Studies show that calorie-reduced diets improve insulin sensitivity, regardless of their fat content.
However, it’s unclear if this is due to the composition of the diet or weight loss.
For this reason, a group of scientists compared the effects of high- and low-fat diets, while maintaining stable weight.
Below is a review of their findings, published in the European Journal of Nutrition.
Insulin sensitivity refers to how sensitive the body is to the effects of insulin.
Low insulin sensitivity, also known as insulin resistance, means the body doesn’t respond to insulin efficiently. This adverse condition characterizes type 2 diabetes and can lead to abnormally high blood sugar after meals.
It’s unclear exactly what causes insulin resistance, but several studies have examined how diet composition affects insulin sensitivity.
Two short-term studies compared the effects of diets high in fat (50–55% of calories) and low in fat (20–25% of calories) on insulin sensitivity. They found no significant differences in insulin sensitivity between diets.
Another study in older individuals showed that a 4-week, high-fat diet (42% of calories), high in saturated fat (24% of calories), did not cause significant changes in insulin sensitivity.
What’s more, an 11-day study found that a very-high-fat diet (83% of calories) had no effects on insulin sensitivity, compared to a diet that contained no fat.
However, the evidence is not entirely conclusive. Some studies indicate that low-fat diets may improve insulin sensitivity.
In short, it seems that eating high amounts of fat does not increase your risk of becoming insulin resistant, but more research is needed.
Researchers from the University of Washington in the US compared the effects of a very high-fat diet and a low-fat diet on insulin sensitivity.
A high‑fat, high‑saturated fat diet decreases insulin sensitivity without changing intra‑abdominal fat in weight‑stable overweight and obese adults.
The main purpose of this small, randomized, controlled trial was to examine the effects of a high-fat diet, rich in saturated fat, on insulin sensitivity.
The study included 13 overweight or obese men and women, aged 18–55 years.
For the first 10 days of the study, all of the participants followed a standardized diet. It provided 47% of calories from carbs, 18% from protein, and 35% from fat, including 12% from saturated fat.
Then, the participants were assigned to two 4-week diets in a random order:
- Low-fat diet: 20% of calories from fat, including 8% from saturated fat, 62% from carbs and 18% from protein.
- High-fat diet: 55% of calories from fat, including 25% from saturated fat, 27% from carbs and 18% from protein.
These two diets were identical, apart from their varying fat and carb content. The major sources of fat in both diets were butter and safflower oil, high in oleic acid.
The study kitchen provided participants with all food, and they were weighed twice weekly to ensure weight stability.
The study had a crossover design, meaning that the participants followed both diets on different study periods, separated by 6 weeks.
At the beginning and end of the diets, the researchers measured insulin sensitivity, abdominal fat and fasting levels of very-low-density lipoproteins (VLDLs).
Only 7 participants completed both diets, or 54% of those who originally started.
Bottom Line: This was a randomized, crossover trial comparing the effects of a very high-fat diet and a low-fat diet on insulin sensitivity.
Finding 1: A High-Fat Diet Reduced Insulin Sensitivity
Insulin sensitivity is often assessed by injecting people with insulin and then measuring blood sugar clearance, or how quickly blood sugar levels drop.
During the high-fat diet, insulin sensitivity decreased, whereas it remained constant during the low-fat diet.
The chart below compares the differences in the change in blood sugar clearance during each of the two diets. The blood sugar response is shown for both low and high doses of insulin.
These results cannot be explained by changes in body weight or abdominal fat, since the participants’ weight remained the same during the study.
The researchers speculated that the saturated fat in the high-fat diet may have contributed to the decrease in insulin sensitivity, which is supported by other studies.
In one study, when healthy participants followed a diet high in saturated fat (17% of calories) for three months, insulin sensitivity decreased by 12.5%.
Other studies have shown that single meals or diets high in polyunsaturated or monounsaturated fat improve insulin sensitivity, compared to saturated fat.
These studies indicate that insulin sensitivity is influenced by the type of fat eaten, rather than the total dietary fat content. However, further studies are needed.
Bottom Line: The high-fat diet reduced insulin sensitivity, whereas it remained constant on the low-fat diet.
Finding 2: Increases in Skin Fat Were Associated With Higher Insulin Sensitivity
Why the high-fat diet reduced insulin sensitivity is unclear. However, the researchers observed a few significant associations.
During the low-fat diet, the researchers found that increases in the amounts of skin fat were associated with better insulin sensitivity.
Although there was a similar trend during the high-fat diet, the association was not statistically significant.
Conversely, on both diets, an increase in the content of omega-6 docosapentaenoic acid in very-low-density lipoproteins (VLDL) was associated with reduced insulin sensitivity.
Yet it should be noted that these were observational findings, and do not prove a causal relationship.
Bottom Line: Increased skin fat was associated with increased insulin sensitivity. Conversely, high blood levels of omega-6 docosapentaenoic acid were associated with reduced insulin sensitivity.
This study was nicely designed, but had a few minor limitations.
First, the study was very small, including only 13 participants. For this reason, it may not have had the statistical power to detect smaller differences as significant.
Second, the dropout rate was very high, or 46%, indicating that dietary adherence may have been difficult.
Third, the study was designed to compare the effects of high-fat and low-fat diets on insulin sensitivity. Therefore, it does not provide any evidence for the effects of saturated fat on insulin sensitivity.
Finally, the study included individuals with normal blood sugar control, and the high-fat diet contained higher amounts of fat than are typically consumed. As a result, the findings may not apply to diabetics or those who consume lower amounts of fat.
Bottom Line: This study was very small and the dropout rate was high. Additionally, the findings may not apply to normal real-life settings or people with type 2 diabetes.
Summary and Real-Life Application
In short, this study suggests that a diet very high in fat, or saturated fat, make the body less sensitive to the effects of insulin.
However, the results may only apply to very high amounts of fat or specific types of fatty acids.