Low-carb, high-fat diets could be to blame for an upsurge in unhealthy cholesterol levels in the blood of Swedes, a new study suggests.

The 25-year study focused on diet and heart disease risk factors in northern Sweden, where men had some of the highest prevalence of cardiovascular disease worldwide in the 1970s.

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Consumption of bread and other carbohydrate-rich foods changed over the course of the study. (Henry Romero/Reuters)

People were counseled to lower their fat intake, which happened at first. 

But starting last decade, there was an explosion in the popularity of low carbohydrate/high fat diet regimes that favour cutting out sugar and starch and increasing fats, including saturated fat, researchers said in Monday's issue of Nutritional Journal, published by BioMed Central.

"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," said Professor Ingegerd Johansson, from the University of Umea, who led the research.

The increase in cholesterol levels in the blood coincided with the increase in fat intake, especially saturated fats and fats for spreading on bread and cooking, the researchers said.

Cholesterol changes overlooked?

Body mass index increased continuously over the course of the study, based on measurements that nurses took of the participants’ heights and weights.

"The long-term deleterious effects of a high blood cholesterol level seem to be neglected in the population and the media, and the interest is centred on diets that promise rapid weight loss," the study's authors wrote.

"For the individual standing on the bathroom scale an increase in blood cholesterol may be overlooked, because it will only be detected by measurements at a medical centre."

The researchers said the most striking diet changes were sharp declines in consumption of boiled potato and crackers like whole grain crisp bread. The changes were balanced by increasing intake of rice and pasta and whole grain soft bread.

Wine consumption increased over the study, particularly among women. Men drank more export beer.

"Our study design does not allow a causal evaluation of the relationship between the increased fat intake since 2004 and the increased cholesterol values after 2007, although the parallel trends would suggest such a relationship," the study's authors concluded.