People with type 2 diabetes "should not be encouraged" to take omega-3 fish oil supplements, according to researchers at the University of East Anglia.
There was some concern that omega-3s could be harmful to people with the disease.
But until the BMJ review of more than 80 studies found evidence of this, they also failed to find any benefit.
Diabetes UK said it was better to get omega-3s from eating oily fish as part of a healthy diet.
Most people with diabetes ̵
Being overweight or obese or having a close relative with the condition increases the risk.
Researchers have found that omega-3 fish oil has little or no effect on the probability of being diagnosed with diabetes or on glucose metabolism, no matter how long humans have been tested.
Dr Lee Hooper, who runs the study, told the BBC that he had concerns that omega-3 supplements could harm people with type 2, making it difficult to control glucose.
But those with the disease or who are at risk of developing it can also have high triglycerides – a type of blood fat – that omega-3 has been shown to reduce.
She said, "We found neither harm nor benefit.
"These are really expensive things. If someone is at risk of diabetes, there are much better things to do to spend money on, such as physical activity – or oily fish."
Douglas Twenfur, M.Sc. Diabetes UK's Head of Care, says: “Eating a healthy and varied diet is extremely important and we know that certain foods – including fruits, vegetables, whole grains, yogurt and cheese – can help reduce the risk of diabetes type 2.
"While omega-3 fatty acids are crucial For our overall health, it is generally better for people with type 2 diabetes to receive their intake by eating at least two servings of oily fish per week than by taking supplements. "
But Dr. Carrie Ruxton, funded by The Health and Nutritional Supplements (HSIS) industry says: "While I would prefer people to follow government advice and eat more fish, this is not the reality and daily omega-3 supplement – whether it's from fish oil or algae – can bridge the gap. "