Introduction
Public health nutrition is facing many major challenges and three of these will shape food-related policy for decades to come. These are the rapidly increasing burden of obesity, the increasing age of populations and the challenge of increasing world food production by some 50% by 2030 to meet the increasing demands (House of Commons, 2009) whilst minimising the effect on the environment. The first two forces in particular will increase the risk of chronic disease substantially and the third will have other, perhaps obvious risks. This means that diet, a key moderator of chronic disease risk will play an increasingly important role. The marked rise in obesity and related type 2 diabetes is of particular concern since it is now also beginning to affect younger people (Nugent, 2003). In the UK, the recent Foresight Report on obesity predicts that by 2050 some 55% of UK adults will be obese (Butland et al., 2007). Similarly, projections to 2050 of the age structure of the EU 25 suggest that its old-age dependency ratio (the number of people aged 65 and over relative to those between 15 and 64) will double to 54% by 2050 (Carone and Costello, 2006). The outcome of these trends will, if not moderated, lead to unsustainable costs of health care.
Food production in the future will have to balance many factors, environmental cost, financial cost but crucially their role in reducing the risk of chronic disease. Animal-derived foods in particular have been highlighted as having a high environmental cost but much less attention has been paid to their role in healthy diets that have impact from childhood to old age.
Milk and children
As early as 1926, the UK Medical Research Council identified that daily consumption of an extra 568 ml of milk per day by boys gave rise to a sizeable increase in growth (Corry Mann, 1926). This formed the basis of the school milk policy which is now almost non-existent in the UK, yet the value of milk for children remains recognised and in 2000 the UN Food and Agriculture Organisation declared a World School Milk Day.
Special concern focuses on the mineral/trace element intakes of children and young people. Adequate dietary calcium, phosphorus and magnesium supply is essential for optimal bone growth. A sub-optimal calcium intake reduces bone density more quickly than it affects growth (Moore et al., 1963) and there are now cases of rickets re-emerging in the UK. In the UK, calcium intake in children aged 7 to 18 years of age is now substantially sub-optimal, especially in girls. (Bates et al., 2012). This is of particular concern because many individuals in the UK also have sub-optimal vitamin D status which would further contribute to reduced bone development. Milk and dairy products are rich sources of dietary calcium and the fall in milk consumption over recent years is no doubt a key contributor to the observed sub-optimal calcium intake by many children.
There is also now evidence that many young women have sub-optimal iodine status, especially during pregnancy. Moreover recent findings show that children born to women of sub-optimal status have a slower cognitive development than those from women of adequate status (Bath et al., 2013). Milk is the main source of dietary iodine and whilst the reduced status in young women is almost certainly the result of reduced consumption, the problem has highlighted how variable milk iodine concentration can be. On average, milk in summer has about half the iodine concentration of winter milk and in equivalent seasons, organic milk has about half the concentration of conventional milk (Bath et al., 2012).
Milk and dairy products in adulthood
Milk consumption and cardiovascular disease
Many conclusions about the risk of cardiovascular disease (CVD) resulting from consumption of milk and dairy products have been based on the interpretation of risk makers for disease, notably blood cholesterol. This probably provides an incomplete picture of risk and led Alvarez-Leon et al. (2006) to highlight that statements about the benefits and risks of dairy product consumption appear to be based on selected physio-pathological data (such as blood cholesterol) and not on valid epidemiological evidence. Indeed, as they commented ‘public health nutrition should not be unaware of the need for evidence-based conclusions’.
Although they do have limitations, for now the best evidence on dairy food consumption, health and survival, comes from long-term cohort studies with disease events and death as the outcomes. Moreover, the most reliable conclusions are those derived by combining the results from many studies in a so-called meta-analysis. To date, the largest meta-analysis for CVD is that of Elwood et al. (2010) based on 17 studies judged of sufficient quality for inclusion. Together these comprised a total of 4.3 million person-years, during which 16,212 heart disease events or deaths attributed to heart disease occurred. Within each study an estimate of the relative risk (RR) of cardiovascular disease and diabetes was made within a group of subjects with the highest milk/dairy consumption compared with a group of subjects with the lowest consumption. Overall, the meta-analyses showed a reduction in risk in the subjects with the highest dairy consumption relative to those with the lowest intake, i.e. 0.87 (95% confidence interval (CI) 0.77, 0.98) for all-cause deaths, 0.92 (95% CI 0.80, 0.99) for ischaemic heart disease, 0.79 (95% CI 0.68, 0.91) for stroke and 0.85 (95% CI 0.75, 0.96) for incident diabetes. Stroke is of particular interest because of the clinically important reduction in blood pressure which has been repeatedly shown to be associated with a high intake of milk/dairy foods. In all types of stroke (ischaemic, haemorrhagic and subarachnoid bleed) raised blood pressure is important and the risks of all three appear to be reduced by high milk and dairy food consumption by roughly the same amount.
