A dieting craze in Sweden has been blamed for a surprise surge in cholesterol levels that may be putting people at increased risk of heart disease.
The link was uncovered by a 25-year study looking at diet and heart disease risk factors in the north of the country.
An unexpected rise in population cholesterol followed an increase in fat consumption in 2004, reversing earlier trends.
At the same time there was an explosion in the popularity of low carbohydrate/high fat (LCHF) diets in Sweden. The diets focus on cutting out sugar and starch while upping intake of fats, including saturated fat.
Its proponents, such as Dr Annika Dahlqvist, who is credited with starting the Swedish craze, argue that it helps maintain normal weight and blood sugar while freeing people to eat their favourite foods.
A poll last year suggested that a quarter of Swedes had at least partly adopted an LCHF diet. Around 5% had taken it up seriously, leading the DietDoctor website to hail a "Swedish low carb revolution". There were even reports of stores running out of butter due to increasing demand.
The new study was launched in 1986 after concerns about the high incidence of heart disease in northern Sweden.
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Scientists analysed data on food and nutrient intake, body weight, height and cholesterol levels compiled from more than 140,000 measurements and questionnaires between 1986 and 2010.
The results showed an initial reduction in fat consumption and cholesterol levels throughout the 1990s. This coincided with the nationwide introduction of an education and food labelling programme aimed at improving diet and health.
The Vasterbotten Intervention Programme (VIP) continues to this day.
After 2004, there was an unexpected change. Levels of total and saturated fat intake began to increase until they were higher than they were in 1986.
Blood cholesterol levels remained roughly unaltered between 2002 and 2007, but then showed a sharp rise.
In 2010 the average cholesterol level for men was around 5.5 millimoles per litre of blood, and for women slightly less. This was despite a significant increase in the number of people taking cholesterol-lowering drugs.
In earlier years, cholesterol levels had declined from a peak of more than six millimoles in 1986.
Over the whole 25-year period there was no sign that dieting of any kind helped people lose weight. Average body mass index (BMI), a measurement relating weight and height, showed a consistent rise in both men and women.
Professor Ingegerd Johansson, from the Umeå University, who led the research published in Nutrition Journal, said: "The association between nutrition and health is complex. It involves specific food components, interactions among those food components, and interactions with genetic factors and individual needs.
"While low carbohydrate/high fat diets may help short-term weight loss, these results of this Swedish study demonstrate that long-term weight loss is not maintained and that this diet increases blood cholesterol, which has a major impact on risk of cardiovascular disease."
In their paper, the researchers said the marked increase in cholesterol levels after 2007 was "a deep concern".
They added: "After 2004 fat intake increased, especially saturated fat and butter-based spread for bread and butter for cooking.
"Supportive opinions in media for high-fat diets seem to have had an impact on consumer behaviours. Initially beneficial and thereafter deleterious changes in blood cholesterol paralleled these trends in food selection, whereas a claimed weight reduction by high-fat diets was not seen in the most recent years.
"In contrast, BMI increased continuously over the 25-year period. These changes in risk factors may have important effects on primary and secondary prevention of CVD (cardiovascular disease)."
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