No evidence Nordic diet prevents heart disease
"Roll out the reindeer and berries for Nordic health fix," The Daily Telegraph tells us, reporting that a Scandinavian diet can lower cholesterol and may reduce the risk of heart disease. Meanwhile, the Mail Online website tells us to "forget the Mediterranean diet" in favour of Nordic foods.
There is a wide range of evidence that the Mediterranean diet, with plenty of fresh fruit and vegetables as well as beans, wholegrains, olive oil and fish, can be good for the heart. But is the same true for the staples of Nordic diets? The current study is not able to answer this question for us.
The study in question involved 200 white Nordic people with metabolic syndrome who had either a ‘healthy’ or ‘average’ Nordic diet for up to six months.
Researchers found that the ‘healthy’ diet had no effect on glucose tolerance and insulin sensitivity, nor did it improve weight or blood pressure. They did find small decreases in ‘bad’ cholesterol levels and fat-binding proteins in the ‘healthy’ group, but these were not the main outcomes investigated and are of limited importance for our health. Because this study of the Nordic diet was quite short, it is not clear whether these changes would have any lasting benefits.
If you are worried about your cholesterol levels you are advised to follow a healthy diet with plenty of fresh fruit and vegetables and low amounts of saturated fat and sugar.
Where did the story come from?
The study was carried out by researchers from the University of Eastern Finland and other academic institutions in Scandinavia. Funding was provided by various sources including NordForsk, the Academy of Finland, the Finnish Diabetes Research Foundation and the Finnish Foundation for Cardiovascular Research.
The study was published in the peer-reviewed Journal of Internal Medicine.
The Daily Telegraph and the Mail Online have both exaggerated the findings of this study. The researchers did not find any significant difference in the outcomes it had set out to examine – insulin sensitivity and glucose tolerance. These are two biological markers used to assess the risk of developing diabetes.
The only significant changes seen were a small increase in non-HDL cholesterol and a change in one inflammatory marker. These slight changes cannot be interpreted to mean that a person is at lower risk of cardiovascular disease as a result of eating a healthy Nordic diet.
What kind of research was this?
This was a randomised controlled trial (the SYSDIET trial) investigating the effect that a Nordic diet might have on levels of cholesterol and other fats in the blood, blood pressure, insulin sensitivity and inflammatory markers. These are all components of what is medically known as ‘metabolic syndrome’ – a collection of risk factors associated with increased risk of cardiovascular disease.
Insulin is the hormone that controls blood glucose levels. It is produced by our bodies when blood glucose levels are high and it causes the body’s cells to take up glucose and use it for energy. Measuring insulin sensitivity means looking at how sensitive the body’s cells are to the action of insulin. People with reduced insulin sensitivity (also called insulin resistance or glucose intolerance) can’t regulate their blood sugar very well, which means they are at risk of developing – or may already have – type 2 diabetes.
A randomised controlled trial such as this is the best way of looking at the short-term effects of the diet (the trial was up to six months long). However, it cannot reliably show what the longer term effects of the diet are, or its effect on disease outcomes such as heart attack or stroke.
What did the research involve?
The SYSDIET trial recruited people by advertisement at six centres – two in Finland, two in Sweden, one in Iceland and one in Denmark.
Eligible participants were required to have features of metabolic syndrome:
- a body mass index categorising them as overweight or obese (BMI 27-38), and
- glucose intolerance (defined by set criteria)
The researchers did not include people with major chronic diseases, excluding metabolic syndrome.
Two hundred people took part in the study. The average age was 55, the average BMI 31.6, 67% were women and all were of white ethnicity. They were randomly allocated to follow either the ‘healthy Nordic diet’ or a control diet for 18-24 weeks (the shorter duration was used in four of the six centres).
The control group was described as following the ‘average Nordic diet’. The control diet was based on the same number of calories as the ‘healthy’ diet, but included higher salt and saturated fat, and lower fibre, fish, fruit and vegetables. The researchers gave the participants the key food items for the diet they were following (for example, the Nordic diet group were given wholegrain cereals, while the control group got low fibre cereals).
At the start of the study, the researchers measured the participants’ height, weight and blood pressure, and did various tests on their blood. The participants also underwent an oral glucose tolerance test. At 12 weeks and at their final visit (18 or 24 weeks) these measurements were repeated. At the time of starting the study, and at weeks two, 12, 18 and 24 participants completed a four-day food diary to check their compliance with their assigned diets. Participants were advised to keep weight and physical activity constant and not to change their smoking and drinking habits or drug treatment throughout the study.
The researchers were mainly interested in insulin sensitivity and glucose tolerance. However, their secondary outcomes of interest were other components of metabolic syndrome including blood fats, blood pressure and inflammatory markers.
What were the basic results?
The study was completed by 92% of those assigned to the ‘healthy’ Nordic diet, but only 73% of those assigned to the control diet.
Over the course of the trial there were no significant changes in body weight within either group, and no differences in weight between groups at the end of 18-24 weeks. There were also no significant differences between groups in glucose tolerance or insulin sensitivity (the main outcomes the trial set out to examine), and neither were there any differences in blood pressure.
There was no significant difference in the actual levels of LDL (often described as ‘bad cholesterol’) and HDL (so-called ‘good cholesterol’) levels.
A significant difference was found in non-HDL cholesterol levels between the healthy and control groups, with non-HDL levels in the healthy Nordic diet group being much lower. Non-HDL cholesterol is a measurement of total cholesterol level minus HDL. While the lower levels of non-HDL cholesterol levels found in the healthy Nordic diet are encouraging in terms of health outcomes, they do not represent the sort of important improvement that would be signified by a drop in LDL levels.
There was a borderline significant decrease in the ratio of LDL to HDL cholesterol in the ‘healthy’ diet group. There was also a significant decrease in the ratio of two fat-binding proteins in the ‘healthy’ diet group, and a significant increase in the level of one inflammatory marker in the control group.
How did the researchers interpret the results?
The researchers conclude that the ‘healthy Nordic diet’ improves blood fat profile and has a beneficial effect on low-grade inflammation.
Conclusion
This was a well-designed randomised controlled trial that took place across several Nordic locations. The study took careful clinical measures of elements of metabolic syndrome at several points during the trial, and used food diaries at regular intervals to check compliance to the assigned diet.
However, it provides no reliable proof that the ‘healthy’ Nordic diet is any better than the ‘average’ Nordic diet at improving components of metabolic syndrome and, in turn, no proof that it reduces the risk of cardiovascular disease.
Importantly, this study found no significant results for its main aim (which was to see if the healthy ‘Nordic’ diet affected glucose tolerance and insulin sensitivity of people with metabolic syndrome). The study also found that the Nordic diet had no effect on weight or blood pressure. The only statistically significant differences were small borderline significant decreases in non-HDL cholesterol levels and fat-binding proteins among people following the healthy Nordic diet. People following the normal Nordic diet were found to have increases in one inflammatory marker.
However, the effects of these two diets on the cardiovascular system have only been assessed in the short-term. It is not clear whether these small changes would have had any real life significance to people (for example, whether they would stop people dying of heart disease) if they were continued for longer.
It is worth noting that the study involved people of Nordic, white ethnicity and those with metabolic syndrome so its results may not be applicable to other groups. The higher dropout rate in the control group also reduces the reliability of the results.
Finally, it is also worth noting that, despite the media hype, this study was not directly comparing a ‘healthy’ Nordic diet with a ‘healthy’ Mediterranean diet. Until there is reliable evidence comparing the two dietary patterns, this research cannot tell us which is the best way to keep the heart healthy.