Hypertension is one of the leading risk factors in the development of heart disease and stroke, and is estimated to be responsible for around 13% of all deaths worldwide (Alwan, 2011). As noted above, there are now many studies which show a reducing effect of milk and dairy product consumption on blood pressure (e.g. Griffith et al., 1999; Livingstone et al., 2013). However, in addition to effects on blood pressure, the effect of milk/dairy foods on new, more holistic markers of vascular health such as vascular stiffness is becoming evident. Both cross-sectional (Crichton et al., 2012) and longitudinal (Livingstone et al., 2013) cohort studies have shown significant negative relationships between dairy product intake and measures of arterial stiffness. Studies are limited in number, but suggest that milk proteins probably play a key part in reducing both blood pressure and arterial stiffness.
Because of its link to obesity, Type 2 diabetes is another major disease of interest. The meta-analysis of Elwood et al. (2010) involving a total of 7,779 new cases of diabetes during 1.7 million person years showed a 15% overall reduction in diabetes (RR 0.85; 95% CI, 0.75, 0.96) associated with a high milk/dairy intake. It is particularly interesting that two recent studies (Mozaffarian et al., 2010; 2013) have suggested a substantial negative relationship between one of the minor fatty acids present in milk fat (trans-palmitoleic acid) and risk of diabetes. Time will tell if this reflects a real mechanism.
Dairy food consumption and cancer
Relationships between dairy food consumption and cancer were examined in detail by the World Cancer Research Fund and the American Institute for Cancer Research (WCRF, 2007). Their report stated that milk consumption probably protects against colorectal cancer (RR 0.78; 95% CI, 0.69, 0.88), possibly bladder cancer (RR 0.82; 95% CI, 0.67, 0.99), but is associated with a possible increase in prostate cancer (RR 1.05; 95% CI, 0.98, 1.14). An updated meta-analysis (Aune et al., 2012) was based on a total of 19 cohort studies with more than one million subjects, of which 11,579 developed colon cancer. The results showed that milk and total (i.e. all types) dairy products, but not cheese or other dairy products (mainly butter, yoghurt, ice cream and fermented milk), were associated with a reduction in colorectal cancer risk. The summary RR were 0.83 (95% CI, 0.78, 0.88) per 400 g/day of total dairy products and 0.91 (95% CI, 0.85, 0.94) per 200 g/day of milk intake.
These relationships with cancer are poorly understood at present, though a beneficial effect on colon cancer risk has also been reported with calcium supplements. Much more work on this subject is needed.
Milk and dairy products and the elderly
Partly because of the increased life expectancy, there is growing concern about increased prevalence of dementia with the numbers in the UK projected to double by 2040 (Jagger et al., 2009). There are few studies which have examined any relationship between milk consumption and risk of dementia. One study which did is the Adult Health Study (Yamada et al., 2003), a prospective cohort of 1,774 subjects in Hiroshima, Japan born before September 1932. Between 1992 and 1997, 1,660 were shown to have no dementia whilst 114 had dementia (51 with Alzheimer’s disease, 38 with vascular dementia). Vascular dementia prevalence increased significantly with age, with higher systolic blood pressure and crucially, with lower milk intake. The mechanism whereby milk provided protection cannot be stated with certainty but given other evidence linking milk consumption with lowered blood pressure, it would seem probable that this mechanism was involved. Clearly much further research is needed in this area.
Another increasing problem of the elderly is loss of skeletal muscle mass and strength (sarcopenia) which is linked with a progressive decline in functional performance and increased risk of bone breakages following a fall. It has been known for some time that resistance exercise training can reduce loss of muscle and improve overall strength but more recently the value of dietary protein has been identified. In particular it was shown that whey protein was more effective at reducing muscle mass than soya protein especially if consumed after exercise (Phillips et al., 2009). Whey protein is widely used by the sports industry but the future may see it have a key role in diets of older people.
Conclusions
Milk and milk products have had a very mixed press in recent times from being blamed for many disorders to more measured considerations of their role in a balanced diet. Whilst most people know they are very important sources of dietary nutrients such as calcium and iodine, there is more uncertainty about whether or not these foods contribute to increased risk of cardiovascular and other chronic diseases. The evidence from long term cohort studies that high milk consumption does not increase cardiovascular disease risk and indeed may provide benefit is now pretty unequivocal, although the effects of butter and cheese as well as benefits of fat reduced milk and saturated fat reduced milk are less certain. There is now good evidence that specific health issues related to children, pregnant women and the elderly can be alleviated by milk or components of milk and these effects are not all explained by traditional nutrition. They must however be considered when debating future sustainable food production; for example, a simplistic replacement of milk proteins by plant proteins may not provide the same long term health benefits.
References
Alvarez-Leon, E-E., Roman-Vinas, B., Serra-Majem, L.S. (2006). Dairy products and health: a review of the epidemiological evidence. Brit. J. Nutr. 96: 594–599.
