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High milk intake and increased mortality - experts respond
High milk intake and increased mortality
- experts respond
A
high milk intake in women and men is not accompanied by a
lower risk of fracture and instead may be associated with a
higher rate of death, suggests Swedish
research.
The authors of a new study, published in the BMJ, say the increased risk of early death may be explained by the high levels of sugars in milk that, in previous animal studies, have been shown to increase oxidative stress and chronic inflammation.
The researchers tracked diet, lifestyle and health data for over 100,000 people in Sweden, finding that women who drank more than three glasses of milk per day had a greater risk of dying during the study period than those who drank just one glass of milk. Similar, although less pronounced, results were also seen in males.
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The SMC collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476; smc@sciencemediacentre.co.nz).
Dr Carol Wham, Senior Lecturer, Institute of Food, Nutrition and Human Health, Massey University, comments:
"This was an observation study and these types of studies are, by their nature, open to dispute. Although high milk intake was associated with higher fracture incidence in Swedish women this wasn't observed in the group of Swedish men. Typically fractures are more of a problem for older women than for older men and the links observed may relate to reporting of milk intake or some other factor. The best evidence would come from a randomised controlled trial, but no sufficiently powered trials of milk consumption have been reported because large numbers of participants would be required and compliance would be difficult to determine. As the authors suggest the results of this study need to be treated with caution and independently replicated before any cause and effect can be confirmed.
"To maintain good bone health New Zealanders are recommended to consume at least three servings of low or reduced fat milk and milk products a day and to maintain a healthy body weight and active lifestyle. Estimation of future fracture risk, including a measurement of bone density is prudent for women in their sixties or men in their seventies as valuable treatments are now available to reduce fractures from occurring."
From the UK Science Media Centre:
Catherine Collins, Principal Dietitian at St George's Hospital NHS Trust, said:
"The study as reported creates more questions than provides answers, and whilst an interesting read adds nothing to influence current UK dietary recommendations related to dairy products and health. It fails to confirm the hypothesis that a component of milk sugar - galactose - is clearly implicated in bone fracture and earlier mortality. The results were dietetically ambiguous, and there is no need to recommend change in peoples' usual consumption of milk and dairy products based on this work - even for those consuming twice the average intake.
"The authors hypothesised that galactose, a simple sugar found mainly in milk, contributes to oxidative stress and inflammation within the body, increasing the risk of fracture and earlier mortality. The naturally lower lactose/galactose content of yoghurt and other fermented dairy products, and the negligible content in cheese, would hypothetically generate different risks if galactose was directly involved in fracture risk and mortality. This hypothesis remains unproven.
"There are several limitations relating to this study. Firstly, the authors did not define the type of physical activity taken by participants, yet physical activity of a weight bearing nature such as swimming has a greater impact on maintaining bone 'strength' than non-weight bearing exercise such as swimming. Only duration and intensity of activity, translated into a standard unit of 'METS' was reported, and this value was insufficient to define the impact of subject activity on bone health.
"Secondly, whilst excluding 'pathological' bone fractures (caused by a health condition, such as breast cancer) from the data, the inclusion of high impact trauma fractures (e.g. fractures resulting from a car crash) with low impact fractures (e.g. person trips on rug at home, falls and sustains a hip fracture) is unusual. Without clarification the additional yearly fracture risk suffered by 1 in every 1000 women who reported being avid milk drinkers could merely reflect poor driving skills and loss of collision damage waiver.
"There is also the concern that dietary habits and exercise levels change over the years in many people, so a 'snapshot' of diet and exercise taken over a decade ago often fails to reflect current habitual diet. Furthermore, women consuming over a pint of semi skimmed milk per day would on average be consuming 550kcals (25-30% calorie needs in this age group) than women consuming the UK average of 1/3 pint a day, yet BMI remained remarkably consistent across all groups irrespective of energy intake. This discrepancy highlights the inaccuracies of longitudinal studies on diet and lifestyle in representing true intakes, and serves only to demonstrate how trends, rather than absolute amounts of a particular food, may influence the health of a population."
Prof Sue Lanham-New, Head of the Department of Nutritional Sciences, University of Surrey, said:
"This new study in BMJ has a number of limitations. Indeed the authors refer to this in the final sentence of their abstract. The study is only observational and the method used to assess diet is weak with no estimation of total energy intake or the extent of under reporting which means we are not sure how accurate the cut off values used for frequency of milk consumption is. Neither do we have a feel for the influence of physical activity or other life style habits important to bone or overall mortality. The effect of increasing BMI has not been fully investigated in this study.
"Also key is that all milk in Sweden is fortified with vitamin A. This may well be an important confounding factor that the authors do not address
"Milk and dairy products in the UK provide 50-60 percent of the calcium in our diet. We know that low calcium intakes (less than 400mg per day) is a risk factor for osteoporosis. Individuals should still be encouraged to consume a balanced diet from the five key food groups of which milk and dairy are key."
