The main findings of our study are: (1) low serum DHA and pravastatin use were associated with atheroma progression in statin-treated patients with DM; (2) plasma glucose and HbA1c levels did not correlate with plaque progression or regression during statin therapy; and (3) volume of the necrotic core component at baseline was associated with atheroma progression during statin therapy.
Residual risk of cardiovascular events during statin therapy
DM is accompanied by extensive atherosclerosis deposition despite the use of medications , and statins are significantly less effective at inducing regression of coronary artery plaque in patients with DM [7, 9]. Not all patients show atheroma regression and enhanced prevention of cardiovascular events while taking statin therapy, particularly diabetic patients. Serum LDL-C levels at the 8-month follow-up did not differ between the progressors and regressors, whereas low serum DHA was associated with progression of coronary atherosclerosis during statin therapy. On the basis of this study’s results, the residual risk of cardiovascular events during statin therapy can be explained in part by n-3 polyunsaturated fatty acids (PUFAs). Indeed, highly purified EPA is very effective in decreasing the incidence of CAD in patients with impaired glucose metabolism .
Surrogate marker of PUFAs
Plasma levels of EPA and DHA have been associated with reduced risk of sudden death  and fatal ischemic heart disease . However, the parameter that is the best surrogate marker of PUFAs for cardiovascular events remains unclear. Several authors tended to explain the PUFAs effects in terms of a balance between n-3 to n-6 PUFAs, rather than the absolute amount of each single molecule. Domei et al. reported that lower EPA/AA ratio, but not EPA, DHA, or DHA/AA ratio, was associated with higher incidence of cardiac events . However, a recent cohort study reported that a lower EPA/AA ratio was associated with a greater risk of cardiovascular events in patients with higher hs-CRP levels, but no clear association was observed in those with lower hs-CRP levels . In addition, the other cohort study reported that plasma EPA and DHA, but not AA, were inversely associated with cardiovascular events . Thus, although serum EPA/AA ratio is one of the useful markers for the risk of cardiovascular events, this ratio does not apply to all patients. Heterogeneity in population characteristics, background dietary fish consumption, and length of follow-up may each contribute to these inconsistent findings from different study designs. The present study found a low DHA to be associated with progression of coronary atherosclerosis, indicating that low DHA could be one of biomarkers for progression of coronary atherosclerosis. Furthermore, our results suggest that EPA and DHA may have different effects on coronary atherosclerosis, particularly in patients with DM. This may be because of the enhanced peroxisomal β-oxidation and reduced n-3 PUFAs content in diabetic heart .
PUFAs and cardiovascular events
The intake of fish is associated with a lower risk of cardiovascular disease [20, 21]. Several mechanisms may explain the favorable association between n-3 PUFAs and cardiovascular risk. One of these mechanisms is that n-3 PUFAs beneficially alter the cardiovascular risk factor profile [22, 23]. In experimental studies, higher n-3 PUFAs levels alter cell membrane fluidity and receptor responses, regulate gene transcription, and serve as metabolic precursors to potent anti-inflammatory molecules . These molecular effects may underline their systemic and cardiac benefits such as inflammatory responses, autonomic control, vascular and cardiac hemodynamics, endothelial function, blood lipids, and possibly thrombosis [24, 25]. Data from intervention studies have also identified a lower risk of cardiovascular events associated with purified EPA (1800 mg/day)  as well as EPA+DHA (1 g/day) . Pharmacological differences between EPA and DHA have become clear in recent years, but it is still unclear as to which of these n-3 PUFAs has a strong antiatherogenic effect. A clinical trial reported that DHA, but not EPA, has been associated with reduced progression of coronary atherosclerosis . Two additional studies reported that DHA, but not EPA, showed a significant inverse association with intima-media thickness [29, 30]. The Japan Eicosapentaenoic Acid Lipid Intervention Study (JELIS) showed that highly purified EPA reduced nonfatal CAD . Although the JELIS study did not report the serum DHA level during trial, the reduction of nonfatal CAD is more likely to be mediated through EPA rather than DHA. In the present study, high serum EPA was associated with atheroma regression in univariate analysis, whereas this correlation did not observe in multivariate analysis. Therefore, our results may suggest that DHA has a more potent antiatherogenic effect than EPA.
Plasma glucose control and cardiovascular events
Although intensive plasma glucose control with sulfonylureas or insulin does not reduce macrovascular disease , pioglitazone reduces the composite of all-cause mortality, non-fatal myocardial infarction, and stroke in patients with DM . Furthermore, pioglitazone prevents progression of coronary atherosclerosis  and stabilizes coronary atherosclerosis , suggesting that this medication may have beneficial effect on coronary artery plaques. Plasma glucose and HbA1c levels did not correlate with atheroma progression or regression in this study. This might be due to the fact that antidiabetic medications were not fixed during the study period.
Statin therapy for secondary prevention of cardiovascular events
Compared with moderate lipid-lowering therapy, intensive lipid-lowering therapy with a statin provides a significantly greater reduction of coronary events [3, 4]. Indeed, atorvastatin 80 mg significantly reduced the rate of cardiovascular events by 25% compared with atorvastatin 10 mg in patients with DM . This indicates that intensive LDL-C control is necessary for secondary prevention of CAD, particularly in patients with DM. The American Heart Association and American College of Cardiology offer an optimal LDL-C goal of 〈 70 mg/dl for patients with a very high risk of CAD . In Japan, a provisional therapeutic goal of LDL-C with statins is 〈 75 mg/dl in patients with the highest risk conditions (those with higher baseline LDL-C level, DM, or chronic kidney disease) . However, in our study, LDL-C levels at the 8-month follow-up were 85 mg/dl in progressors and 88 mg/dl in regressors. More vigorous reduction in LDL-C levels would be necessary to produce regression of coronary atherosclerosis in patients with DM as previously reported . Serum PUFAs have less impact on coronary atherosclerosis in patients treated with strong statin because a greater reduction in LDL-C mainly affects coronary atherosclerosis . The results of this study suggest that diabetic patients should be treated with strong statin at intensive lipid-lowering dose rather than weak statin at moderate lipid-lowering doses for secondary prevention of CAD.
We also observed that necrotic core and calcified plaque volume at baseline was significantly greater in progressors than in regressors. This may reflect ongoing, irreversible plaque progression once the disease has reached a critical point. It may also suggest that treatment with delayed statin therapy would potentially be a risk for atheroma progression and lead to future cardiovascular events. The findings from previous trial support a strategy of the early initiation of an aggressive LDL-C lowering to prevent death and major cardiovascular events .
The present study had several limitations. First, it is a post-hoc subanalysis of the TRUTH trial. Second, diagnosis of DM was made by the attending physician and a glucose tolerance test was not conducted for all potential study subjects. Patients with only impaired glucose tolerance or borderline diabetes were not diagnosed as having DM. Third, antidiabetic and antihypertensive medications were not fixed during the study period. Changes in medications may have affected the results of this study. Fourth, patients were not prohibited from making lifestyle changes. Changes in PUFA composition of the diet likely reflect changes in serum PUFA levels. Finally, the statistical power was insufficient to characterize the determinants of atheroma progression because of the small number of patients. For these reasons, a prospective, randomized, multicenter study is necessary to compare intensive versus moderate lipid-lowering statin therapy on coronary artery plaque volume in patients with DM.