Aune, D., Lau, R., Chan, D.S.M., Vieira, R., Greenwood, D.C., Kampman, E. and Norat, T. (2012). Dairy products and colorectal cancer risk: a systematic review and meta-analysis of cohort studies. Ann. Oncol. 23: 37–45.
Bates, B., Lennox, A., Prentice, A., Bates, C. and Swan, G. (2012). National Diet and Nutrition Survey Headline results from Years 1, 2 and 3 (combined) of the Rolling Programme (2008/2009–20010/11). A survey carried out on behalf of the Department of Health and the Food Standards Agency.
Bath, S.C., Button, S. and Rayman, M.P. (2012). Iodine concentration of organic and conventional milk: implications for iodine intake. Brit. J. Nutr. 107: 935–940.
Bath, S.C., Steer, C.D., Golding, J., Emmett, P. and Rayman, M.P. (2013). Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet, Published Online May 22, 2013 http://dx.doi.org/10.1016/S0140–6736(13)60436–5.
Butland, B., Jebb, S., Kopelman, P., McPherson, K., Thomas, S., Mardell, J. and Parry, V. (2007). Foresight Future Choices – Project Report Tackling Obesities: 2nd Edition. Retrieved 2 June 2009, from: www.foresight.gov.uk
Carone, G. and Costello, D. (2006). Can Europe Afford to Grow Old? Finance and Development, 43. Retrieved 5 June 2009, from: www.imf.org/external/pubs/ft/fandd/2006/09/carone.htm
Jagger, C., Matthews, R., Lindesay, J., Robinson, T., Croft, P. and Brayne, C. (2009). The effect of dementia trends and treatments on longevity and disability: a simulation model based on the MRC Cognitive Function and Ageing Study (MRC CFAS). Age and Ageing 38, 319–325.
Livingstone, K.M., Lovegrove, J.A., Cockcroft, J.R., Elwood, P.C., Pickering, J.E. and Givens, D.I. (2013). Does dairy food intake predict arterial stiffness and blood pressure in men?: Evidence from the Caerphilly Prospective Study. Hypertension 61: 42–47.
Moore, T., Impey, S.G., Martin, P.E.N. and Symonds, K.R. (1963). Meat diets. II. Effect of the age of rats on their ability to withstand the low calcium intake induced by a diet of minced beef. J. Nutr. 80: 162–170.
Mozaffarian, D., Cao, H., King, I.B., Lemaitre, R.N., Song, X., Siscovick, D.S. and Hotamisligil, G.S. (2010). Trans-Palmitoleic acid, metabolic risk factors, and new-onset diabetes in U.S. adults. Ann. Intern. Med. 153: 790–799.
Mozaffarian, D., de Oliveira Otto, M.C., Lemaitre, R.N., Fretts, A.M., Hotamisligil, G., Tsai, M.Y., Siscovick, D.S. and Nettleton, J.A. (2013). trans-Palmitoleic acid, other dairy fat biomarkers, and incident diabetes: the Multi-Ethnic Study of Atherosclerosis (MESA). Am. J. Clin. Nutr. 27: 854–61.
Corry Mann, H.C. (1926). Diets for boys during Phillips, S.M., Tang, J.E. and Moore, D.R. the school age. Medical Research Council (2009). The role of milk- and soy-based protein Special Report Series No. 105, London: HMSO. in support of muscle protein synthesis and Available via The James Lind Library, http:// muscle protein accretion in young and elderly www.jameslindlibrary.org/. persons. J. Am. Coll. Nutr. 28: 343 –354.
Crichton, G.E., Elias, M.F., Dore, G.A., Yamada, M., Kasagi, F., Sasaki, H., Masunari, Abhayaratna, W.P. and Robbins, M.A. (2012). N., Mimori, Y. and Suzuki, G. (2003). Relations between dairy food intake and arterial Association between dementia and midlife risk stiffness. Hypertension 59: 1044–1051. factors: the radiation effects research foundation Adult Health Study. J. Am. Geriatr. Soc. 51: 410–414.
Elwood, P.C., Pickering, J.E., Givens, D.I. and Gallacher, J. (2010). The consumption of milk and dairy foods and the incidence of vascular disease and diabetes: An overview of the evidence. Lipids 45: 925–939.
Griffith, L.E., Guyatt, G.H., Cook, R.J., Bucher, H.C. and Cook, D.J. (1999). The influence of dietary and non-dietary calcium supplementation on blood pressure: an update meta-analysis of randomised controlled trials. Am. J. Hypertens. 12, 84–92.
House of Commons (2009). Securing food supplies up to 2050: the challenges faced by the UK. Environment, Food and Rural Affairs Committee, Fourth Report of Session 2008–09 Volume I. London: The Stationery Office Limited, 68 pp.
WCRF (2007). World Cancer Research Fund/American Institute for Cancer Research: Food, Nutrition, Physical Activity and the Prevention of Cancer: A Global Perspective. AICR (American Institute for Cancer Research), Washington, DC, 517 pp.