A diet rich in milk products is promoted to reduce the likelihood of osteoporotic fractures. Milk contains 18 of 22 essential nutrients, including calcium, phosphorus, and vitamin D of especial importance for the skeleton. Intestinal uptake of these nutrients is enhanced by the enzymatic capacity to digest lactose into D-glucose and D-galactose by mutation in the lactase gene, a variant common in those with northern European ancestry.1 2 An intake of dairy foods corresponding to three or four glasses of milk a day has been suggested to save at least 20% of healthcare costs related to osteoporosis.3
A high intake of milk might, however, have undesirable effects, because milk is the main dietary source of D-galactose. Experimental evidence in several animal species indicates that chronic exposure to D-galactose is deleterious to health and the addition of D-galactose by injections or in the diet is an established animal model of aging.4 5 6 7 Even a low dose of D-galactose induces changes that resemble natural aging in animals, including shortened life span caused by oxidative stress damage, chronic inflammation, neurodegeneration, decreased immune response, and gene transcriptional changes.5 7 A subcutaneous dose of 100 mg/kg D-galactose accelerates senescence in mice.5 This is equivalent to 6-10 g in humans, corresponding to 1-2 glasses of milk. Based on a concentration of lactose in cow’s milk of approximately 5%, one glass of milk comprises about 5 g of D-galactose. The increase of oxidative stress with aging and chronic low grade inflammation is not only a pathogenetic mechanism of cardiovascular disease and cancer in humans8 9 but also a mechanism of age related bone loss and sarcopenia.9 10 The high amount of lactose and therefore D-galactose in milk with theoretical influences on processes such as oxidative stress and inflammation makes the recommendations to increase milk intake for prevention of fractures a conceivable contradiction.
Because of the high content of lactose in milk, we hypothesised that high consumption of milk may increase oxidative stress, which in turn affects the risk of mortality and fracture. Meta-analyses of cohort studies for the association between dairy and milk intake in relation to mortality11 and fractures12 13 have displayed no clear pattern of risk, and evidence from randomised trials are lacking. Separating milk intake from the consumption of other dairy products may be of importance since a less pronounced induction of oxidative stress and inflammation in humans is expected with cheese and fermented dairy products (for example, soured milk and yogurt) because of their lower or non-existent lactose and galactose content,14 15 possible probiotic antioxidant and anti-inflammatory effects,16 17 18 and effects on gut microbiota.19 20 21 Indeed, a high intake of fermented milk products has been associated with a decreased risk of cardiovascular diseases,18 22 23 24 whereas a high milk intake is related to a tendency of an unfavourable risk profile for the development of diabetes and cardiovascular disease.18 23 24 We therefore assessed the relation between high milk intake with risk of death and fractures in women and men. We also studied biological markers of oxidative stress and inflammation in relation to milk intake in humans.
Objective To examine whether high milk consumption is associated with mortality and fractures in women and men.
Design Cohort studies.
Setting Three counties in central Sweden.
Participants Two large Swedish cohorts, one with 61 433 women (39-74 years at baseline 1987-90) and one with 45 339 men (45-79 years at baseline 1997), were administered food frequency questionnaires. The women responded to a second food frequency questionnaire in 1997.
Main outcome measure Multivariable survival models were applied to determine the association between milk consumption and time to mortality or fracture.
Results During a mean follow-up of 20.1 years, 15 541 women died and 17 252 had a fracture, of whom 4259 had a hip fracture. In the male cohort with a mean follow-up of 11.2 years, 10 112 men died and 5066 had a fracture, with 1166 hip fracture cases. In women the adjusted mortality hazard ratio for three or more glasses of milk a day compared with less than one glass a day was 1.93 (95% confidence interval 1.80 to 2.06). For every glass of milk, the adjusted hazard ratio of all cause mortality was 1.15 (1.13 to 1.17) in women and 1.03 (1.01 to 1.04) in men. For every glass of milk in women no reduction was observed in fracture risk with higher milk consumption for any fracture (1.02, 1.00 to 1.04) or for hip fracture (1.09, 1.05 to 1.13). The corresponding adjusted hazard ratios in men were 1.01 (0.99 to 1.03) and 1.03 (0.99 to 1.07). In subsamples of two additional cohorts, one in males and one in females, a positive association was seen between milk intake and both urine 8-iso-PGF2α (a biomarker of oxidative stress) and serum interleukin 6 (a main inflammatory biomarker).
Conclusions High milk intake was associated with higher mortality in one cohort of women and in another cohort of men, and with higher fracture incidence in women. Given the observational study designs with the inherent possibility of residual confounding and reverse causation phenomena, a cautious interpretation of the results is recommended.
A higher consumption of milk in women and men is not accompanied by a lower risk of fracture and instead may be associated with a higher rate of death. Consequently, there may be a link between the lactose and galactose content of milk and risk as suggested in our hypothesis, although causality needs be tested using experimental study designs. Our results may question the validity of recommendations to consume high amounts of milk to prevent fragility fractures.3 71 72 The results should, however, be interpreted cautiously given the observational design of our study. The findings merit independent replication before they can be used for dietary recommendations.
What is already known on this topic
A high milk intake is recommended for the prevention of osteoporotic fractures
Milk is the major dietary source of galactose intake
The addition of galactose by injection or in the diet is an established animal model of aging by induction of oxidative stress and inflammation
Results of previous research on the importance of milk intake for the prevention of fractures and the influence on mortality rates are conflicting
What this study adds
A high milk intake in both sexes is associated with higher mortality and fracture rates and with higher levels of oxidative stress and inflammatory biomarkers
Such a pattern was not observed with high intake of fermented milk products