High milk intake linked with higher fractures and mortality, research suggests

Women who drank more than three glasses of milk a day had a higher risk of death than women who drank less than one glass of milk a day. Credit: © Africa Studio / Fotolia

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 observational research published in The BMJ this week.

This may be explained by the high levels of lactose and galactose (types of sugar) in milk, that have been shown to increase oxidative stress and chronic inflammation in animal studies, say the researchers.

However, they point out that their study can only show an association and cannot prove cause and effect. They say the results "should be interpreted cautiously" and further studies are needed before any firm conclusions or dietary recommendations can be made.

A diet rich in milk products is promoted to reduce the likelihood of osteoporotic fractures, but previous research looking at the importance of milk for the prevention of fractures and the influence on mortality rates show conflicting results.

So a research team in Sweden, led by Professor Karl Michaëlsson, set out to examine whether high milk intake may increase oxidative stress, which, in turn, affects the risk of mortality and fracture.

Two large groups of 61,433 women (aged 39-74 years in 1987-1990) and 45,339 men (aged 45-79 years in 1997) in Sweden completed food frequency questionnaires for 96 common foods including milk, yoghurt and cheese.

Lifestyle information, weight and height were collated and factors such as education level and marital status were also taken into account. National registers were used to track fracture and mortality rates.

Women were tracked for an average of 20 years, during which time 15,541 died and 17,252 had a fracture, of whom 4,259 had a hip fracture.

In women, no reduction in fracture risk with higher milk consumption was observed. Furthermore, women who drank more than three glasses of milk a day (average 680 ml) had a higher risk of death than women who drank less than one glass of milk a day (average 60 ml).

Men were tracked for an average of 11 years, during which time 10,112 died and 5,066 had a fracture, with 1,166 hip fracture cases. Men also had a higher risk of death with higher milk consumption, although this was less pronounced than in women.

Further analysis showed a positive association between milk intake and biomarkers of oxidative stress and inflammation.

In contrast, a high intake of fermented milk products with a low lactose content (including yoghurt and cheese) was associated with reduced rates of mortality and fracture, particularly in women.

They conclude that 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, although causality needs be tested.

"Our results may question the validity of recommendations to consume high amounts of milk to prevent fragility fractures," they write. "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."

Michaëlsson and colleagues raise a fascinating possibility about the potential harms of milk, says Professor Mary Schooling at City University of New York in an accompanying editorial. However, she stresses that diet is difficult to assess precisely and she reinforces the message that these findings should be interpreted cautiously.

"As milk consumption may rise globally with economic development and increasing consumption of animal source foods, the role of milk and mortality needs to be established definitively now," she concludes.

Source: BMJ-British Medical Journal

Boy moms more social in chimpanzees: Watching adult males in action may help youngsters prepare

Infant chimpanzee in Gombe National Park, Tanzania. Nearly four decades of observations of Tanzanian chimpanzees has revealed that the mothers of sons are about 25 percent more social than the mothers of daughters. Boy moms were found to spend about two hours more per day with other chimpanzees than the girl moms did. Credit: © Impala / Fotolia

Nearly four decades of observations of Tanzanian chimpanzees has revealed that the mothers of sons are about 25 percent more social than the mothers of daughters. Boy moms were found to spend about two hours more per day with other chimpanzees than the girl moms did.

Chimpanzees have a male-dominated society in which rank is a constant struggle and females with infants might face physical violence and even infanticide. It would be safer in general to just avoid groups where aggressive males are present, yet the mothers of sons choose to do so anyway.

"It is really intriguing that the sex of her infant influences the mother's behavior right from birth and that the same female is more social when she has a son than when she has a daughter," said Anne Pusey, chair of Evolutionary Anthropology at Duke.

The researchers believe that the mothers are giving the young males the opportunity to observe males in social situations, even while still clinging to their mothers. This gives the youngsters a start on developing the social skills they'll need to thrive in the competitive world of adults.

The findings are based on an analysis of 37 years of daily observations of East African chimpanzeess from the Gombe National Park in Tanzania. Duke University houses all of the data from the famous Kasekela chimpanzee community in the Jane Goodall Institute Research Center, which contains more than 50 years of observational data all the way back to Jane Goodall's first hand-written observations from the early 1960s.

The data largely consist of "follows," in which a researcher focuses on one chimpanzee and notes her behaviors and interactions with others throughout the day. Duke scholars led by Pusey are now working on digitizing the entire collection of Gombe data in the Goodall archive to enable more longitudinal studies of this kind.

"Drawing from the long-term datasets, we were able to investigate patterns within the same mother, examining how she behaved with her sons versus with her daughters," said lead author Carson Murray, an assistant professor at George Washington University, who was a PhD student under Pusey. "These results are even more compelling than a general pattern, demonstrating that the same female behaves very differently depending on the sex of her offspring."

For this study, researchers measured gregariousness based on three kinds of analyses. They looked at how much time a mother spent with other adults who were not immediate family members; the average size of the mother's party and its composition; and the proportion of time a mother spent in mixed-sex and female-only parties.

For the most part, mothers with offspring spend their time alone or with adult daughters and other dependents. Adult males are the more gregarious sex, forming coalitions with other males to assert rank, defend their territory and hunt as a group.

Mothers with sons were found to spend more time with others and to associate with more of their kin. During the first six months of an infant's life, mothers with sons spend significantly more time in mixed-sex parties than mothers with daughters.

At 30 to 36 months, chimpanzee infants start moving around more on their own without being carried and spend most of their time out of mother's reach. At this age, the male infants start having more interactions with unrelated chimpanzees, especially adult males. Their female counterparts are significantly less social.

As the offspring get older and range further from their mothers, the young males have more social partners over the course of the day. Juvenile and adolescent males watch their adult counterparts carefully and often mimic the behaviors they see, including charging displays and copulation.

"Mothers obviously increase social exposure for their young male infants," Murray said. "This finding leads to a larger question about how social exposure might shape gender-typical behavior in humans as well."

This study also suggests it is possible the sons themselves are driving the increased gregariousness later in life. In early infancy, the boy mothers spend about the same time in female-only groups that the girl moms do. But as their sons become older, boy moms spend more time in female-only, nursery groups, probably because the young males are attracted to the offspring of other females as playmates.

"One of the most surprising results to me was that mothers with young females still have lower association with their relatives," Murray said. "As we argue in the paper, this suggests that social exposure is less critical to females in general."

Social exposure has a potential downside too. Females with low rank are known to experience more social stress in large groups, and there is always a risk of infanticide against the young chimpanzees. Perhaps the best way to avoid having infants killed is to steer clear of groups, which the mothers do up to 70 percent of the time.

"Mothers with infant daughters were likely to be avoiding competitive and stressful situations," Murray said. "While mothers with sons seem willing to incur those costs for the benefit of having their sons socialized."

Source: Duke University.