The kind of saturated fat found in dairy products such as yoghurt is likely to reduce type 2 diabetes risk, say scientists.
But other types obtained from red meat and fried food, or generated after consuming carbohydrates or alcohol, may be harmful.
The differences depend on the number of carbon atoms chain-like saturated fatty acid molecules contain, according to the research.
Those with an even number — 14, 16 and 18 —were associated with a higher risk of type 2 diabetes, the most common form of the disease. Molecules with odd numbers of carbon atoms, 15 and 17, led to a protective effect.
Saturated fat is typically found in fatty animal products such as butter, cheese and red meat.
It is generally considered unhealthy and linked to high levels of cholesterol and heart disease, as well as type 2 diabetes.
Lead researcher Dr Nita Forouhi, from the Medical Research Council Epidemiology Unit at Cambridge University, said: "Our findings provide strong evidence that individual saturated fatty acids are not all the same.
"The challenge we face now is to work out how the levels of these fatty acids in our blood correspond to the different foods we eat."