Should we be wary of dairy?
It’s commonly accepted that calcium is necessary for us to build and maintain healthy bones. Dairy has long been recognised as an excellent source of many nutrients, including calcium. Yet, over recent years the reputation of dairy products, particularly milk, has been under fire – including claims that dairy could be a trigger for asthma and increases the risk of bone fractures, heart disease, premature ageing and even cancer.
In this edition of In Brief we examine the claim that dairy is bad for our health.
Why we consume dairy
Dairy milk has been linked to the diet of many human societies for centuries, with records suggesting that milk drinking first started in central Europe some 7500 years ago. Dairy (especially milk, yoghurt and cheese) provides an excellent source of nutrients including calcium, potassium, vitamin D and protein. These essential nutrients have been linked to a range of health benefits.
Why not dairy?
People who choose not to consume dairy do so for a variety of reasons, including taste or preference, or for religious, cultural or ethical reasons. But for some people, it is not a matter of choice, with approximately 4 per cent of Australian adults reporting in 2015 that they were lactose intolerant.
Lactose is a milk sugar in dairy products that is broken down by the enzyme lactase. Lactase is naturally present in the gut of newborn infants but its production declines after weaning, which can lead to lactose intolerance.
Lactose intolerance is the reduced ability to digest milk sugars. Commonly reported symptoms include bloating, gas, abdominal pain and diarrhoea.
Up to 75 per cent of the human population is affected by lactose intolerance, but this figure varies considerably across communities. Compared to Australia’s 4 per cent, lactose intolerance is reported to be as high as 95 per cent in countries such as China. Lactose intolerance in Australian adults has also been reported to vary according to gender; women appear to be more susceptible.
‘Lactase persistence’ is a genetic mutation that gives people the ability to produce lactase throughout their lives. Genetic mutations can be beneficial, neutral or harmful. In the case of lactase persistence, the ability to produce lactase – and consume dairy – has been highly beneficial. Milk, a rich source of nutrition, offered early societies that began to harvest dairy a selective advantage by helping to sustain communities when harvests failed and water supplies were contaminated.
What are the effects of dairy?
The effects of dairy consumption are best determined by reviewing findings from randomised controlled trials (RCTs) – the gold standard in research. Unfortunately, RCTs are not always a practical (or ethical) option when investigating a phenomenon such as diet. For this reason, few RCTs to date have been able to examine the effects of dairy consumption on population health outcomes. In the following we explore some of the ‘next best’ available evidence on the effects of dairy on human health.
Dairy products are often cited as increasing mucus production in the respiratory tract and thereby triggering or exacerbating asthma.
An early RCT, undertaken by South Australian researchers in 1993 found that participants experienced a ‘milk–mucus effect’ after consuming dairy milk. This finding, however, was not unique to cow’s milk, with participants who consumed the soy milk placebo reporting the same effect.
In a RCT a few years later, researchers from Monash University and Alfred Hospital found no evidence for a relationship between dairy consumption and changes in asthma symptoms.
Osteoporosis is a major cause of bone fracture, deformity and mobility limitations among older Australians. Osteoporosis occurs when our bones lose minerals, such as calcium. Dairy has long been recognised as an excellent source of calcium and thus we’ve been encouraged over the years to drink milk to grow and maintain healthy bones.
A 2014 study challenged this practice, suggesting that high milk intake was associated with higher mortality in one cohort of Swedish women and in another cohort of Swedish men, and with higher fracture incidence in women. This finding should be interpreted with caution, however, given the observational design of the study. People at high-risk of developing osteoporosis often deliberately increase their calcium intake as a preventative strategy. In this instance, higher milk intake may actually be associated with lower bone mass density, which may be caused by other factors such as family history, medication, underlying health conditions and lifestyle factors.
A 2016 review of the literature found no conclusive evidence to suggest that we should reduce our dairy intake.
As noted in an earlier In Brief article, a diet high in saturated fat can contribute to raised blood cholesterol levels and has been shown to be a risk factor for heart disease. Dairy foods, particularly full-fat versions, are often high in saturated fats, leading to the assumption that dairy is therefore bad for our heart health.
