VoedingsMagazine - nummer 4, december 2011, 24e jaargang
NutritionMagazine, No.4, 2011, Vol.24
Milk consumption fits in with a healthy dietary pattern
Chinese meta-analysis: dairy products lower the risk of type 2 diabetes
Chinese researchers have weighed seven thoroughly conducted cohort studies of milk consumption. The diabetes studies had been conducted in various countries (including four in the USA). This meta-analysis was the first milk meta-analysis with type 2 diabetes as endpoint.
The conclusion drawn from a recent meta-analysis conducted by Chinese researchers is that dairy products lower the risk of type 2 diabetes (1). They conclude that milk consumption fits in with a healthy dietary pattern. That conclusion had been drawn in previous publications; this is the first meta-analysis that confirms that relationship. Nutritionists of Soochow University in Suzhou, a university city north of Shanghai (southeast China), searched Medline (www.ncbi.nlm.nih.gov/pubmed/) for papers about the health effects of milk consumption on development of type 2 diabetes. They found in that database of medical-scientific publications seven usable studies that met their selection criteria (2—7). Only papers with original cohort data were eligible. When multiple publications had been devoted to the same study only the most recent one was included in the weighting analysis.
Study results based on a total of 330, 000 people The seven publications about such cohort studies (studies with many participants followed for many years) come from diverse countries: four from the USA, two from Asia (Japan and China) and one from Europe (England). It is remarkable that just one of the seven studies have been conducted in Europe (University Hospital of Wales, Cardiff), a continent with a high level of milk consumption. In the seven studies people aged 39—57 were followed for 5—25 years. A total of almost 330,000 people with a body mass index of 23.5—27.6 kg/m2 (i.e. with a normal weight or slight overweight) participated in the seven cohort studies. In six of the seven studies milk consumption was found to lower the risk of type 2 diabetes (1). Only the Japanese study reported a slightly higher risk for men (see Figure 1). In contrast, women were found in that study to benefit strongly from milk consumption. The authors suggest that this difference is probably attributable to the quantities of milk drunk: women drank much more milk than men. As is usual in all meta-analysis, the Chinese researchers of the meta-analysis weighted all study results. That weighting process revealed that people who drink much milk (more than 2—3 glasses per day) have a 14% lower risk of developing type 2 diabetes than people who drink little milk (1—2 glasses a week). A sensitivity analysis showed that all seven studies could be weighted on an average level: no single study played first fiddle in the eventual weighted means. The authors state that the risk benefits disappear when people use full-cream milk or other full-cream dairy products. They calculated that yoghurt could be given a high score, with a risk reduction of 17% (see Table 1). The Chinese nutritionists assume that vitamin D and calcium play a role in this risk reduction: their role in human metabolism leads to a lower body weight, which is an important factor in the development of type 2 diabetes. The researchers refer in this regard to one of the seven studies, the American cohort study of Anastassios Pittas of the Tufts-New England Medical Center in Boston (5). That study placed a specific focus on the roles of vitamin D and calcium.
Healthy trans fatty acids have a protective effect Other studies have shown that other milk components presumably influence the reduction in risk of type 2 diabetes. Milk components like whey proteins have often been suggested. In animal experiments, for example, whey proteins have been found to strengthen insulin resistance. The underlying notion is that milk proteins induce satiation and suppress appetite. It reduces the risks of overweight, hypertension and obesity all of which are risk factors for type 2 diabetes. A milk component that entered the stage recently is the trans fatty acid (TFA) trans-palmitoleic acid, a healthy TFA found in milk, cheese and yoghurt which is produced in the cow's rumen, in contrast with unhealthy TFA generated during industrial hydrogenation of fats (9). Milk contains small amounts of palmitoleic acid, which protects against the development of type 2 diabetes. Researchers of Harvard School of Public Health in Boston found in 2010 that trans-palmitoleic acid accounts for 0.5% of fatty acids in milk (8). They analysed blood from almost four thousand men and women aged 65 and over for the presence of palmitoleic acid. The medical data of this cohort of health workers have been collected for over 20 years. The researchers subsequently tried to find a correlation with type 2 diabetes and found that the prevalence of type 2 diabetes was much lower in the group of participants with high blood palmitoleic acid levels than in the group with lower levels. ‘This substance might be the missing link and help to explain why consumption of dairy products lowers the risk of type 2 diabetes,' the Harvard researchers state.