Gaynor Bussell, Dietitian and Public Health Nutritionist, said:
"An interesting study and well thought out, but as the authors say, some caution required here. Firstly food frequency questionnaires are not the gold standard for estimating food intake, as the authors point out. Secondly, we cannot assume cause and effect as there may be another factor that is hard to identify causing the increased mortality and fracture rate in women, but seems to track with milk consumption.
"We do know that increase in inflammation is associated with reduced bone density and so the effect of galactose certainly needs to be looked at in further studies. Calcium is required in the diet for bone health and we require approximately the daily amount of calcium that's in about a pint of milk. So milk is a very convenient source of calcium as well as many other vitamins and minerals. So as the author said, some caution is required here in interpreting the results and so I would urge some more research in this area that can back or refute these findings. One such study is insufficient to base public health decisions on."
Prof Brian Ratcliffe, Professor of Nutrition, Robert Gordon University, Aberdeen, said:
"This is a thought-provoking study. The authors seem to have taken care with the design and analysis of the study and they have clearly pointed out that their conclusions should not be over interpreted because of the possibilities of confounding of the data for which they have not accounted. The effects that they report for the highest consumption of milk compared with the lowest seem to be most marked in women compared with men. Although the authors have taken various factors into account such as smoking (the prevalence was higher in the high milk drinkers), BMI, and use of vitamin D supplements (lowest in the high milk consumers), they seem to have understated the possible role of alcohol intake.
"From their data, it looks as though most of the extra mortality in the highest milk drinkers is related to increased death rates from cardiovascular disease (there is little difference across the categories of intake for cancer) but the lowest level of milk consumption was associated with the highest consumption of alcohol (63% higher for women and 42% higher for men) with a graded inverse relationship with milk intake. There is considerable evidence for a cardio-protective effect of alcohol at modest intakes and this may be the source of more confounding than the authors' acknowledge.
"Furthermore, the lowest category of milk intake also had the lowest absolute intake of saturated fatty acids (and, in women, this was the lowest intake expressed as a percentage of energy intake). The results showing higher levels of biomarkers of oxidative stress and inflammation in relation to higher milk consumption is worthy of further investigation. Certainly the effects reported are only for milk, since the authors did not see detrimental effects for other dairy products, and the consumption of liquid milk in the UK has fallen considerably over the last three decades."
From the Australian Science Media Centre:
Associate Professor Natalie Sims, Associate Director of the St Vincent's Institute, St Vincent's Hospital, at the University of Melbourne, comments:
"This study seeks to answer a question that has been debated for many years: Is it possible for older people to increase their bone strength by eating more calcium? Recent studies have suggested that calcium tablets could be bad for your health, and many have thought that the same high levels of calcium in food or drink would be a safer alternative.
"This new study hasn't tested that directly, but has asked people about their dietary habits, and tracked information about their fractures and noted how many people died in each group. They found that those who drank three or more glasses of milk a day were not protected from fractures, and a higher proportion of people from that group happened to die during the course of the study. Importantly, those who drank less milk, or who consumed yogurt, did not have an increased risk of death. Three glasses of milk a day is three times higher than the NHMRC recommended guidelines for milk consumption. It's not suggesting that it's dangerous to drink one cup of low fat milk a day."
Associate Professor Kerrie Sanders, Honorary (Principal Fellow) at the NorthWest Academic Centre within the Department of Medicine at The University of Melbourne, comments:
"The study is conducted by a very well regarded research group in Sweden and included very large participant numbers (~100,000 women and men). Results from the same study published last year in The BMJ suggested that in these Swedish women a low intake of dietary calcium (less than 700mg/day; ie about two and half serves of dairy food daily) was associated with an increased risk of hip fracture, any fracture and of osteoporosis. This week The BMJ publishes further analysis from the same group . This time the researchers have separated the calcium intake from milk and the calcium intake from other milk products such as yoghurt, cheese and fermented milks. These results show that in Swedish women quite high intakes of milk but not these other dairy foods, is associated with an increased risk of hip fracture and death. Stronger associations of risk with milk were found in women compared to men.
"As the authors correctly point out results should be interpreted with caution and further research is needed to confirm these findings. The results may not be relevant to many Australian women because of genetic differences in the way milk is ingested in populations of northern European ancestry. Furthermore, the risk only appears to be increased in women who had a high intake of milk corresponding to three to six glasses per day. The study did not distinguish between low fat or regular milk. The bottom line is that adults should consume a wide variety of foods. If Australians want to be cautious before more research on the topic is published then it would be prudent to obtain their calcium requirement from a variety of foods such as yoghurt, cheese, fermented milk, almonds and other non-dairy high calcium foods as well as milk."
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