It is commonly asserted in journal articles and on the internet that saturated fat reduces insulin sensitivity. The idea is that saturated fat reduces the body's ability to handle glucose effectively, placing people on the road to diabetes, obesity and heart disease. Perhaps this particular claim deserves a closer look.
I found a review article from 2008 that addressed this question (1). I like this review because it only includes high-quality trials that used reliable methods of determining insulin sensitivity*.
On to the meat of it. There were 5 studies in which non-diabetic people were fed diets rich in saturated fat, and compared with a group eating a diet rich in monounsaturated (like olive oil) or polyunsaturated (like corn oil) fat. They ranged in duration from one week to 3 months. Four of the five studies found that fat quality did not affect insulin sensitivity, including one of the 3-month studies.
The fifth study, which is the one that's most commonly cited, requires some discussion. This was the KANWU study (2). Over the course of three months, investigators fed 163 volunteers a diet rich in either saturated fat or monounsaturated fat.
The SAFA diet included butter and a table margarine containing a relatively high proportion of SAFAs. The MUFA diet included a spread and a margarine containing high proportions of oleic acid derived from high-oleic sunflower oil and negligible amounts of trans fatty acids and n-3 fatty acids and olive oil.Yummy. After three months of these diets, there was no significant difference in insulin sensitivity between the saturated fat group and the monounsaturated fat group. Yes, you read that right. Even the study that's commonly cited as evidence that saturated fat causes insulin resistance found no significant difference between the diets. I'll be generous and acknowledge that the small difference was almost statistically significant (p = 0.053).
What the authors focused on is the fact that insulin sensitivity declined slightly but significantly on the saturated fat diet compared with the pre-diet baseline. That's why this study is cited as evidence that saturated fat impairs insulin sensitivity. But those of you with a science background may be able to spot the problem here. You need a control group for comparison, to take into account normal fluctuations caused by such things as the season, eating a new diet provided by the investigators, and having a doctor poking at you. That control group was the group eating monounsaturated fat. The comparison between diet groups was the comparison that matters most, and it wasn't quite significant. I think the most you can say about this study is that it provides weak evidence that saturated fat decreases insulin sensitivity.
So we have five studies through 2008, which overall offer little support the idea that saturated fat reduces insulin sensitivity in non-diabetics. Since the review paper was published, I know of one subsequent study that asked the same question (3). Susan J. van Dijk and colleagues fed volunteers with abdominal overweight a diet rich in either saturated fat or monounsaturated fat. I e-mailed the senior author and she said the saturated fat diet was "mostly butter". After 8 weeks, insulin sensitivity was virtually identical between the two groups. This study appeared well controlled and used the gold standard method for assessing insulin sensitivity, called the euglycemic-hyperinsulinemic clamp technique***.
The evidence from controlled trials is rather consistent that saturated fat has no major effect on insulin sensitivity in humans, at least on time scales of a few months.
UPDATE: other trials have added to this finding. The large European LIPIGENE randomized controlled diet trial found that substantial differences in SFA intake had no effect on insulin sensitivity over 12 weeks in people with the metabolic syndrome (3b).
* For the nerds: euglycemic-hyperinsulinemic clamp (the gold standard), insulin suppression test, or intravenous glucose tolerance test with Minimal Model. They didn't include studies that reported HOMA as their only measure, because it's not very accurate.
*** They did find that markers of inflammation in fat tissue were higher after the saturated fat diet.