Explosive worldwide growth of prevalence of type 2 diabetes Type 2 diabetes has grown in the Netherlands like in practically the whole Western world in recent years into the most prevalent chronic disease. In the Netherlands, roughly one million people are suffering from diabetes. Figures of the Dutch Diabetics Fund, the main fund raiser for diabetes research, show that nine out of ten Dutch diabetes patients have type 2 diabetes (1). The estimate of one Dutch million diabetes patients includes the people (an estimated quarter) who do not know yet that they have diabetes. Those people have not yet examined for diabetes or received a diagnosis of diabetes. People in that category are hence not taking measures anyhow (pills, insulin) to keep the disease under control. If the prevalence continues to boom, some 1.3 million Dutch will have a diagnosis of diabetes by the year 2025 according to a prognosis of the National Institute for Public Health and the Environment RIVM (2). Since the population of the Netherlands will have been grown in 2025 to 16.9 million, 8% of the Dutch will be diabetic by then. The Netherlands would then follow the course of the USA, where 8% of the people are already diabetic now according to recent figures of the Centers for Disease Control and Prevention (3). The increase of the diabetes prevalence is stronger for men than for women. The 1.3 million diabetics by the year 20025 is not the end of the story. Expectations are that there will still be in 2025 some hundreds of thousands of diabetic people who have not yet received a diagnosis of diabetes. Ageing is one important cause of the explosive growth of the prevalence of diabetes. A possibly even stronger role is played by the growing prevalence of overweight. A third underlying factor identified by RIVM is that people engage less and less in physical activity.
The Dutch do not drink enough milk The latest Dutch National Food Consumption Survey (DNFCS) of RIVM (1) shows that the diet of most of the Dutch is too low in dairy products such as milk and yoghurt. The survey of consumer behaviour was conducted from 2007 to 2010. RIVM analysed data on consumption behaviours of ca. 3800 children and adults (aged 7—69) on the basis of questionnaires and interviews. The survey data revealed that most of the Dutch do not meet the Dutch guidelines for a healthy diet. Their diet is too poor in vegetables, fruit, fibre (from wholemeal bread and vegetables) and fish. Older people eat more healthy products and comply more with the dietary guidelines than younger people. The picture is different for dairy products: children almost meet the recommendations for milk and dairy products but adults use (far) too little of dairy products. As people age their diet becomes (far) too low in dairy products although they need more of it in that stage of life. People in their thirties drink 297—344 g of milk per day (two glasses) while three glasses of milk per day are recommended. People over 50 years of age drink 266—307 milk and milk products per day while, according to RIVM, 500—550 g per day is needed to prevent osteoporosis and other complaints. The Dutch Nutrition Council states that milk is an important source of vitamins and minerals. Low-fat milk and milk products are an important element of a healthy dietary pattern and are therefore included in the Food Guide Pyramid (called ‘Schijf van Vijf' in the Netherlands): two glasses a day for youth and three for adults, notably for people over 50 years of age. In addition, people should have one slice of cheese per day.
C. Van Rossum, H. Fransen, J. Verkaik-Kloosterman, E. Buurma-Rethans, M. Ocke (National Institute for Public Health and the Environment (2011) Nederlandse voedselconsumptiepeiling (VCP) [Dutch National Food Consumption Survey]. www.rivm.nl/vcp. (In Dutch.)
K. Tong, J.-Y. Dong, Z.-W. Wu, W. Li, L.-Q. Qin (2011) Dairy consumption and risk of type 2 diabetes mellitus: a meta-analysis of cohort studies. Eur. J. Clin. Nutr. 65: 1027–1031.
H. Choi, W. Willett, M. Stampfer, E. Rimm, F. Hu (2005) Dairy consumption and risk of type 2 diabetes mellitus in men: a prospective study. Arch. Intern. Med. 165: 997–1003
S. Liu, H. Choi, E. Ford, Y. Song, A. Klevak, J. Buring (2006) A prospective study of dairy intake and the risk of type 2 diabetes in women. Diabetes Care 29: 1579–1584.
R. van Dam, F. Hu, L. Rosenberg, S. Krishnan, J. Palmer (2006) Dietary calcium and magnesium, major food sources, and risk of type 2 diabetes in US black women. Diabetes Care 25: 417–424.
A. Pittas, B. Dawson-Hughes, T. Li, R. van Dam, W. Willett, J. Manson, F. Hu (2006) Vitamin D and calcium intake in relation to type 2 diabetes in women. Diabetes Care 29: 650–656.
P. Elwood, J. Pickering, A. Fehily (2007) Milk and dairy consumption, diabetes and the metabolic syndrome: the Caerphilly prospective study. J. Epidemiol. Commun. Health 61: 695–698.
K. Kirii, T. Mizoue, H. Iso, Y. Takahashi, M. Kato, M. Inoue (2009) Calcium, vitamin D and dairy intake in relation to type 2 diabetes risk in a Japanese cohort. Diabetelogia 52: 2542–2550.
D. Mozaffarian, H. Cao, I. King et al. (2010) Trans-palmitoleic acid, metabolic risk factors, and new-onset diabetes in US Adults. Ann. Intern. Med. 153: 790–799.