Don't take ordinary fish oil to prevent heart and blood vessel disease

High value
Evidence A
Saves money
Disputed
Free
Takes minutes
No willpower

In plain words

Two large trials enrolled 26,000 (2.6万) people and 15,000 (1.5万) people with diabetes. Among those who took 1 gram of fish oil a day, as many had a major heart or blood vessel event or died as among those who did not. “Eating fish is good for you” does not mean “fish oil capsules are good for you.” Skipping it saves 400 to 1000 CNY a year.

Cost: About 1 to 3 CNY a day, 400 to 1000 CNY a year. It takes no time.
What you get: The VITAL trial enrolled 25,871 people who took 1 gram of fish oil a day and followed them for 5.3 years (median). No difference was seen on either side in major heart or blood vessel events or in death. Major cardiovascular events HR 0.92 (HR is the hazard ratio; 1 means the two sides are the same). 95% CI 0.80–1.06 is the plausible range for that number. It crosses 1, which means no difference can be seen. All-cause death HR 1.02 (0.90–1.15). The ASCEND trial enrolled 15,480 people with diabetes, also at 1 gram a day, and followed them for an average of 7.4 years; it likewise found no difference. Serious vascular events RR 0.97 (0.87–1.08). All-cause death RR 0.95 (0.86–1.05). It looks like a good deal because many people take “eating fish is good for you” to mean “fish oil capsules are good for you”
Notes: Dispute: one trial did find an effect. The REDUCE-IT trial enrolled 8,179 people who already had cardiovascular disease or diabetes, were all on a statin, and still had elevated triglycerides (135–499 mg/dL). They took a prescription drug of 4 g of pure EPA a day, and the share who had an event was 17.2% versus 22.0%. The risk was about a quarter lower (HR 0.75, 0.68–0.83). But that was a high-dose prescription drug for high-risk patients, which is not the same as ordinary people taking fish oil supplements. Eating fish itself is also not what is being discussed here
Evidence grade: A : Specific numbers from meta-analyses, large cohorts or randomized trials; for law and policy, the statute or official text.
Sources: Manson JE et al. (2019). Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer. NEJM. https://doi.org/10.1056/NEJMoa1811403
ASCEND Study Collaborative Group (2018). Effects of n-3 Fatty Acid Supplements in Diabetes Mellitus. NEJM. https://doi.org/10.1056/NEJMoa1804989
Bhatt DL 等 (2019). Cardiovascular Risk Reduction with Icosapent Ethyl for Hypertriglyceridemia. New England Journal of Medicine. https://doi.org/10.1056/NEJMoa1812792（备注里那项高纯度处方鱼油试验）

Translated from the Chinese original. Original book, section 6, tip 2. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer