Metabolic Flexibility

Your body is amazingly flexible. Maybe not in a touch-your-toes-from-standing kind of way, but deep inside at the cellular level. This metabolic flexibility enables your body to fuel itself using different kinds of macronutrients such as carbohydrates or fats.

This ability was especially important when humans’ food sources were unpredictable, but it is still useful today. However, our easy access to calorie-high processed foods can impair metabolic flexibility and lead to health problems.

What is metabolic flexibility?

Metabolic flexibility is your body’s ability to easily switch between energy sources depending on access to food and activity level.

Glucose is your body’s primary fuel source, a simple carbohydrate found in foods like table sugar, candy, and white bread. Your body breaks down complex carbohydrates in grains, vegetables, and other foods into glucose.

After eating, your body tends to use glucose as its fuel source, storing excess as glycogen and fatty acids. “Any time we eat when we’re not hungry and don’t have capacity in our muscles to store excess fuel [as], we convert that energy to fat,” says Kim Shapira MS, RD, a nutrition therapist and author of This Is What You're Really Hungry For: Six Simple Rules to Transform Your Relationship with Food to Become Your Healthiest (BenBella, 2023).

After extended periods without food, your body shifts gears to rely on different fuel sources, such as glycogen and fatty acids. And during exercise? Your body uses glucose as well as stored glycogen and fatty acids.

Benefits of metabolic flexibility

One of the benefits of metabolic flexibility is your body’s ability to meet shifting energy demands throughout the day, even when you haven’t eaten for a while. Early studies also suggest that metabolic flexibility is associated with better weight management, a lower risk of developing metabolic syndrome, and healthier blood glucose levels.

Metabolic syndrome is a group of conditions that increase your risk of heart disease, diabetes, stroke, and other health problems. A person is diagnosed with metabolic syndrome if they have three or more of these risk factors:

• high blood glucose
• low blood levels of HDL (“good”) cholesterol
• high blood levels of triglycerides
• large waist circumference
• high blood pressure

In contrast, impaired metabolic flexibility is associated with insulin resistance, which is linked to type 2 diabetes, obesity, cardiovascular disease, and other health problems.

Insulin resistance is when cells in your muscles, fat, and liver don’t respond normally to insulin—the hormone that signals cells to accept glucose and produce energy, lowering your blood sugar. As a result, your blood sugar remains high, your cells produce less energy, and your pancreas produces even more insulin to try and get through to your cells.

Metabolic flexibility vs. metabolism

Metabolism refers to all the chemical reactions that take place in the body’s cells to provide it with energy. This energy is used to carry out everyday functions, from activating nerves and muscles to building bones and warming the body. As your ability to switch between energy sources, metabolic flexibility is just one aspect of your metabolism.

The number of calories needed to sustain your body at rest is called your basal or resting metabolic rate. Basal metabolic rates are affected by factors such as sex, weight, height, age, ethnicity, body composition, and genetic factors.

Of these, you can only change your weight and body composition. One way to increase your basal metabolic rate is to increase muscle mass through strength training. Another is to decrease your weight through exercise and dietary change. However, neither method guarantees immediate change to your metabolism.

Ways to boost metabolic flexibility

There are several things you can do to maintain metabolic flexibility:

• Eat a whole-foods diet with minimal intake of processed foods and added sugars. “Every single person alive needs a combination of fermented foods, fruits and vegetables, whole grains, fiber, and a good probiotic,” says Shapira.
• Avoid excess caloric intake, especially eating when you are not hungry, which can lead to insulin resistance even in people who are not overweight.
• Stay physically active, which can help improve metabolic flexibility and insulin sensitivity. “In order to maintain a healthy weight, you need at least 10,000 steps a day or an active lifestyle,” says Shapira. “You also need a variety of movements—strength training, cardiovascular fitness, flexibility, and endurance.”
• Get better sleep. Sleep deprivation can also lead to insulin resistance as well as increased inflammation. “If you’re not able to fall asleep, stay asleep, or you wake up feeling unrested, it creates stress, which causes more inflammation,” says Shapira.
• Manage your stress. Exposure to chronic stress can lead to insulin resistance and contribute to type 2 diabetes.

While exercise can help you manage your stress, Shapira cautions that people who are chronically stressed should focus on gentle activities, such as moderate walking, yoga, or Pilates. “If you are stressed, doing a vigorous workout only amplifies that stress and becomes more disruptive to sleep,” she says.

This article was originally published in the January-February 2025 issue of alive magazine (US edition).