A large meta-analysis of 29 prospective cohort studies recently found no association between consumption of high and low-fat dairy milk with coronary heart disease and cardiovascular disease. A separate review of prospective cohort studies found an association between dairy consumption and reduced risk for ischaemic heart disease, stroke and incident diabetes.
According to the Australian Dietary Guidelines, published in 2013, consumption of at least two serves of dairy per day is associated with reduced risk of ischaemic heart disease, myocardial infarction and stroke.
Animal studies examining the effects of D-galactose have prompted the argument that consuming dairy milk accelerates the ageing process. Lactose is made up of two simple sugars: glucose and galactose. D-galactose is the metabolite that results when galactose is absorbed and metabolised.
A 2004 study found a causal relationship between D-galactose and a decrease in the lifespan of flies and a later study, conducted by the same researchers, found that chronic exposure of D-galactose in mice induced neurodegeneration.
Despite these findings, to date there is no evidence to suggest that D-galactose produces the same effects in humans. Indeed, a longitudinal study of an elderly Japanese population found that consumption of dairy reduced the risk of dementia, especially Alzheimer’s disease.
Diet is a lifestyle factor related to the risk of developing cancer and other chronic conditions. Specifically, eating a healthy balanced diet is thought to reduce cancer risk, whereas a poor diet increases cancer risk. The Australian Dietary Guidelines describe a healthy balanced diet as consuming foods from each of the five food groups (in the amounts recommended), one of which is dairy. However, several scientific studies challenge this recommendation, reporting an increased risk of ovarian and prostate cancer associated with dairy consumption.
A prospective, population-based cohort study of 61 084 Swedish women, published in 2004, found that high intakes of lactose and dairy products, particularly milk, were associated with an increased risk of serous ovarian cancer. However, there is no suggestion of a causal relationship. Indeed, as part of an ongoing review, the World Cancer Research Fund 2014 Ovarian Cancer Report found that evidence of a relationship between dairy products and ovarian cancer is limited and no conclusions could be drawn at that time.
That same year, a systematic review of 32 studies examining dairy product and calcium intakes found that high intakes of dairy products, including whole milk, low-fat milk and cheese, could increase total prostate cancer risk. Interestingly, supplemental calcium was associated with risk of fatal prostate cancer; however, this finding was based on only two studies.
Conversely, consumption of dairy, particularly milk, has been suggested as a protective factor for other types of cancer. According to a report by the World Cancer Research Fund, and the evidence summarised in the Australian Dietary Guidelines, milk is a probable protective factor against colorectal cancer.
In summary, consumption of dairy milk has been reported as both protective (colorectal) and occasionally harmful (ovarian and prostate) in cancer. A lack of robust clinical trials examining the long-term effects of dairy milk consumption makes it difficult to reach definitive conclusions.
It has been suggested that hormones present in cow’s milk and potentially harmful to human health may be responsible for adverse health outcomes associated with consumption of dairy. All milk contains a range of natural bioactive hormones, such as insulin-like growth factor 1 (IGF-1), which is not considered harmful to humans.
Bovine somatotropin (bST), also known as bovine growth hormone (bGH), occurs naturally in cows to help them produce milk. Sometimes a synthetic copy of this hormone is given to cows to boost milk production. While synthetic bGH is used in the United States to increase milk production, this practice is prohibited in Australia.
Nonetheless, it appears unlikely that bST (natural or synthetic) has adverse effects on human health, given that growth hormones from cows have not been shown to be biologically active in humans. Pasteurisation of milk destroys approximately 90 per cent of hormones, and the remaining trace amounts are broken down into inactive amino acids – just like any other protein.
To date, few RCTs examining the effects of dairy on health outcomes have been conducted. An assessment of the best available evidence indicates that dairy consumption is not associated with adverse health outcomes in healthy adults. While some studies have reported associations between dairy consumption and adverse outcomes, such as some cancers and fractures, these links are not conclusive.
On balance, to date no compelling evidence is available to indicate that dairy products should be avoided. Dairy provides a convenient source of nutrients for many people, particularly children and young people. When consumed in moderation, and in accordance with the Australian Dietary Guidelines, dairy poses no significant risks to our health.