Cardiovascular diseases are among the most common diseases worldwide and include cerebrovascular pathologies (stroke, transient ischemic attack, etc.), ischemic heart disease (including angina pectoris, myocardial infarction, etc.), peripheral arterial diseases (upper and lower extremities), thrombosis and embolism, as well as rheumatic and congenital heart disease. Of the 17.7 million deaths due to cardiovascular disease in 2015, 7.4 million were due to coronary heart disease and 6.7 million were due to stroke. Of the 17 million non-communicable disease-related deaths in 2015, 82% occurred in low- and middle-income countries, and 37% were due to cardiovascular disease.
The abundance of omega-3 fatty acids in the diet is thought to be associated with low mortality due to cardiovascular disease. These acids are most abundant in fish (especially fatty varieties), as well as in some plant foods. Putative cardioprotective mechanisms include: reduction in blood pressure levels, modification of the lipid profile (especially blood triglyceride levels), modulation of arterial lipoprotein lipase levels, tendency to reduce thrombosis, antiarrhythmic and anti-inflammatory effects, improvement in vascular endothelial function and insulin sensitivity, and increased stability of atherosclerotic plaques.
Given that most people obtain omega-3 fatty acids from fatty fish and fish oil, it should be noted that there is evidence of high levels of various toxic compounds (mercury, dioxins, polychlorinated biphenyls, etc.) in these products. All of these compounds are fat-soluble and thus can gradually accumulate in the body over time, manifesting after prolonged consumption of fish or fish oil. Since many people consume oily fish or take fish oil supplements, it is important to examine the potentially harmful effects of consuming omega-3 fatty acids from these foods. It is also possible that omega-3 fatty acids themselves may be detrimental to health, for example, by increasing bleeding time or suppressing normal immune responses.
The cardiovascular effects of consuming more oily fish may differ from those of fish oil supplementation because fish (rather than fish oil) is a rich source of nutrients including selenium, iodine, zinc, calcium, and protein. Fish in the diet can also displace various other foods, including sources of saturated or trans fats, so it may influence cardiovascular disease in other ways as well. It is noted that different international organizations have slightly different views on omega-3 fatty acid intake. For example, the UK’s National Institute for Health and Clinical Excellence supports fish consumption but not supplements. The American Heart Association also recommends consumption of fish (especially fatty fish), but does not reject the possibility of omega-3 fatty acid supplementation, especially in patients with coronary heart disease.
Aim, methods and study design
In this regard, scientists in the UK conducted the most extensive systematic review and meta-analysis of randomized controlled trials to date to assess the effects of omega-3 fatty acids (long- and short-chain separately) on cardiovascular disease, obesity, lipid profile and all-cause mortality. Moreover, the researchers evaluated the potentially adverse effects of these acids. The results of this work were published in the Cochrane Database Systematic Reviews on July 18, 2018. The World Health Organization is currently updating its recommendations for polyunsaturated fatty acid intake in children and adults, and the results of this systematic review will be taken into account.
Primary outcomes: all-cause mortality, cardiovascular mortality, cardiovascular events, mortality due to coronary heart disease, incidence of coronary heart disease, stroke, arrhythmia (atrial fibrillation). Secondary outcomes: Major adverse cerebrovascular, cardiovascular events or individual cardiovascular events (total/fatal/non-fatal myocardial infarction, sudden cardiac death, angina, heart failure, revascularization, peripheral arterial disease, acute coronary syndrome); body weight and other indicators of obesity; lipid profile (total cholesterol, low and high density lipoprotein cholesterol, triglycerides). Tertiary outcomes: blood pressure levels, significant adverse events (any other diseases), side effects, exclusion from the study, quality of life indicators, and economics. The World Health Organization has identified the following key indicators for its recommendations: all-cause mortality, cardiovascular mortality, coronary heart disease incidence, cardiovascular events, coronary heart disease-related mortality, stroke, arrhythmias (atrial fibrillation), lipid profile, obesity indicators (body weight and BMI).
Results
Eligible studies were those involving adults (18 years of age or older, male and/or female) at any risk of cardiovascular disease; subjects at increased risk of malignancy; subjects who had undergone revascularization surgery (coronary artery bypass grafting or angioplasty); patients who have current (or history of) cardiovascular disease, nephritis with systemic lupus erythematosus, breast cysts, diabetes mellitus, rheumatoid arthritis, multiple sclerosis, psoriasis, hay fever, bronchial asthma, or ulcerative colitis. The inclusion of these populations was driven by the need to develop an understanding of both the development and progression of cardiovascular disease (primary and secondary prevention). Researchers excluded pregnant participants with acute or severe medical conditions (acute stage cancer, heart or kidney transplantation, HIV/AIDS, hemodialysis, IgA glomerulonephritis, or any other kidney pathology other than of diabetic genesis).
As a result, 79 eligible studies (112,059 participants) were identified. It was found that increased intake of long-chain omega-3 fatty acids had little or no effect on the risk of cardiovascular events, coronary heart disease and death due to it, stroke, arrhythmias, all-cause mortality, and death due to any cardiovascular pathology. No dose-dependent effects were found, and no difference was found depending on the source of omega-3 fatty acids. On the other hand, the researchers found some evidence that increasing intake of short-chain omega-3 fatty acids (alpha-linolenic acid) probably slightly reduces the risk of cardiovascular disease, arrhythmias, and mortality due to coronary heart disease, but does not affect all-cause mortality or cardiovascular pathologies in general.
