Insulin is a hormone that drives glucose and other nutrients from the bloodstream into cells, among other things. A loss of sensitivity to the insulin signal, called insulin resistance, is a core feature of modern metabolic dysfunction and can lead to type II diabetes and other health problems. Insulin resistance affects a large percentage of people in affluent nations, in fact the majority of people in some places. What causes insulin resistance? Researchers have been trying to figure this out for decades.*
Since saturated fat is blamed for everything from cardiovascular disease to diabetes, it's no surprise that a number of controlled trials have asked if saturated fat feeding causes insulin resistance when compared to other fats. From the way the evidence is sometimes portrayed, you might think it does. However, a careful review of the literature reveals that this position is exaggerated, to put it mildly (
1).
The glycemic index, a measure of how much a specific carbohydrate food raises blood sugar, is another common concept in the diet-health literature. On the surface, it makes sense: if excess blood sugar is harmful, then foods that increase blood sugar should be harmful. Despite evidence from observational studies, controlled trials as long as 1.5 years have shown that the glycemic index does not influence insulin sensitivity or body fatness (
2,
3,
4). The observational studies may be confounded by the fact that white flour and sugar are the two main high-glycemic foods in most Western diets. Most industrially processed carbohydrate foods also have a high glycemic index, but that doesn't imply that their high glycemic index is the reason they're harmful.
All of this is easy for me to accept, because I'm familiar with examples of traditional cultures eating absurd amounts of saturated fat and/or high-glycemic carbohydrate, and not developing metabolic disease (
5,
6,
7). I believe the key is that their food is not industrially processed (along with exercise, sunlight exposure, and probably other factors).
A large new study just published in the American Journal of Clinical nutrition has taken the evidence to a new level (
8). At 6 months and 720 participants, it was both the largest and one of the longest studies to address the question. Participants were assigned to one of the following diets:
- High saturated fat, high glycemic index
- High monounsaturated fat, high glycemic index
- High monounsaturated fat, low glycemic index
- Low fat, high glycemic index
- Low fat, low glycemic index
Compliance to the diets was pretty good. From the nature of the study design, I suspect the authors were expecting participants on diet #1 to fare the worst. They were eating a
deadly combination of saturated fat and high glycemic carbohydrate! Well to their dismay, there were no differences in insulin sensitivity between groups at 6 months. Blood pressure also didn't differ between groups, although the low-fat groups lost more weight than the monounsaturated fat groups. The investigators didn't attempt to determine whether the weight loss was fat, lean mass or both. The low-fat groups also saw an increase in the microalbumin:creatinine ratio compared to other groups, indicating a possible deterioration of kidney function.
In my opinion, the literature as a whole consistently shows that if saturated fat or high glycemic carbohydrate influence insulin sensitivity, they do so on a very long timescale, as no effect is detectable in controlled trails of fairly long duration. While it is possible that the controlled trials just didn't last long enough to detect an effect, I think it's more likely that both factors are irrelevant.
Fats were provided by the industrial manufacturer Unilever, and were incorporated into margarines, which I'm sure were just lovely to eat. Carbohydrate was also provided, including "bread, pasta, rice, and cereals." In other words, all participants were eating industrial food. I think these types of investigations may be limited by reductionist thinking. I prefer studies like Dr. Staffan Lindeberg's paleolithic diet trials (
9,
10,
11). The key difference? They focus mostly on diet
quality, not calories or specific nutrients. And they have shown that quality is king!
* Excess body fat is almost certainly a major cause. When fat mass increases beyond a certain point, particularly abdominal fat, the fat tissue typically becomes inflamed. Inflamed fat tissue secretes factors which reduce whole-body insulin sensitivity (
12,
13). The big question is: what caused the fat gain?
