Omega-3s supplements don’t help people with diabetes control their blood sugar levels, a 2008 systematic review found. A 2012 study that combined a meta-analysis and a systematic review looked at the possible link between eating seafood or plants with omega-3s and the risk of developing type 2 diabetes. The study found little evidence that these dietary sources of omega-3s affected the risk of developing diabetes.
Omega-3 supplements usually do not have negative side effects. When side effects do occur, they typically consist of minor gastrointestinal symptoms, such as belching, indigestion, or diarrhea.
Omega-3 supplements may extend bleeding time (the time it takes for a cut to stop bleeding). People who take drugs that affect bleeding time, such as anticoagulants (“blood thinners”) or nonsteroidal anti-inflammatory drugs (NSAIDs), should discuss the use of omega-3 fatty acid supplements with a health care provider.
Studies (including a 2010 research review and 2009 clinical trial) have found no evidence that taking vitamin C supplements is helpful for diabetes.
The research on diabetes and vitamin D and calcium supplements is not conclusive.
Supplementing with vitamin D combined with calcium appears to lower the risk of developing type 2 diabetes, according to a 2007 systematic review and meta-analysis.
In a 2008 clinical trial studying 33,951 post-menopausal women over 7 years, calcium plus vitamin D supplements did no better than a placebo at reducing the risk of developing diabetes.
The lower risk seen in some studies in people who consume more calcium may be because those individuals are also getting more magnesium, a 2012 meta-analysis reported.