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Is the consumption of large quantities of milk in adulthood helpful or harmful?

Marinko Bilušić, MD, specialist in clinical pharmacology and toxicology
Polyclinic Bonifarm

Once again people's truth that any exaggeration is not good is scientifically validated. Namely, a rooted myth that is transmitted for generations both in preventive and in curative medicine, how drinking a lot of milk strengthens bones and contributes to vitality, is definitely called into question.

Although the recommendation that taking 3 cups of milk a day for the prevention and alleviation of osteoporosis seems quite logical, with respect to the ideal ratio of essential ingredients necessary for the development of a bone, such as calcium, phosphorus, vitamin D it appears that taking a larger quantity of milk per day does not reduce the risk of fractures, moreover can be harmful and even increase mortality.

We know that the research results in animal cannot be directly mapped to people, but it is significant that it was demonstrated in several animal species how chronic exposure to D-galactose causes faster aging of cells and their shorter life expectancy. And the milk is the richest source of galactose, both in the form of monosaccharides and in combination with glucose in form of disaccharide lactose.

Given this fact, numerous studies have been prompted to investigate the actual impact of milk on the human body. One of the largest researches of that kind started back in the 80's of last century in Sweden. It is a large epidemiological cohort study involving 61,433 women (aged 39-74 years) and who were followed for a little more than 20 years and 45,339 men (aged 45-79 years) who were followed for about 11.2 years. The aim of this study was to determine how the use of large quantities of milk affects the overall mortality and the number of bone fractures in women and men1.

Results of this research were recently published and are very interesting, possibly even quite surprising (maybe not for scientists). In fact, during these 20 years 15,541 women of the total number were killed and 17,252 of them had a fracture, of which 4259 were hip fractures. As for the group of men, over a period of 11.2 years, when they were followed 10,112 died and 5,066 had a fracture, of which 1,166 hip fracture.

In the primary analysis the connection between the consumption of dairy products and the total mortality, i.e. the number of fractures coincided and no obvious connection was noticed. Then a special analysis of the effect of milk on mortality and the occurrence of fractures was made, especially the analysis of the impact of dairy products (cheese, milk and yogurt), which due to the low fermentation contain an extremely small amount of lactose and galactose, and very significant difference were obtained.

Analyses made in the group of women indicate that already a daily consumption of a glass of milk over a prolonged period slightly increased the risk of total mortality, however daily consumption of 3 or more glasses of milk almost doubles the overall mortality (risk of 1.93) compared to the average consumption of less than 1 cup of milk daily. A significant increase risk of fractures in this group was noticed, especially hip fractures (risk 1.63).

Analyses made in the group of men who daily consume three or more glasses of milk the risk of total mortality compared to those who consume up to one cup a day indicate that the risk is much less elevated (risk 1.10), unlike to what is observed in women and is mainly related to higher mortality risk of cardiovascular diseases. As for the effect on fractures, consuming large quantities of milk in a man did not have any correlation with the number of fractures. The risk of their occurrence was neither prevented nor increased.

Unlike the consumption of milk, consumption of fermented dairy products showed a completely opposite, a protective effect. Thus, women who consumed higher amounts of cheese and other fermented dairy products recorded lower mortality and lower number of fractures by 10-15%. As for the men, adequately less increased risk when taking large quantities of milk and this protective effect of taking larger amounts of fermented dairy products in them is less pronounced.

Multivariate analysis showed that the joint taking of large quantities of milk, and fermented milk products makes the risk of total mortality and fractures smaller than what is seen with predominant consumption of only milk.
Certainly, it is logical and reasonable to ask why is the connection between milk consumption and increased mortality and a larger number of fractures in women so pronounced and obvious, and with men just indicated, and whether this means that men are not at higher risk of mortality, of various diseases and fractures with increased consumption of milk, or it is about something else. It is possible that gender differences play a role, however, much more likelier explanation is the small number of men involved in the study, especially the much shorter period of observing men than women, which means it could be expected that in a larger sample and after prolonged follow-up period risk in men would be very similar to that of women.

What is it in the milk that is responsible for these adverse effects? On the basis of previous studies and findings it is considered that a high content of galactose is the main cause of these negative effects of milk. The increased concentration of galactose probably has a variety of negative effects in cells. Thus, the laboratory analyses established the positive correlation between increased intake of milk and an increase in markers of oxidative stress (8-iso-PGF2α in urine) as well as markers of inflammation (serum interleukin 6), while at the intake of fermented dairy products, these markers are not increased 2,3. It is believed that the interleukin-6 is responsible for cardiovascular diseases4 but it also affects the reduction in mineralization of bones and formation of osteoporosis 5.

Finally it should be mentioned that the results of the Swedish research cannot be automatically mapped to the other races and ethnic groups, and especially not on children and adolescents. However, similar results of numerous other studies confirm the high probability of this correlation. So some ecological studies indicate increased mortality and increased number of cardiovascular diseases in countries with high milk consumption6,7. Other studies directly indicate an association between higher consumption of milk with some types of malignant disease and increased risk of cardiovascular diseases 8,9,10. Meta-analysis of cohort studies similar to this Swedish one, that investigated the relation of consuming of large quantities of milk with mortality and number of fractures showed significant heterogeneity.

Also, no matter that the results of all of these previous studies with distinct probability indicate the negative effects of long-term consumption of large quantities of milk, for a definite confirmation and official dietary recommendations at least another similar-sized, independent epidemiological study would be needed, since at this point there are no results of randomized, controlled clinical studies.

Literature:

  1. Michaëlsson K, Wolk A, Langenskiöld S, Basu S, Warensjö Lemming E, Melhus H, Byberg L. Milk intake and risk of mortality and fractures in women and men: cohort studies. BMJ. 2014 Oct 28;349
  2. Basu S, Michaëlsson K, Olofsson H, Johansson S, Melhus H. Association between oxidative stress and bone mineral density. Biochem Biophys Res Commun 2001;288:275-9
  3. Östman B, Michaëlsson K, Helmersson J, Byberg L, Gedeborg R, Melhus H, et al. Oxidative stress and bone mineral density in elderly men: antioxidant activity of alpha-tocopherol. Free Radic Biol Med 2009;47:668-73.
  4. IL6R Genetics Consortium Emerging Risk Factors Collaboration, Sarwar N, Butterworth AS, Freitag DF, Gregson J, Willeit P, et al. Interleukin-6 receptor pathways in coronary heart disease: a collaborative meta-analysis of 82 studies. Lancet 2012;379:1205-13.
  5. Clowes JA, Riggs BL, Khosla S. The role of the immune system in the pathophysiology of osteoporosis. Immunol Rev 2005;208:207-27.
  6. Hegsted DM. Fractures, calcium, and the modern diet. Am J Clin Nutr 2001;74:571-3.
  7. Segall JJ. Hypothesis: is lactose a dietary risk factor for ischaemic heart disease? Int J Epidemiol 2008;37:1204-8.
  8. Soedamah-Muthu SS, Ding EL, Al-Delaimy WK, Hu FB, Engberink MF, Willett WC, et al. Milk and dairy consumption and incidence of cardiovascular diseases and all-cause mortality: dose-response meta-analysis of prospective cohort studies. Am J Clin Nutr 2011;93:158-71
  9. Song Y, Chavarro JE, Cao Y, Qiu W, Mucci L, Sesso HD, et al. Whole milk intake is associated with prostate cancer-specific mortality among U.S. male physicians. J Nutr 2013;143:189-96.
  10. Lampe JW. Dairy products and cancer. J Am Coll Nutr 2011;30(5 Suppl 1):464S-70S.