Abstract and Introduction
Abstract
Objective To study the long term consequences of low carbohydrate diets, generally characterised by concomitant increases in protein intake, on cardiovascular health.
Design Prospective cohort study.
Setting Uppsala, Sweden.
Participants From a random population sample, 43,396 Swedish women, aged 30-49 years at baseline, completed an extensive dietary questionnaire and were followed-up for an average of 15.7 years.
Main outcome measures Association of incident cardiovascular diseases (ascertained by linkage with nationwide registries), overall and by diagnostic category, with decreasing carbohydrate intake (in tenths), increasing protein intake (in tenths), and an additive combination of these variables (low carbohydrate-high protein score, from 2 to 20), adjusted for intake of energy, intake of saturated and unsaturated fat, and several non-dietary variables.
Results A one tenth decrease in carbohydrate intake or increase in protein intake or a 2 unit increase in the low carbohydrate-high protein score were all statistically significantly associated with increasing incidence of cardiovascular disease overall (n=1270)—incidence rate ratio estimates 1.04 (95% confidence interval 1.00 to 1.08), 1.04 (1.02 to 1.06), and 1.05 (1.02 to 1.08). No heterogeneity existed in the association of any of these scores with the five studied cardiovascular outcomes: ischaemic heart disease (n=703), ischaemic stroke (n=294), haemorrhagic stroke (n=70), subarachnoid haemorrhage (n=121), and peripheral arterial disease (n=82).
Conclusions Low carbohydrate-high protein diets, used on a regular basis and without consideration of the nature of carbohydrates or the source of proteins, are associated with increased risk of cardiovascular disease.
Introduction
Overweight and obesity are risk factors for several common chronic diseases, and they have become endemic in most economically developed countries and beyond. Increased physical activity is one way of counteracting excessive energy intake, but reducing this intake is also important. Many dietary regimens have been proposed as conducive to weight control, invoking various mechanisms including increased satiety. The most popular among these diets emphasise reduction of carbohydrate intake, thereby encouraging high protein intake, as high fat diets are generally avoided in most Western societies. Low carbohydrate-high protein diets may have short term effects on weight control, but concerns have also been expressed, notably with respect to cardiovascular outcomes. Although low carbohydrate-high protein diets may be nutritionally acceptable if the protein is mainly of plant origin and the reduction of carbohydrates applies mainly to simple and refined ones, the general public do not always recognise and act on these qualifications.
During the past few years, several cohort studies have evaluated the long term health effects of low carbohydrate-high protein diets, with emphasis on cardiovascular diseases. In the Nurses’ Health Study in the United States, diets lower in carbohydrate and higher in protein were not associated with increased incidence of ischaemic heart disease. Three smaller studies in Europe, however, indicated statistically significant increases in cardiovascular mortality in relation to low carbohydrate-high protein diets. One of these studies was from the same cohort of relatively young Swedish women as the investigation reported here (but presented data only on 75 cardiovascular deaths from shorter follow-up), the second study was based on middle aged Greek men and women, and the third study investigated elderly men in Sweden. Willett has considered possible reasons for the discrepant results of the European studies and the American one, including differences in the main dietary sources of protein (animal versus plant) and the prevalence of obesity in the studied populations.
Given the importance of the topic and the widespread use of low carbohydrate diets for weight control, particularly among women, we followed up the women in the Swedish Women’s Lifestyle and Health Cohort, focusing on incidence of cardiovascular diseases and using the valuable registry resources available in Sweden.