"I am curious whether there is any detail provided in these studies regarding the types of fat, or is all fat, i.e., saturated, unsaturated, etc. lumped together?"The type of fat matters. The more saturated the fat, the more often it's associated with reductions in insulin sensitivity. The following study is often cited:
Substituting Dietary Saturated For Monounsaturated Fat Impairs Insulin Sensitivity In Healthy Men And Women: The KANWU Study, Diabetologia, 2001
In this study, 162 people ate isocaloric diets for 3 months, containing either a high proportion of saturated fat (SFA: 17.6% of calories) or monounsaturated fat (MUFA: 21.2% of calories). SFA fared worse than MUFA. (MUFA raised lipoprotein (a) though). And fish oil had no benefit in either group, as regards insulin at least. (I'm more interested in insulin levels and insulin sensitivity at the moment.)
- LDL increased on the SFA diet by 4.1%
- LDL decreased on the MUFA diet by 5.2%
- Lp(a) increased on the MUFA diet by 12%
When total fat intake was kept to no more than 37% of calories:
- Insulin sensitivity decreased on the SFA diet by 12.5%
- Insulin sensitivity increased (this is good) on the MUFA diet by 8.8%
There was a subassignment for fish oil (3.6 grams/day) in both groups:
"The addition of n-3 fatty acids influenced neither insulin sensitivity nor insulin secretion."A later analysis of this population1 found:
(n-3 fatty acids are omega-3 fatty acids, a type of polyunsaturated fatty acid found abundantly in fish oil.)
"VLDL cholesterol and triacylglycerol were significantly reduced and LDL cholesterol significantly increased by fish oil supplementation."
The above was an intervention trial from Sweden. Below are three large epidemiological studies from the US that implicate saturated fat in diabetes development.
SFA = saturated fatty acid
PUFA = polyunsaturated fatty acid
MUFA = monounsaturated fatty acid
1. Dietary Fat and Incidence of Type 2 Diabetes in Older Iowa Women, Diabetes Care, 2001.
This is the Iowa Women's Health Study -- 35,988 women followed for 11 years. It found:
"Polyunsaturated fatty acid was inversely related to diabetes risk when substituted for saturated fatty acid." There was up to a 17% reduction is diabetes risk when PUFA was substituted for SFA. Consumption of vegetable fats reduced risk for diabetes by 22% for the highest consumers.2. Dietary Fat Intake And Risk Of Type 2 Diabetes In Women, American Journal of Clinical Nutrition, 2001.
This is the Nurses' Health Study -- 84,204 women followed for 14 years. It found:
"Replacing 5% of energy from SFA with energy from PUFA was associated with a 35% lower risk [of type 2 diabetes]."3. Dietary Fat And Meat Intake In Relation To Risk Of Type 2 Diabetes In Men, Diabetes Care, 2002.
This is the Health Professionals Follow-up Study -- 42,504 men followed for 12 years. It found:
"Total and saturated fat intake were associated with a higher risk of type 2 diabetes, but these associations were not independent of BMI." (RRs 1.27 and 1.34 respectively.)
Fats that are less saturated have been shown to enhance the action of insulin, conversely, fats that are more saturated interfere with insulin's action. Here are some mechanisms thought to explain this:
- "The type of fat in the diet may affect insulin sensitivity by changing the fatty acid composition of membrane lipids. A higher proportion of unsaturated fat may improve insulin signaling by increasing membrane fluidity." (From the Health Professionals Study above.)
- PUFAs (that would include the omega-3 in fish oil) have been shown to lower triglyceride in muscle and pancreatic beta-cells, improving insulin sensitivity.
- This one is a little involved... Fatty acids themselves can act as ligands, affecting the expression of genes. For example, they can bind to proteins in the nuclear membrane that act as transcription factors. (A group of these transnuclear proteins are known as PPARs. Some of the best known PPAR ligands are the thiazolidiediones ... a class into which the diabetes drugs Avandia and Actos fall.)
So, by controlling gene expression, PUFAs direct fatty acids away from storage (lipid synthesis) and towards oxidation (lipid breakdown). (This is exactly how the diabetes drug Avandia, a PPAR-gamma agonist works). PUFAs also encourage total body glycogen storage. These actions in concert improve insulin sensitivity.