Quality of diet is important for the prevention of cardiovascular disease


Matti Uusitupa, MD, PhD, Professor, Institute of Public Health and Clinical Nutrition, University of Eastern Finland, PO Box 1627, 70211 Kuopio, Finland. (fax: 358-17-162792; e-mail: matti.uusitupa@uef.fi).

It has been known since the late 1950s that elevated serum cholesterol, high blood pressure and smoking constitute the three main risk factors for atherosclerotic vascular disease (ASVD) and coronary artery disease (CAD) in particular. Today, it is generally accepted that low-density lipoprotein (LDL)-cholesterol and other apolipoprotein-B particles are involved in the atherosclerotic process, in which inflammation also plays a central role [1–3]. It is also known that lowering of LDL-cholesterol by diet or drugs results in decreased morbidity and mortality from ASVD and may lead to regression of atherosclerotic plaque [4, 5]. Recent genome-wide association studies have shown that many genetic variants that increase the risk of CAD mediate their effects through an elevation of LDL-cholesterol, thus strengthening the causality of LDL-cholesterol in the pathogenesis of ASVD [6]. Despite this convincing evidence, the role of cholesterol in, and the optimum diet for, the prevention of ASVD has been much debated. In particular, the debate has focused on the role of quality of fat in the prevention of ASVD. This discussion has been particularly active in Nordic countries, where milk and dairy fat have been an important part of the traditional Nordic diet. Nonetheless, great success with respect to a reduction in ASVD has been achieved, for instance in Finland, through lifestyle changes including dietary habits [7].

In this issue of the Journal of Internal Medicine, Adamsson et al. [8] report the results from a randomized controlled trial of the effects of a healthy Nordic diet (NORDIET) on cardiovascular risk factors in individuals with hypercholesterolaemia. This parallel trial lasted for 6 weeks, and the study participants in the experimental diet group received all foods free of charge. The key elements of the NORDIET were low-fat milk products, margarine rich in polyunsaturated fat, fatty fish, psyllium and flax seeds, berries, vegetables and fruit. In addition, salt intake was reduced. The diet was based on the current Nordic nutrition recommendations with some modifications [9]. Changing to this diet resulted in a marked decrease in the average intake of saturated fats from 14.3% to 5.2% of total energy intake and a moderate increase in polyunsaturated fats from 4.9% to 6.3% of energy intake. Dietary cholesterol intake also decreased markedly, but the intake of dietary fibre increased from 30 to 54 g day−1. Of note, the mean carbohydrate intake increased from 47% to 52% of energy intake.

The changes in the habitual diet resulted in dramatic improvements in serum total and LDL-cholesterol levels, which decreased by 16% and 21%, respectively. Furthermore, systolic blood pressure decreased and there was an improvement in insulin sensitivity. It is interesting that the mean body weight also decreased by 4% in the group that received the healthy diet; this largely explained the improved insulin sensitivity and reduced systolic blood pressure.

What is the main conclusion from these results? A Mediterranean diet has been regarded as superior to the general western diet with regard to ASVD prevention since the Seven Countries Study was conducted in the late 1950s[10]. The Mediterranean diet has gained an almost unbeatable status as it has also been suggested to increase life expectancy [11]. Now, we have clinical evidence that it is possible to modify the Nordic diet in a way that markedly improves the main cardiovascular risk factors, and in particular leads to a clinically meaningful reduction in LDL-cholesterol [4, 5]. Furthermore, a reduction in systolic blood pressure also has a substantial beneficial effect, as long as the reduction is permanent.

The reduction in serum total cholesterol is of the same magnitude (1 mmol L−1) as expected from the equations based on the changes in the intakes of saturated fats, polyunsaturated fats and dietary cholesterol in the large number of well-controlled dietary interventions [12]. Approximately two-thirds of the reduction can be attributed to the decrease in dietary saturated fats and a small increase in polyunsaturated fats, and one-third to the lowered dietary cholesterol intake, weight reduction and increased intake of dietary fibre. Of course, any long-term reduction in LDL-cholesterol may not be of similar magnitude in the real-life setting. Of note, the reduction in serum total and LDL-cholesterol is dependent on other influences, such as the extent of dietary changes, the levels of serum lipids before intervention and genetic factors.

One important result of this trial was a clinically meaningful reduction in body weight in those in the healthy Nordic diet group. This was because of a lower mean energy intake, even though the diet was provided ad libitum (i.e. there was no restriction in energy intake). Also, adherence to the diet was excellent and no participants withdrew from the study. These results offer a fresh look at different weight-reduction diets and indicate that weight reduction is mainly dependent on total energy intake; carbohydrate modification may not be the key issue when it comes to long-term health effects [13]. The current results by Adamsson et al. [8] are in line with the results of the Finnish Diabetes Prevention Study, in which the high-fibre/low-fat group had the greatest weight loss and lowest incidence of diabetes [14].

It has been suggested that inflammation plays a role in the development of CAD [1, 2]. In this study, high-sensitivity C-reactive protein did not change significantly after initiation of the healthy Nordic diet. This may be because of a relatively short intervention time. Furthermore, other inflammatory markers may be more sensitive to the impact of dietary changes on inflammation in various pathological situations.

In summary, these results by Adamsson et al. [8] are encouraging and emphasize the great potential of the healthy Nordic diet for the prevention of chronic diseases, including ASVD and type 2 diabetes. They also raise an important issue of the significance of well-controlled dietary interventions with regard to the effects of quality of fat on human health. Longer-term controlled trials in the real-life setting are needed to determine the long-term adherence to and sustainability of the health effects of the Nordic healthy diet with monitoring of various surrogate outcome measures. Such trials have now begun in four Nordic countries within the Sysdiet consortium.

Conflict of interest

None to declare.