LACTOSE INTOLERANCE

Lactose, or milk sugar, is found in all mammalian milks and certain dairy products made from these milks (e.g., yoghurt, some cheeses and ice cream). Lactose may also be added to foods and beverages as an ingredient, such as doughnuts, biscuits, breakfast bars, and hamburger buns. Whey may also be used as an ingredient in foods, and it has a relatively high lactose content. Finally, lactose is also used in pharmaceutical products as a filler because it is inexpensive and easy to compress. The table lists the amount of lactose in a typical serve of commonly consumed foods:

Like all disaccharides, lactose needs to be broken down (digested) into its component sugars (glucose and galactose), before it can be used by our bodies (see figure, below). Lactase—a lactose breaking, genetically controlled enzyme located in the microvilli of the small intestine—does this for us. Until we are three or four years of age, most of us have sufficient lactase to digest lactose. After this, lactase production slowly grinds to halt, leading to lactose intolerance, which affects 65% of adults, globally. However, in Western, Southern and Northern Europe (and people with this heritage), lactose intolerance affects only 19% to 37% of adults.

The ability to digest lactose after early childhood changed for some groups of people about ten thousand years ago when they settled down, became farmers and herders, and added milk from their domesticated animals to their diet. Within a thousand years or so of their doing so, a genetic mutation allowed many of them to keep producing lactase beyond childhood, thus enabling them to continue digesting the lactose in milk. It is thought that the same genetic mutation occurred independently in North Africa and the Middle East when the camel became domesticated and camel milk became part of the diet.

What is lactose intolerance?

If there is insufficient lactase in the small intestine, undigested lactose passes down into the large intestine (bowel), where it can be fermented by intestinal bacteria, producing gases such as hydrogen, methane and carbon dioxide. Undigested lactose in the intestine can also create an osmotic load which draws water and electrolytes into the intestine from the body and causes a loose stool and rapid transit time (i.e., diarrhoea).

Lactose intolerance is therefore characterised by symptoms resulting from the malabsorption of lactose including flatulence (flatus), gas, bloating, cramps, diarrhoea and (rarely) vomiting. However, in most people, lactose intolerance is mostly a deficiency, not a complete absence of the enzyme lactase.

Additionally, lactose intolerance may develop transiently, when the small intestine is injured following acute infection or in protein-energy malnutrition. It also occurs in association with coeliac disease and tropical sprue, where the small intestinal villi that contain the lactase enzyme are damaged.

A recent systematic review of randomised crossover trials in women (55%) and men (45%), found that people with lactose intolerance can ingest from 10-15 g (average 12 g) of lactose (approximately one cup of milk) per day, particularly if taken with other food, with no or only minor symptoms. When people with lactose intolerance consumed more than 12 g, symptoms became more prominent, and they increased appreciably after 24 g of lactose, and 50 g induced symptoms in the vast majority of people.

To help put this into perspective, in 2011/2, the average Australian adult consumed approximately 10 g of lactose per day and the average Australian child consumed approximately 14 g of lactose per day.

Today, a large range of low lactose and lactose free dairy foods can be found in local supermarkets, corner stores and even baristas. Additionally, lactase can be added to milk/yoghurt in the form of drops or tablets to pre-digest the lactose before you consume it. Therefore, people with various degrees of lactose intolerance can continue to enjoy their favourite dairy foods without fear of unpleasant consequences.

Read more:

1. Barclay, Sandall and Shwide-Slavin. The Ultimate Guide to Sugars and Sweeteners. The Experiment. 2014.
2. Dairy Australia and The Gut Foundation. Lactose Intolerance. 2020.
3. Wilt and colleagues. Lactose Intolerance and Health. No. 192. Minnesota Evidence-based Practice Center, 2010.

Dr Alan Barclay, PhD, is a consultant dietitian and chef with a particular interest in carbohydrates and diabetes. He is author of Reversing Diabetes (Murdoch Books), and co-author of 40 scientific publications, The Good Carbs Cookbook (Murdoch Books), Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment Publishing).