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How Carb Intake, Metabolic Flexibility, and BMI Are Connected

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The ability to maintain a healthy weight is closely linked to our overall metabolic health, and specifically, our metabolic flexibility. Metabolic flexibility refers to how efficiently your mitochondria switch from burning carbs or fats for fuel depending on their availability and your body’s needs. Body mass index (BMI) is often used to assess whether one’s weight is in the healthy range. It is an indirect measure of your body fat based on your weight and height

A new study by researchers at Lumen involving 2,607 participants found a link between carb intake, BMI, and metabolic flexibility. When measuring metabolism post-meal, the obese BMI group in the study had a reduced carbs response (as measured by %CO₂ in the breath), indicating they were not efficiently using the carbs they ingested during the meal and likely had reduced metabolic flexibility as compared to those in the healthy BMI group. 

People with better metabolic flexibility (in the healthy BMI group) demonstrated a higher post-meal carb response, burning more of the carbs they ate for energy, rather than storing them as fat. This is important because carbs are more likely to be stored as fat when not used immediately after a meal. So people with reduced metabolic flexibility cannot eat as many carbs without gaining weight. 

Improving metabolic flexibility is not only essential for weight management, but it also reduces the risk of type 2 diabetes, cardiovascular disease, and hypertension.

3 Ways to improve your metabolic flexibility 

The good news is that you can improve your metabolism flexibility and make it easier to manage your weight by implementing consistent habits and lifestyle adjustments, such as balanced nutrition, exercise, and quality sleep.

Track your macros

Unlike counting calories, a macro plan involves tracking the quantity and balance of three macronutrients: carbohydrates, proteins, and fats.

In general, the Dietary Guidelines for Americans recommends the following for adults:

  • Carbohydrates: 45–65% of total caloric intake 
  • Proteins: 10–35% of total caloric intake 
  • Fats: 20–35% of total caloric intake 

However, your body’s needs and dietary requirements are unique. So, it’s best to tailor your macros based on your health status, nutritional requirements, and fitness goals. Tracking your macros will help you identify whether you’re eating carbs in excess or in balance. Consuming more carbs than you need, especially added sugars and refined carbs, can cause sharp glucose spikes and trigger high insulin production. 

Over time, this can lead to low insulin sensitivity or insulin resistance. This means your body can’t efficiently uptake glucose from your bloodstream and move it into your cells. When blood glucose levels remain chronically high because of insulin resistance, the body flags it as harmful and stores excess glucose as fat.

Build muscle mass

Physical inactivity is one of the main drivers of metabolic inflexibility. Research suggests aerobic and anaerobic activities provide health benefits, such as improving mitochondrial capacity and glucose control. High-intensity interval training (HIIT), in particular, can enhance your mitochondrial health and body composition.  

Moreover, building and maintaining muscle mass contributes to strength and metabolic flexibility. That’s because your muscles are packed with mitochondria. Having more mitochondria in your muscles means you can burn more fat at rest and shift to burning carbs when you need a quick boost.

Prioritize quality sleep 

Science shows that a lack of quality sleep is linked to an increased risk of metabolic diseases like diabetes and obesity. Sleep hygiene is essential because it keeps your circadian rhythm balanced. Your circadian rhythm is like your body’s internal clock and lets you know when it’s time to work and when to rest. When your internal clock loses its natural rhythm, your mitochondria work in high gear, which leaves less time for them to rest and repair while sleeping. On the flip side, rested mitochondria can function better and enable you to wake up in fat burn. 

To get enough quality sleep, start by implementing several simple sleep hygiene habits:

  • Follow a regular sleep schedule 
  • Put your devices away an hour before bed
  • Create a dark, comfortable, and relaxing bedroom environment for sleep 
  • Avoid alcohol, coffee, or large meals near bedtime 

These steps can kickstart a virtuous cycle. Prioritizing sleep enhances your metabolic flexibility, and a healthier metabolism can, in turn, help you get more restful sleep. 

People also ask

Can stress influence metabolic flexibility and BMI? 

Being in a constant state of stress increases the release of cortisol, a stress hormone. In turn, cortisol signals your mitochondria to switch from burning fats to carbs. In the long run, chronic stress keeps your mitochondria in overdrive mode, damaging them and cutting their lifespan short. Your mitochondria might also become used to over-relying on carbs and “forget” how to burn fat efficiently, which can contribute to weight gain. 

Does fasting improve metabolic flexibility and BMI? 

Fasting is a popular strategy for weight management. When you fast, your mitochondria burn through your body’s glycogen stores until they begin to burn fat. This depletes your glycogen, triggering mitochondrial biogenesis, which can be great for both metabolic flexibility and weight management. However, it’s important to understand your optimal fasting window, because, when you fast for too long, and your glycogen levels get critically low, your body begins to break down muscle into carbs for energy, which has a negative effect on your metabolic health. 


The link between carb intake, BMI, and metabolic flexibility sheds light on overall metabolic health. If you want to take the guesswork out of improving your metabolic health, Lumen can help. With a single breath into the Lumen device, you find out in real-time whether you’re burning fats or carbs for fuel. Lumen uses your metabolic measurements to tailor lifestyle recommendations for your unique body and help you achieve metabolic flexibility over time.

Dror Maayan is the head of organic growth at Lumen.

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