP diet”… Ice cream come at me! (I hear the dairy lovers scream)… But remember even if you are lactose intolerant (or unsure if you are and therefore restricting lactose on the low FODMAP diet), you can still have up to 4g of lactose in one sitting. This means one scoop of normal ice-cream is still on the menu (although check for other FODMAPs).

Until my next blog, happy low FODMAPing (or FOMAP for those lactose tolerant ;))

Related articles by Dr Megan Rossi in the irritable bowel syndrome (IBS) series:

  1. Introduction to a low FODMAP diet - By Dr Megan Rossi 
  2. The research behind the low FODMAP diet -  By Dr Megan Rossi 
  3. Lactose guidelines for the FODMAP diet by Dr Megan Rossi
  4. Beyond the low FODMAP diet By Dr Megan Rossi
  5. Why the low FODMAP diet may not be working for you

By Dr Megan Rossi 

Instagram: Dr_Megan
Twitter: @DrMegan_RD


  • 1. Lomer, MCE, Parkes, GC, Sanderson, JD: Review article: lactose intolerance in clinical practice – myths and realities. Aliment Pharmacol Ther, 27: 93-103, 2008.
  • 2. Staudacher, HM, Irving, PM, Lomer, MC, et al.: Mechanisms and efficacy of dietary FODMAP restriction in IBS. Nat Rev Gastroenterol Hepatol, 11: 256-66, 2014.