Image: iStock. By  Veronique Chachay , The University of Queensland and Marina Reeves , The University of Queensland .

Decades of public health messages have encouraged us to drink milk to strengthen our bones and reduce the risk of fractures as we age.

But dairy products have recently come under fire – and not just from paleo dieters and animal welfare supporters. Researchers have linked high milk intakes to bone fractures, cancer and premature ageing.

Only last month two research reviews, published in the British Medical Journal, reported that calcium supplementation and dietary calcium intake didn’t substantially reduce the risk of hip fractures or increase bone mineral density in adults aged over 50 years. (Looking for a healthy quinoa recipe? Check out Paper Tiger’s in the video below. Post continues after video.)

We would love to give a definitive answer on whether milk is healthy or harmful, and whether you should include it in your diet. But the reality is, it all depends.

It depends on how much and what kind of milk products you consume. It depends on whether you’re an omnivore or vegetarian. And it depends on your age and current health status. So, whether you’re underweight or carrying excess weight, elderly or convalescent, or have food allergies or intolerance.

Let’s look at whether the arguments against cow’s milk stack up.

Milk increases your risk of chronic diseases

The most recent reviews of the evidence have shown that drinking milk is not associated with increased risk of premature death.

Dairy consumption is associated with a reduced risk of heart disease and stroke, type 2 diabetes and colon (bowel) cancer.

Milk, dairy and calcium have not been conclusively associated with an increased or decreased risk of any other cancer. But there is limited evidence to suggest dairy increases the risk of prostate cancer.

Dietary calcium and dairy products are important for bone development up until the end of adolescence and for maintaining peak bone mass in adulthood.

"Recently dairy products have recently come under fire." (Image: iStock)

But there seem to be few benefits for older adults, in terms of fracture prevention, neither for dietary intake or supplementation. There are also possible adverse effects from too much calcium, such as kidney stones, heart problems and gastrointestinal symptoms.

It’s important to keep in mind, however, that there are few robust clinical trials of the long-term effects, especially in the context of chronic disease. And milk intake can be vastly diverse (quantity and form). So it’s difficult to compare study conditions and provide definitive conclusions.

Cow’s milk is for calves and not for humans

Human breast milk is arguably the first food the human gastrointestinal tract is exposed to. It provides a large selection of vital macro- and micronutrients to promote growth and development of the newborn.

Cow’s, goat’s and other mammals' milk provide approximately the same selection of nutrients but in different ratios. Here’s how 100 millilitres of human milk compares with cow’s milk:

Note the larger concentration of lactose in human milk, but the significantly lower protein, calcium, phosphorus and slightly lower fat concentration.

Lactose (the naturally occurring sugar found in milk) is broken down into glucose and galactose in the gut by an enzyme, lactase.

Lactase is present in the gut of babies but production declines after weaning in two-thirds of the world’s population. This is known as lactase non-persistence. The natural decline of lactase production is the basis for the argument that we’re not “designed” to continue to consume milk. (Dairy is in a variety of delicious dishes. Post continues after gallery.)

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However lactase persistence (tolerance) is an inherited trait thought to be associated with dairy farming and has been identified in dairy-consuming populations around the world.

People with lactase non-persistence are largely lactose-intolerant. When these people consume milk, lactose remains unabsorbed in the gut, causing bloating (due to bacterial fermentation) and build-up of fluid leading to diarrhoea.

It’s worth noting that products derived from milk, such as yogurt and cheese, have significantly lower concentrations of lactose due to the yogurt and cheese making processes, so they may be well tolerated.

Milk accelerates the ageing process

Lactose is made of two simple sugars: glucose and galactose. When lactose is digested, galactose is absorbed and metabolised, resulting in the metabolite D-galactose.

Animal studies have found that D-galactose increases oxidative stress in mice and accelerates the ageing process in flies. Oxidative stress essentially means that the body’s antioxidant “capacity” (dietary antioxidants compounds and the body’s antioxidant enzymes) is less than than the amount of free radicals. This imbalance leads to oxidative damage and ageing.

These animal studies have prompted some scientists to argue that drinking milk accelerates ageing in humans, by increasing oxidative stress and inflammation from galactose exposure.

But there is insufficient evidence for the theory and a lack of association between chronic disease and milk consumption, as outlined above in humans.

"Lactase is present in the gut of babies but production declines after weaning in two-thirds of the world’s population. " (Image via iStock.)

It is worth pointing out that human breast milk contains a much greater concentration of lactose – therefore galactose – than cow’s milk. If the link with increased oxidative stress was a possibility, it would have perhaps been observed earlier.

A subset of children, however, are born with the genetic disease galactosaemia. This is where the body is unable to metabolise galactose because it lacks one or more functional enzymes. The build up of galactose causes serious health problems. Total galactose (also present in other than dairy foods) avoidance is currently the only treatment.

Cut down on milk because it’s high in saturated fat

Milk and dairy products, especially full-fat varieties, are high in saturated fat. Two-thirds (68%) of milk fat is saturated fat.

Saturated fat has been suggested to increase concentrations of “bad” cholesterol in the blood and to increase the risk of heart disease.

For decades, dietary guidelines have recommended consuming low-fat dairy products to reduce saturated fat intake.

More recently, though, the evidence linking saturated fat to cholesterol production and to heart disease has been debated. Specifically, in relation to milk intake, the most recent literature review and a cross-sectional study suggested that milk intake was not associated with an overall increased risk of heart disease. And those consuming full-fat dairy were less likely to have metabolic syndrome (a condition that includes risk factors for heart disease and type 2 diabetes). (Post continues after gallery.)

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