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Lactose – Have we always been ‘Intolerant’?

Lactose is a form of sugar, more specifically a disaccharide (two monosaccharides joined together) found in mammals’ milk, including human milk.[1]  As a disaccharide, it cannot be absorbed unless it is broken down by the enzyme lactase in the intestine (when a word ends in ‘ase’, it is usually an enzyme).

So far, so good. Thus, if you have enough lactase, you simply digest it into glucose (blood sugar), and another monosaccharide called galactose (if the word ends in ‘ose’, it is usually a type of sugar or carbohydrate). These sugars get absorbed and metabolized like other sugars. But what if you don’t have enough lactase? What happens to lactose then? This is where things get exciting and we may develop a condition commonly known as Lactose Intolerance.

What is ‘Lactose Intolerance’?

Intolerance occurs when lactose is not digested by lactase due to a deficiency of lactase.[2] As humans are born needing to suckle on our mother’s milk, which is rich in lactose, babies rarely are born with lactose intolerance. However, as humans age, our lactase levels decline as the need to suckle mothers’ milk also ceases to be a need because we grow our teeth to chew our own food.[3] Thus, adults are commonly intolerant as we don’t need to drink milk. Some adults do choose to consume other mammals’ milk as they age such as drinking cow’s milk. Lactase persistence in the gut is thought to be related to the domestication of dairy cattle which has occurred relatively recently (in the last 10,000 years).[4]

What are the symptoms of Intolerance?

This depends on what age you are when you develop lactose intolerance. At the most severe end of the scale and if the inability to digest lactose occurs during the period of breast-feeding, the infant may die if not recognized very early after birth. In adults, symptoms of intolerance generally do not occur until there is less than 50% of lactase activity.[5] The good news is that most people with lactose intolerance can tolerate small amounts of lactose (less than 12 g), especially if it is combined with other foods or spread throughout the day.[6] This equates to about one cup of whole milk per day.

Thus, if the lactose individual consumes more lactose than they can tolerate, the lactose finds its way to the small intestine where it interacts with the gut’s microbiome. All in all, intolerance affects approximately 80% of the world’s population, and it is quite prevalent in the non-Western community. Intolerant individuals present numerous bowel-related symptoms, including abdominal pain, diarrhea, nausea, increased peristalsis, meteorism (massive gas in the bowel), flatulence, and abdominal distension.[7]

Who gets Lactose Intolerance?

Below is a graphical representation of what countries has elevated (and low levels) of lactose intolerance:



The graph shows that countries such as England and some Nordic countries have low (around 4%) lactose intolerance, whereas some African and Asian countries have almost 100% lactose intolerance. This is because the English, for example, consume high amounts of dairy, which keeps their lactase enzymes active whereas some African countries consume low amounts of lactose, which aids in the atrophy of lactase.

The take-home message

Lactose intolerance is a widespread evolutionary ‘condition’. It occurs as humans don’t usually consume mammal’s milk as they age in some countries. Westerners, on the other hand, consume cow’s milk into adulthood, which preserves the activity of lactase. The latter may not be ideal, as there are numerous other adverse effects of cow’s milk on human health, which may manifest later in life.


[1] Asia Pac J Clin Nutr. 2018;27(1):29-46. doi: 10.6133/apjcn.022017.05. Prevalence, cause and diagnosis of lactose intolerance in children aged 1-5 years: a systematic review of 1995-2015. Louise Harvey 1, Thomas Ludwig 2, Alice Qian Hou 1, Quak Seng Hock 3, Michelle Ln Tan 3, Seksit Osatakul 4, Jacques Bindels 3, Leilani Muhardi 5

[2] Nutrients. 2015 Sep; 7(9): 8020–8035. Lactose Intolerance in Adults: Biological Mechanism and Dietary Management. Yanyong Deng,1 Benjamin Misselwitz,2 Ning Dai,1 and Mark Fox.

[3] Lactose intolerance. Vesa TH, Marteau P, Korpela R. J Am Coll Nutr. 2000 Apr; 19(2 Suppl):165S-175S.

[4] Identification of a variant associated with adult-type hypolactasia. Enattah NS, Sahi T, Savilahti E, Terwilliger JD, Peltonen L, Järvelä I Nat Genet. 2002 Feb; 30(2):233-7.

[5] Systematic review: effective management strategies for lactose intolerance. Shaukat A, Levitt MD, Taylor BC, MacDonald R, Shamliyan TA, Kane RL, Wilt TJ. Ann Intern Med. 2010 Jun 15; 152(12):797-803.

[6] Review article: lactose intolerance in clinical practice–myths and realities. Lomer MC, Parkes GC, Sanderson JD

Aliment Pharmacol Ther. 2008 Jan 15; 27(2):93-103.

[7] Mattar R, Mazo DF de C. Lactose intolerance: changing paradigms with molecular biology. Rev Assoc Médica Bras. 2010; 56: 230-6.

[8] Country, regional, and global estimates for lactose malabsorption in adults: a systematic review and meta-analysis. Christian Løvold Storhaug, MS †Svein Kjetil Fosse, MS †Dr Lars T Fadnes, PhD Published:July 06, 2017DOI: VOLUME 2, ISSUE 10, P738-746, OCTOBER 01, 2017