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Diabetes claim for low-fat yoghurt not proven

“Yoghurt is key to beating diabetes,” is the front page headline from the Daily Express.

The news is based on a study looking at the association between dairy intake and the risk of developing type 2 diabetes.

It found that the risk of the disease was reduced by 28% in people who reported eating large amounts of low fat yoghurt, compared to those who ate none. Similar results were found in people who ate more of all low fat fermented products such as fromage frais and cottage cheese.

A strength of this study is that it was based on people using food diaries to report their dairy intake around the time of eating. This is arguably better than using the traditional approach of asking people to recall their diet using a food frequency questionnaire, as this can be prone to inaccuracies.

However, the diaries were only used over a seven day period – hardly long enough to make an accurate assessment of dairy intake. It is quite possible that people’s diets did not stay the same during the 11 year follow-up.

At present it is not clear if fermented low fat products such as yoghurt, fromage frais and cottage cheese may help prevent diabetes, although it is an interesting theory.

There are many steps you can take to help prevent this condition developing. They include regular exercise, a balanced healthy diet, stopping smoking and reducing alcohol intake.

It’s important to note that many low fat products also contain large amounts of sugar which can contribute to obesity and is a risk factor for both diabetes and cardiovascular disease. Check food labels carefully.

Where did the story come from?

The study was carried out by researchers from the University of Cambridge. The research was part of a larger study funded by the Medical Research Council and Cancer Research UK.

The study was published in the peer-reviewed journal Diabetologia and has been provided on an open access basis so it is free to download (Zip, 173kb).

Both The Independent and the Daily Express failed to point out the study’s limitations or to obtain comments from independent experts on its findings. The Express claimed that people could help protect themselves from the condition by “adding a small but regular amount of these products to their diet.” 

Adding low fat fermented products to the diet (as opposed to these products replacing calories from other sources, such as snacks) is not supported by the study’s findings and it could contribute to weight problems.

What kind of research was this?

This was a nested case cohort study looking at the association between total dairy intake and the risk of developing type 2 diabetes. In this type of study, researchers identify one group of people who have the condition and randomly select a second group. The two groups are then compared for specific exposures (in this case, dairy intake). 

A nested case study is one based on a larger population, where only a subset of participants are used. This type of study is useful for helping to identify certain factors which may contribute to a particular outcome but it cannot prove cause and effect. 

The researchers say that dairy products have been associated with a lower risk of type 2 diabetes in previous research. Although whole fat dairy is also a source of unhealthy saturated fats, intake of which should be limited.

They also say their previous studies have found fermented dairy foods such as yoghurt to be associated with a lower risk of the disease. However they point out that to date the research has made use of retrospective food frequency questionnaires (FFQs) to estimate dairy intake, in which people have to recall what they ate, sometimes up to a year previously. This kind of recall can be unreliable.

The researchers also say that FFQs are limited by using predefined food lists. They say that food diaries are not restricted in the amounts or types of foods recorded and that people can record the food they eat at around the same time it is consumed.

What did the research involve?

The research was part of a larger study looking at diet and the risk of cancer and which is following over 25,000 participants aged 40-79 years. The people in the present study were followed up for 11 years.

From the 25,000, researchers randomly selected a subcohort of 4,000 participants for their study, of whom 143 had a diagnosis of diabetes. In addition, they identified 892 people who had been diagnosed with diabetes.

After excluding those who did not meet study criteria – for example, cases where diabetes status was uncertain, or who had missing food diary data and those with cardiovascular disease or cancer – 3,502 in the random cohort group and 753 people with diabetes remained for the final analysis.

The researchers ascertained diagnosis of type 2 diabetes using multiple sources including linkage with GP and local hospital registers and admissions data.

Researchers collected data on people’s dietary intake using a seven day food diary, which participants were advised how to use by trained nurses. These were completed around the time of eating. From the diaries, the researchers then estimated

  • Total dairy intake. These were food items where dairy was the main ingredient (not including dairy products used in cooking or as a minor ingredient in a bigger dish).
  • High-fat and low-fat dairy intake, using 3.9% fat (the fat content of whole milk in the UK) as a cut off point.
  • Dairy subtypes – yoghurt, cheese and milk.
  • Total fermented dairy products, subdivided into high and low fat.

Participants were divided into three groups (or tertiles), depending on their dairy product intakes, in the different categories.

They also collected details of participants’ social and economic background, lifestyle and health. This data included a four point activity index (to categorise participants as active, moderately active, moderately inactive or inactive); height, weight, waist circumference, body mass index (BMI) and blood pressure; and blood tests to determine vitamin C level which they say is a marker for fruit and vegetable intake.

They analysed the data using standard statistical methods and adjusted their results for factors which might influence the risk of diabetes (confounders) such as physical activity and BMI.

What were the basic results?

The researchers found that people who ate the most low fat fermented products were 24% less likely to have diabetes type 2 than those who ate the least (Tertile 3 vs Tertile1, hazard ratio (HR) 0.76 ,95% confidence Interval (CI) 0.60, 0.99). 

Those who ate the most low fat yoghurt were 28% less likely to have diabetes than those who ate the least (HR 0.72 95% CI 0.55, 0.95).

Total dairy, high-fat dairy, milk, cheese and high-fat fermented dairy product intakes were not associated with the development of diabetes. Nor was low fat dairy, after adjustment for confounders.

How did the researchers interpret the results?

The researchers say their findings suggest that in public health terms, the 24% lower risk of diabetes equates to 4.5 standard size portions (125g) per week of low fat fermented dairy products, largely comprising yogurt but also low fat cottage cheese and fromage frais.

They also calculate that substituting one portion of yoghurt for one portion of another dessert or snack (such as chocolate or cake) was associated with a 47% lower risk of type 2 diabetes. This suggests that some of the association between low fat yoghurt and a lower risk of diabetes may be due to not consuming these unhealthy alternatives.

They speculate that there are several ways in which low fat fermented dairy products might reduce diabetes risk. These include the presence of vitamin K (previously associated with a reduced risk of type 2 diabetes) in these products. They point out that probiotic bacteria found in these products may improve lipid (fat) levels in the blood as well as antioxidant status. These foods are also low in calories, consumption of which may also be associated with a lower risk of type 2 diabetes.

Conclusion

This study has several limitations.

  • People’s reported dairy intake was only collected once, at baseline, over a seven day period. It is quite possible that people’s diets did not stay the same during the 11 year follow-up period.
  • People’s dietary intake was self reported, which could affect reliability.
  • In their analysis, the researchers did not take account of dairy products included in cooking composite dishes.

In addition, although the researchers tried to take account of factors (confounders) which might affect the results, it is always possible that measured and unmeasured confounders had an influence.

It is unclear at present whether low fat fermented products might help cut the risk of diabetes, although it’s an interesting theory. There are many steps you can take to help prevent this condition developing. They include regular exercise, a balanced healthy dietstopping smoking and reducing alcohol intake. It’s important to note that many low fat products contain large amounts of sugar which can cause obesity, a risk factor for diabetes.

Though the results do suggest that swapping sugary snacks and sweets for low-fat yoghurts could improve your overall health; just make sure you find a low-fat yoghurt that is also low-sugar.  

 

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