To compare the metabolic effect of coingestion of saturated and monounsaturated fats with potato, 12 subjects with non-insulin-dependent diabetes mellitus (NIDDM) received 300 g mashed potato alone or in combination with 40 g olive oil, 80 g olive oil, 50 g butter, or 100 g butter, respectively. The blood glucose response area to potatoes with 100 g butter (448 +/- 68 mmol . 240 min/L) was significantly lower than after the four other meals: 596 +/- 63 (potato alone), 649 +/- 82 (potato + 40 g olive oil), 587 +/- 80 (potato + 50 g butter), and 604 +/- 81 (potato + 80 g olive oil) mmol . 240 min/L, P < 0.05, respectively. The insulin response was significantly increased by adding 50 and 100 g butter, whereas addition of 40 and 80 g olive oil had no effect. The fatty acid concentration was higher when 100 g butter was added to the potato meal than when it was not (0.67 +/- 0.05 compared with 0.48 +/- 0.07 mmol/L, P < 0.05). Fatty acid concentrations were similar to those found for the other meals. The triacylglycerol response increased in a dose-dependent manner with the fat content of the meals irrespective of the type of fat. We conclude that butter increases the insulin response more than does olive oil, and large amounts of butter also increase fatty acid and triacylglycerol concentrations. [References: 30].