Can dairy cut diabetes risk?
"A natural substance found in dairy products could help to prevent diabetes," reported the Daily Express. It said that research has found that people who had high levels of palmitoleic acid, a fatty acid, in their blood were 60% less likely to develop type 2 diabetes than those who had low levels.
This study found that whole-fat dairy consumption was linked with increasing levels of trans-palmitoleate in the blood and that this, in turn, was associated with lower fat, higher levels of good cholesterol, lower insulin resistance and a reduced risk of diabetes.
The Daily Express says that the study found benefit from low-fat dairy, but this is incorrect. Researchers only found an association between whole-fat dairy and reduced risk of diabetes.
Overall, the study is not robust evidence that dairy products can lower diabetes risk. The researchers say that their results support the need for additional, detailed experimental and clinical investigation to assess the potential health effects of trans-palmitoleate. At present, the best advice is to eat dairy products as part of a balanced diet.
Where did the story come from?
The study was carried out by researchers from Brigham and Women's Hospital, Harvard Medical School and Harvard School of Public Health, the National Institutes of Health, the University of New Mexico and the University of Washington. The study was published in the peer-reviewed medical journal Annals of Internal Medicine.
The newspapers are quick to suggest that a causal link has been proven here, and are overly optimistic about these findings. Though the Express focuses on low-fat dairy rather than whole-fat dairy, this appears to be in response to a statement by Diabetes UK, which cautioned that high-fat products can lead to weight gain. In fact, the study found an association between whole-fat dairy and reduced diabetes risk and the researchers found no such benefit with low-fat dairy consumption.
What kind of research was this?
Palmitoleic acid is a fatty acid found in most human tissues including fat tissue and in the liver. It is a component of fatty tissue. The fatty acid can be obtained from eating animal, vegetable and fish oils.
Animal experiments suggest that palmitoleic acid may directly protect against insulin resistance and problems with metabolic regulation, the researchers say. Insulin resistance describes the condition in which insulin becomes less effective at lowering the levels of sugar in the blood. The condition is a precursor to type 2 diabetes, in which blood glucose concentrations increase and cannot be controlled.
In this cohort study, researchers investigated the role of a particular form of palmitoleic acid, called trans-palmitoleate, in metabolic health. This particular variety was chosen because it can be distinguished from a type made in the liver and so it is easier to measure the effects of diet on its levels in the body. Trans-palmitoleate is derived from naturally occurring dairy trans fats and so intake of these through dairy products should affect the levels in the blood. The researchers wanted to test whether more palmitoleate in the diet would lower the incidence of diabetes.
What did the research involve?
For this study, the researchers used data from a previous study called the Cardiovascular Health Study. That study began in 1992, and included 5,201 adults over 65 years of age who had been selected randomly from communities in the USA. The participants had several examinations and evaluations and completed a number of questionnaires about their physical and mental health over the subsequent 10 years.
This research used 3,736 blood samples that had been collected in 1992. The researchers assessed levels of fatty acid in the blood and used laboratory methods to measure how much trans-palmitoleate was present in the samples. They also assessed insulin and fasting blood lipid levels, and measured a range of other compounds that might indicate possible confounding factors. The participants' height, weight and waist circumference were included in the analysis, as were any medications they were taking and whether they had been diagnosed with diabetes over the 10-year follow-up period.
The researchers validated (checked) their findings from this first group of people by performing the same analysis in a separate group of 327 women from another study called the Nurses' Health Study.
What were the basic results?
Higher levels of trans-palmitoleate were associated with a lower BMI, a lower waist circumference, lower total cholesterol and lower levels of C-reactive protein (a marker of inflammation). Whole-fat dairy consumption was most strongly associated with higher trans-palmitoleate levels.
In people who did not have diabetes at the start of the study, greater levels of trans-palmitoleate were associated with a lower risk of new-onset diabetes over the subsequent 10 years. Both of these analyses were adjusted for possible confounding factors including demographic, clinical, diet and other lifestyle factors.
How did the researchers interpret the results?
The researchers say that their findings suggest that intake of trans-palmitoleate, rather than the consumption of specific foods, is associated with a reduction in the risk of diabetes.
This large cohort study found an association between levels of trans-palmitoleate in the blood and a lower risk of metabolic problems and incidence of diabetes. These links appeared to be independent of a number of lifestyle, clinical and dietary factors. The study has a number of important limitations, some of which the researchers highlight:
- The analysis of the link between levels of trans-palmitoleate at the start of the study and metabolic risk was "cross-sectional". This means that the study took readings of blood levels of the acid and metabolic risk factors at the same time. This type of analysis cannot show causation because it cannot establish which came first. The researchers do say that reverse causality was unlikely, however.
- The researchers acknowledge that, although they took into account several major possible confounders, there may be other unmeasured confounders.
- The researchers attempted to validate some of these links in a separate group of nurses. However, because of the small sample size and consequent lack of study power, they were unable to validate the link with diabetes.
- Blood levels of trans-palmitoleate were only measured once at the beginning of the study and it is unlikely that they would have remained constant over a 10-year period.
- The researchers conclude that whole-fat dairy was most strongly associated with higher levels of trans-palmitoleate. They looked at the effects of low-fat dairy and found that consumption of this was actually associated with lower levels of trans-palmitoleate. They also noted that consumption of whole-fat dairy was associated with a reduced risk of type 2 diabetes while consumption of low-fat dairy was not. Although the Express focuses on the benefits of low-fat dairy, this appears to be in response to a statement by Diabetes UK, which cautions that high-fat products can lead to weight gain.
Overall, the study is not conclusive evidence that dairy products can lower diabetes risk. The researchers say that their results support the need for additional, detailed experimental and clinical investigation to assess the potential health effects of trans-palmitoleate. At present, the best advice is to eat dairy products as part of a balanced diet.
"A natural substance found in dairy products could help to prevent diabetes," reported the Daily Express. It said that research has found that people who had high levels of palmitoleic acid, a fatty acid, in their blood...
Links to Headlines
How drinking milk could protect you from diabetes. Daily Mail, December 21 2010
Low-fat milk and cheese can help prevent diabetes. Daily Express, December 21 2010
Links to Science
Mozaffarian D, Cao H, King IB, et al. Trans-Palmitoleic Acid, Metabolic Risk Factors, and New-Onset Diabetes in U.S. Adults. A Cohort Study. Annals of Internal Medicine, 2010: 790-799
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