In healthy young people, eating more trans fat for a month doesn’t make their insulin work worse than eating the same amount of butter or olive oil.

People in Japan who have more of a specific fat (elaidic acid) in their blood—usually from processed foods—tend to have higher insulin resistance, which means their bodies don’t use insulin well, but this link gets weaker when you account for high triglyceride levels.

Japanese people with diabetes have more of a certain unhealthy fat in their blood than those with normal or slightly elevated blood sugar, hinting that processed foods might play a role in their condition.

Eating more saturated fats instead of sugary refined carbs doesn't seem to clearly make you more or less likely to get insulin resistance or diabetes.

Japanese people living in the U.S. have similar levels of this unhealthy fat in their blood whether they have diabetes or not, suggesting that their overall diet might mask any link between trans fat and blood sugar problems.

Trans fats from dairy and meat (vaccenic acid) don’t seem to make insulin resistance worse in overweight women, even when eaten at levels higher than typical industrial trans fats.