Understanding Metabolic Flexibilty and the Role of Insulin

Metabolic flexibility is our body’s ability to burn either carbs or fat for fuel. High carb diets and high insulin can lead to metabolic inflexibility, which means metabolic diseases like diabetes.

As a nutritionist one thing I’ve been focused on lately is something called “metabolic flexibility.” Metabolic flexibility is the ability of the human body to ramp up the use of fuels that are more available in the diet. Eat more fat – the body will burn more fat. Eat more carbs – the body will favor carbs for fuel. The body is an amazing machine, don’t you think? My car can’t do something like that, at least not with great results (or, more likely, without disastrous results).

One of the major ways the body achieves this flexibility is through insulin. Insulin has powerful effects on the use of fuels. Namely, high insulin means carbs will be the preferred source of fuel, and low insulin means fat will be preferred. However, it is known that chronic consumption of sugary foods and high body fat is associated with something called “insulin resistance.”

In a nutshell, insulin resistance is when your cells don’t respond as strongly, or in a normal way, to circulating insulin. Insulin resistance is a facet of metabolic diseases like diabetes. Chronic insulin resistance isn’t a good thing, but it may play a role in how the body alters its utilization of fuels.

In a recent study in Nutrition and Metabolism, researchers compared the use of carbs and fats as fuels in regards to insulin resistance in a group of 180 women. They wanted to find out if insulin resistance led to metabolic inflexibility creating a situation in which the body could not react appropriately to changes in diet. They discovered that both insulin resistance and a family history of diabetes led to metabolic inflexibility even when they factored out both body fat levels and age.

Because of this, when the participants would consume higher levels of fats their fat oxidation (utilization of fat as a source of fuel) rates were lower than normal. They also indicated that the levels of mitochondria, the parts of your cells that create energy, were reduced, and total metabolic activity consequently went down. This means reduced energy levels, even while you’re at rest. Metabolic inflexibility reduces your overall ability to make energy at all. Big problem.

So what can be done to prevent this downward spiral into eventual metabolic disease? The answer is simple and makes a lot of sense, but it’s worth putting in print. First of all, exercise not only helps reduce your body fat, it also increases insulin sensitivity, and not just in the short term. Exercise also increases your mitochondrial levels, which means a greater capacity overall for the use of fat as a fuel and greater metabolic flexibility. A higher fat diet also favors the use of fat as fuels and ultimately will help improve insulin sensitivity in metabolically inflexible people. In particular, omega 3 fatty acids cause a strong preference for the use of fat as a fuel.

If you have a consistent high carbohydrate diet, especially a sugary one that yields a big insulin response, or a family history of diabetes it’s time to start being mindful of metabolic disease. Exercise frequently, especially with a focus on aerobic exercise, and eat healthy fats while reducing sugar intake. I personally believe that regularly fasting may help as well.


1. Madelaine Carstens, et. al., “Fasting substrate oxidation in relation to habitual dietary fat intake and insulin resistance in nondiabetic women: a case for metabolic flexibility?,” Nutrition & Metabolism 2013, 10:8

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