Myth: Most people can’t digest milk.
The fact that milk is a commonly eaten food throughout the world suggests we can and do digest it. The story of how this came to be is fascinating, and I was introduced to it by Glenn Cardwell, dietitian and fellow myth-buster, via Gregory Cochran and Henry Harpending’s book, The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. The authors reveal that the ability to digest milk in adulthood started around 10,000 years ago just after humans first started to keep animals for food. Geneticists believe by pure chance a mutation occurred that maintained lactase levels into adulthood and this mutation spread through the gene pool because it conveyed a survival advantage. That is, it provided an important source of nutrition that kept adults healthy and able to reproduce—and thus carry on the genes for digesting milk. This ability spread through Europe and India. A similar scenario was also happening in Arabia where the milk came from camels. It was so successful that fossilised human remains shows 80 per cent of Europeans were able to digest milk 7,000 years ago.
Lactose intolerance is when a person does not produce sufficient amounts of lactose-digesting enzyme (called lactase). Lactose is digested by the enzyme lactase found in the cells that line the digestive tract. All of us have sufficient lactase to digest lactose until three–five years of age, after which it undergoes a genetically determined decline in the majority of people. In fact, all mammals (including dogs, cats, rats, mice, etc.) show this decrease. An Australian review estimated that lactose maldigestion (poor digestion of lactose) affects as few as 4% of adult Caucasians. But figures are higher among people of Asian origin, Aboriginal people and African-Americans. The amount of lactose that can be comfortably tolerated varies from person to person, but generally people with lactose intolerance can digest small amounts of lactose (such as the amount in a small glass of milk) without symptoms, especially if consumed as part of a meal.
The amount of lactose in yoghurt is much lower because the bacterial cultures break the lactose down into lactic acid (giving yoghurt its characteristic tart flavour). Hard cheese contains negligible lactose. For the supersensitive there are lactose-free milks and yoghurts available, and even lactase enzyme powder you can add to milk to do the digesting for you.
There are also those with milk allergy who must stay well away from anything dairy-based or they become ill; however, this unlucky group makes up less than 1 per cent of the adult population.
Milk intolerance (which is different to lactose intolerance) also occurs but no data is available on the numbers of people affected, although it is expected to be higher those with milk allergy (food intolerance is notoriously difficult to diagnose, requiring an elimination diet and food re-challenge supervised by an experienced dietitian).
Getting enough calcium is important for healthy bones and the prevention of osteoporosis as you get older. You might think there are negligible consequences to avoiding dairy foods but a study published in the American Journal of Clinical Nutrition found people with self-perceived lactose intolerance had a much lower intake of calcium because they ate less milk, cheese and yoghurt.
Nicole Senior is an Accredited Practising Dietitian and Nutritionist and author of Food Myths available in bookshops and online and from www.greatideas.net.au