Evidence Level
Cod liver oil has one of the longest research histories of any supplement, dating to controlled studies on rickets in the 18th century. The evidence base draws from both cod liver oil-specific studies and the broader omega-3 and vitamin D literature. For bone health, vitamin D's role in calcium absorption and fracture prevention is well-established, with the VITAL study (Manson et al., 2019) being the largest randomized trial. For cardiovascular outcomes, Aung et al. (2018) published a major meta-analysis of 10 trials (77,917 participants) showing omega-3 supplementation reduced coronary heart disease events by 8%. Martineau et al. (2017) meta-analyzed 25 RCTs and found vitamin D supplementation reduced acute respiratory tract infections, particularly in deficient individuals. For joint inflammation, Goldberg & Katz (2007) confirmed fish oil reduces pain and stiffness in rheumatoid arthritis. The Rajakumar (2003) historical review documented cod liver oil's critical role in eliminating rickets as a public health problem. The key distinction from regular fish oil — the vitamin A content — is both an advantage (synergistic immune and bone benefits) and a limitation (toxicity ceiling). Evidence is classified as Moderate because while omega-3 and vitamin D evidence is individually strong, cod liver oil-specific RCTs are fewer than those for purified fish oil or vitamin D alone.