All the dietary intervention studies determined that a Nordic diet had an effect on blood lipids.
In the NORDIET study (Table 1), total cholesterol levels were reduced by 0.98 mmol/l and the LDL cholesterol level by 0.83 mmol/l in the group that ate a Nordic diet. In the control group, total cholesterol levels increased by 0.23 mmol/l, whereas the LDL cholesterol level increased by 0.10 mmol/l. The changes were significantly different in the two groups (p < 0.0001 and p < 0.001 respectively). At baseline, the diet was more or less identical. It was the reduction in serum concentration of the saturated fatty acids C14:0, C15:0 and C18:0 and the increase in the polyunsaturated omega-3 fatty acid 22:6, n-3 that was related to the improvement in the lipid profile (22).
In the SYSDIET study (Table 1) the non-HDL cholesterol level was reduced by 0.22 mmol/l and 0.06 mmol/l among those who ate a Nordic diet and in the control group respectively. The change was significantly different in the two groups (p = 0.04) (15).
In the NND study (Table 1), the levels of triglycerides, total cholesterol and VLDL cholesterol were reduced by 0.04 mmol/l, 0.17 mmol/l and 0.02 mmol/l respectively in the group that ate the Nordic diet. In the control group, the levels of triglycerides, total cholesterol and VLDL cholesterol increased by 0.16 mmol/l, 0.08 mmol/l and 0.06 mmol/l respectively. The changes were significantly different in the two groups (p = 0.004, p = 0.010 and p = 0.008 respectively) (18). Weight reductions were found in both groups at the end of the intervention, but the change was greater among those who had eaten a Nordic diet, and it was significantly greater than in the control group (p < 0.001). After adjustment for weight change, there was a significant reduction in the triglyceride (p < 0.046) and VLDL cholesterol levels (p = 0.05) only in the group that had eaten a Nordic diet, when compared to the control group. The weight change was maintained after another 52 weeks (21).
The total cholesterol and LDL cholesterol levels are key risk factors for development of cardiovascular disease (30, 31), and these studies show that a healthy Nordic diet has a favourable effect on the risk profile among persons who are at risk of developing cardiovascular disease. Statin treatment reduces the LDL cholesterol level by 30 – 60 % (32). A 21 % reduction in the LDL cholesterol level after a change of diet in the NORDIET study (13) is clinically relevant, and the effect is considerable in light of the fact that the diet includes a number of components that give rise to biological effects.
The strong reduction in LDL cholesterol level in the NORDIET study may be caused by the fact that the intake of saturated fat was reduced from 14 per cent of energy to 5 per cent of energy in the group that ate a Nordic diet, in addition to an increased intake of fibre, as recommended by previous studies (31). Such a change in the intake of saturated fat may be difficult to achieve unless the participants are provided with all the food they are intended to consume.
In the SYSDIET study, in which the participants were given advice on replacing food items or reducing the intake of certain products, the change in intake of saturated fats was smaller than in the NORDIET study. In the SYSDIET study, post hoc analyses were made of the participants’ diet on the basis of biomarkers in blood linked to intake of oily fish, vegetables, wholegrain cereals and linseed oil. Judging from biomarker levels in the bloodstream, those who maintained the highest degree of compliance achieved a favourable effect on their total cholesterol and LDL cholesterol levels (24).
This corroborates the findings that the better the compliance with the dietary advice, the greater the reduction in risk of cardiovascular disease. In addition, a significant change in the plasma lipidomic profile (all types of lipid metabolites in plasma) was observed between the groups in the SYSDIET study (25). Profiling of all lipids in the blood can more sensitively register metabolic changes and specific lipid metabolites linked to risk of disease (33). In the SYSDIET study, the level of metabolites (plasmalogens) with antioxidant properties increased, and the level of metabolites (ceramides) that are linked to the development of insulin resistance was reduced in the group with a Nordic diet (25). The meaning of these results is uncertain.
In the SYSDIMET study (Table 1), an increased intake of fish was associated with an increase in large HDL particles (17). In the NORDIET study, the level of HDL cholesterol was reduced by 0.08 mmol/l in the group with a Nordic diet, while increasing by 0.11 mmol/l in the control group. The effect was significantly different in the two groups (p = 0.001) (13). In the SYSDIET and NND studies, the HDL cholesterol level did not change significantly (15, 18).
The impact of HDL cholesterol levels on the risk of cardiovascular disease remains unclear. Although observational studies have shown that a high level of HDL cholesterol is associated with a lower risk of cardiovascular disease (34), the clinical importance of a reduction in HDL cholesterol levels is unknown, since studies of drugs that increase HDL cholesterol levels have not found any reduction in cardiovascular disease (35, 36). Compared to the positive effect of reduced total cholesterol and LDL cholesterol levels, this small reduction in HDL cholesterol levels most likely plays a minor role for the risk of cardiovascular disease.