A diet rich in dairy products, particularly milk, is promoted to reduce the likelihood of osteoporotic fractures. However, a new study published in the BMJ suggests that a high intake of milk is associated with a higher risk fractures and a higher rate of death in men and women.
Milk contains 18 out of 22 essential nutrients. The relationship between the dynamic duo of calcium and vitamin D in milk and their importance in maintaining bone health has long been promoted in nutritional education, especially in terms of child development.
The United States Department of Agriculture (USDA) recommend a daily dose of 3 cups of dairy to support good health and improve bone mass. An intake of three or four glasses of milk a day has been suggested to save at least 20% of health care costs related to osteoporosis.
Milk is also presented to have many other benefits, including:
- Maintaining healthy blood pressure
- Reducing the risk of cardiovascular disease and type 2 diabetes
- Protection from colorectal cancer
- Providing natural, high-quality protein for muscle mass.
Observational research conducted in Sweden, led by Prof. Karl Michaëlsson, disputes some of the health benefits of milk, in particular providing conflicting data for the most cited connection between milk and prolonged bone health.
The researchers hypothesized that a high consumption of milk may, in fact, increase oxidative stress, leading to an effect on the risk of mortality and bone fracture.
The hypothesis is based on the indication that milk provides the main dietary source of D-galactose. Galactose makes up half of lactose, the sugar found in milk.
Experimental evidence in various animal species indicates that a chronic exposure to D-galactose is detrimental to health, a source of aging and shortened lifespan. These consequences are due to oxidative stress damage, chronic inflammation, neurodegeneration, decreased immune response and gene transcriptional changes.
To test the hypothesis, the investigators used two community-based cohorts to analyze the association of milk consumption and mortality and fracture rates:
- The Swedish Mammography Cohort: a group of 61,433 women, aged 39-74 in 1987-1990
- The Cohort of Swedish Men: a group of 45,339 men, aged 45-79 in 1997.
In answer to a questionnaire, the participants reported their average consumption of up to 96 common foods and beverages, including milk, fermented milk, yogurt and cheese. Lifestyle information, weight and height were gathered and factors relating to education level and marital status were also taken into account. National registers were utilized to track fracture and mortality rates.
Three or more glasses of milk a day could lead to an increased risk of death
The researchers used the underlying cause of death from the Swedish cause of death registry to define mortality from all causes.
The Swedish Mammography Cohort
Over the 20 years that the women were tracked:
- 15,541 died
- 17,252 had a fracture; of these, 4,259 had a hip fracture.
No reduced risk of fracture with higher milk consumption was observed. Women who drank more than three glasses of milk a day (average 680 ml) had a higher risk of death than women who drank less than one glass of milk a day (average 60 ml).
The Cohort of Swedish Men
Over the duration of the 11 years that the men were tracked:
- 10,112 died
- 5,066 had a fracture; of these, 1,116 had a hip fracture.
Although less pronounced than in the female group, men also had a higher risk of death with higher milk consumption.
Further investigation was undertaken to determine whether milk was associated with oxidative stress and inflammation. The results proved positive in both sexes. However, consumption of fermented milk products, yogurt and cheese indicated a negative relation with both the oxidative stress and the inflammatory markers and was associated with reduced rates of mortality and fracture, particularly in women.
The researchers say:
"Our results may question the validity of recommendations to consume high amounts of milk to prevent fragility fractures."
The research concludes that a higher consumption of milk in both sexes is not accompanied by a lower risk of fracture and may instead be linked with a higher rate of death. There may also be a link between the lactose and galactose content of milk and risk as suggested in the hypothesis, although causality has yet to be tested.
Should dietary recommendations regarding milk consumption be changed?
The researchers note that given the observational design of the study, the results should be interpreted cautiously. They point out that the study can only show an association and cannot prove cause and effect, adding that further replication studies should take place before the results can be used for dietary recommendations.
In an accompanying editorial, Prof. Mary Schooling, at City University of New York, comments:
"As milk features in many dietary guidelines and both hip fractures and cardiovascular disease are relatively common among older people, improving the evidence base for dietary recommendations could have substantial benefits for everyone."
She closes, "As milk consumption may rise globally with economic development and increasing consumption of animal source foods, the role of milk in mortality needs to be established definitively now."
Medical News Today recently reported that a glass of milk a day may delay knee osteoarthritis in women. Although the degenerative disease currently has no cure, researchers say drinking milk every day has been linked to reduced progression of the disease.