Do Carbs Cause Insulin Resistance?

Considering the cumulative evidence, it does seem clear that low-carb diets consumed ad libitum usually lead to weight loss in people with overweight or obesity. We also have strong evidence that weight loss is clearly associated with an improvement in insulin sensitivity, particularly in people who lose the most visceral and ectopic fat.

However, such an insulin-sensitizing effect is seen with any weight loss, including weight loss from bariatric surgery, medication, or other diets, and it’s therefore critical to assess the impact of low- vs. high-carb diets on insulin sensitivity under weight-stable or weight-matched conditions. In studies that controlled body weight or matched the degree of weight loss, insulin sensitivity is not clearly higher on low-carb diets, and high-carb diets do not clearly cause insulin resistance.

Many of the available published studies are small, and others employed imperfect measures of insulin sensitivity, so the evidence isn’t perfect at this point. However, a relatively large number of studies used the gold standard hyperinsulinemic-euglycemic clamp technique to measure insulin sensitivity. Also, all food was provided in many studies, and important variables, such as energy or protein intake, were well-controlled in most studies. Therefore, while not fully conclusive, the available evidence suggests that the carbohydrate (and fat) content of the diet is not per se a major determinant of insulin sensitivity.

Now, why may it be that hyperinsulinemia in clamp studies or patients with insulinoma triggers insulin resistance, while eating a high-carb diet does not? Considering the substantial differences in carb content of the tested study diets in these studies, it is pretty certain that insulin concentrations throughout the day must have been higher on the high-carb diet arms.

For one, the nature of hyperinsulinemia in clamp studies and insulinoma patients is chronic, with insulin levels elevated for many hours to days. That’s very different from the regular ups and downs in blood insulin levels that occur with eating a high-carb diet.

It is also possible that other components of high-carb or low-carb diets balance out any effect the carb-induced elevations in blood insulin may have. Because when people eat these low-carb and high-carb diets, in these research studies as well as in real life, they don’t eat isolated carbs or fats. They eat lots of bread, rice, pasta, and fruit on the high-carb diet, or dairy, meat, nuts, oils, butter, and cream on the low-carb diet. We should be clear that even though investigators designed these diets to differ in their carb and fat content, the resulting diets also differ in numerous other ways, many of which could also affect insulin sensitivity. In other words, it is possible that the normal up and down in blood insulin levels that results from eating more carbohydrates does cause some insulin resistance, but that this effect is balanced out by some other component of the diets. That may also explain why there is so much variability in the study results. It basically suggests that some hidden factor, or factors, are at play, and that may well be simply because each study team may have implemented these diets slightly differently.

For this analysis, I have reviewed 21 scientific publications based on 20 clinical trials, and I cannot detect a specific pattern that explains why the results are so heterogeneous. I had a few hypotheses, such as that studies that used more refined carbs in their high-carb diet arms may have had worse outcomes than those that incorporated mostly complex fiber-rich sources of carbs in their high-carb diet. This does not seem to be the case, as most of the studies, as much as can be determined from the papers, seem to have used mostly whole grains, legumes, fruit, and vegetables as high-carb foods in their high-carb diets. I also wondered whether the fat, protein, or fiber contents may have affected the overall results. I cannot confidently rule this out, but I also could not detect a specific pattern.

In other words, from the existing studies, I cannot confidently determine the mechanisms that explain the wide-ranging findings. What we can say clearly, however, is that in weight-stable people, eating more or less carbs will have very little to no consistent impact on insulin sensitivity.

That may be disappointing to some because if lowering carb intake reliably improved insulin sensitivity, we would have a fairly easy tool in our toolbelt to treat insulin resistance. It could also be seen as a positive, however, because these data give people more options in their dietary choices without risking insulin resistance. In other words, if your diet is high in carbs, for whatever reason, there is no good reason to think that this will make you insulin-resistant per se, as is often asserted in low-carb circles.

I also want to be clear that these data should not be seen as suggesting that all high-carb diets are good choices for everyone. Many food sources of carbohydrates, such as refined grains and added sugars, are highly glycemic, low in fiber, and provide little in terms of micronutrients. The data discussed here should not give anyone the impression that eating such foods in large amounts has a neutral effect on their health. Most of the studies discussed above included whole food-based sources of carbohydrates, such as whole grains, legumes, vegetables, and fruit. It shouldn’t come as a surprise, but is worth explicitly stating that investigators in these dietary intervention trials were not feeding their participants mostly candy and soda on their high-carb diets. It is therefore important to be clear that the impact on insulin sensitivity (and other health metrics) of a high-carb diet rich in refined grains and added sugars may well be worse than these data suggest. It is also clear that high-carb diets may be problematic for people with glucose intolerance, such as pre-diabetes and particularly diabetes mellitus.