• Heart disease: In a Dutch study of people who had already had a heart attack, omega-3 fish oil supplements did not reduce the risk of cardiovascular events over a 40-month period. This conflicts with some prior studies that did find a protective effect, and with advice from the American Heart Association that heart attack patients should take omega-3s. But unlike the older research, this New England Journal of Medicine study included mainly people on "state-of-the-art" medication, such as statins and blood pressure drugs, which could help explain the lack of effect of the supplements.
• Atrial fibrillation: In a study of people with this heart rhythm abnormality, high-dose omega-3 fish oil supplements did not reduce the risk of recurrence over a six-month period. It has been theorized that one way omega-3s from fish or supplements may reduce the risk of cardiovascular disease is by preventing heart rhythm problems, and some previous studies have suggested this. This study, in the Journal of the American Medical Association (JAMA), was the largest clinical trial to test this theory.
• Pregnancy: Though pregnant women are often advised to take omega-3 supplements (DHA, in particular, the main omega-3 fat in the brain) to boost their children's mental development and prevent postpartum depression, DHA capsules have neither effect, according to a large Australian clinical trial in JAMA.
• Alzheimer's disease: Omega-3 fish oil supplements do not help slow the progression of the disease, according to a study (also in JAMA) of Alzheimer's patients in Oregon, who took either DHA or a placebo for 18 months. Some previous observational studies suggested that high intakes of DHA can help prevent or slow dementia, including Alzheimer's.
• Cognitive decline: In contrast, another placebo-controlled study, in Alzheimer's & Dementia, found that DHA supplements, taken for 24 weeks, helped improve memory and brain function in people over 55 with mild cognitive impairment. This suggests that, to help the brain, the supplements need to be started early, before mental decline progresses too much.
• Gum disease: A Harvard study in the Journal of the American Dietetic Association found that people who consumed moderate amounts of fish oil omega-3s had a lower risk of developing periodontitis, an inflammatory disease that causes gum recession and tooth loss. Omega-3s have anti-inflammatory effects.
