‘Metabolic flexibility’ is a term that’s been thrown around a lot recently.
But what does it really mean?
It might sound complex and confusing, but, in simple terms, ‘metabolic flexibility’ means having spare capacity in your body’s fuel tanks – space to store the energy from your diet, either in the form of fat or glucose (from carbs).
Your blood glucose is invaluable to validate your hunger, but it can also be useful to guide your food choices and maximise your metabolic flexibility!
This article will:
- guide you on to become more ‘metabolically flexible’,
- help you understand when you might benefit from fuelling with more carbs vs more fat, and
- outline the potential benefits of occasionally switching between fuel sources.
- How to become metabolically flexible
- What is metabolic flexibility?
- How to use intermittent fasting to increase your metabolic flexibility
- What to eat if you are NOT metabolically flexible
- Why do you want to be metabolically flexible?
- When would you want to fuel with carbs rather than fat?
- Your natural cravings… with a little nudge
- How to use your glucose to make better food choices
- What if I’m insulin resistant or have diabetes?
- Nutritious higher carb lower-fat food choices
- Don’t forget the protein
- How Data-Driven Fasting guides your food choices based on your blood glucose
- Start optimising your metabolic flexibility now
- Further reading
How to become metabolically flexible
We’ll get into more detail in this article, but the process is simply:
- drawdown your excess glucose stores (by reducing carbs and/or waiting a little longer to eat),
- deplete your body fat stores (by dialling back dietary fat),
- prioritise nutrients to maximise satiety and reduce cravings,
- choose your fuel (i.e. carb or fat) depending on your activity and current glucose levels.
What is metabolic flexibility?
While we can use alcohol, ketones and even protein, our main energy sources are carbs and fat.
Being metabolically flexible means that we can quickly burn available fuels in the food we eat.
Ideally, when we don’t eat, we should be mainly drawing on our body fat stores.
But sadly, many of us are burning predominately glucose most of the time, even when we sleep or haven’t eaten for a while.
As detailed in Oxidative Priority: THE SECRET to Effective Fat Loss, our modern diet of hyperpalatable fat+carb combo foods causes us to overfill both our fat and glucose fuel tanks at the same time. Before long, this drives energy toxicity and, before long, progresses to prediabetes and Type 2 diabetes.
Because we have to use the more volatile fuels that can’t be stored, we have less storage capacity for them. As shown in the image below of your body’s fuel tanks, we end up stuck burning glucose most of the time, and the other fuels are stored away in our body.
Imagine a balloon pumped full to the point of bursting. If you touch the balloon, it’s rock hard.
The balloon is not very flexible because it is so full.
Even if you ate a high fat keto meal, you’d likely still be burning glucose, with most of the fat from your meal stored as your body tries to burn the stored carbs.
If your fat fuel tank is full to overflowing, it’s likely you’ll still be burning glucose. Research shows that people with a higher respiratory quotient (i.e., using more glucose at rest) are much more likely to gain weight.
The first step to restoring flexibility is simply to reduce the pressure of stored fuels by waiting for a little longer until we need to refuel. It’s when once you reduce your stored energy to below your Personal Fat Threshold that you will regain metabolic flexibility.
How to use intermittent fasting to increase your metabolic flexibility
People who are insulin resistant and not metabolically flexible tend to find it harder to go too long without eating. It’s best to start small and work up to longer ‘fasts’. Data-Driven Fasting is an effective way to guide your body to progressively deplete excess stored glucose. As shown in this next image, once glucose is depleted, your body can turn to fat for fuel.
What to eat if you are NOT metabolically flexible
If both your glucose (in your blood and muscle) and fat stores (in your blood and adipose tissue) are already full, it makes sense to prioritise foods with more nutrients (i.e., vitamins, minerals, essential fatty acids and amino acids/protein) with less energy energy (i.e., carbs and fat).
Once you have depleted your excess energy stores, you will no longer be stuck trying to burn off the glucose that is backed up in your body. You will have become metabolically flexible and now able to use and store carbs and fat.
Why do you want to be metabolically flexible?
I’m not about to tell you that you need to eat potatoes all the time if you enjoy a low carb diet or drink bulletproof coffee every day if you are thriving on a low-fat diet. But it’s helpful to understand how we can switch up our food choices based on what our body requires at any given time.
In the past, nature forced us to be metabolically flexible, with different macronutrients more available at different times of the year:
- fat and protein in winter,
- lean protein and fibre in spring (which allowed us to lose the winter blubber),
- more carbs in summer, and
- fat+carb in autumn (to fatten us up for the coming winter).
Unfortunately, these days our food system is packed with fat+carb foods. We’re stuck in perpetual autumn. Winter (and spring) never comes.
When would you want to fuel with carbs rather than fat?
Athletes are at the forefront of pushing their bodies to become more metabolically flexible to train their bodies to burn both glucose and fat more effectively. They have learned to push back on their body’s natural craving to train their bodies to become more efficient at burning fat.
By training in a glycogen depleted state, we can force our mitochondria to burn fat more efficiently and preserve glucose for when we need it. Then, when we’re about to race, we can fuel up on glucose and fat, able to burn both fuels efficiently.
- Some of the most interesting research in this area has been done by Dr James Moreton, who worked as Team Sky’s nutritionist when Chris Froome won the Tour De France in 2018. Moreton talks about ‘fueling for the work required’.
- Some bodybuilders practice carb cycling in which they intentionally deplete their glucose for a number of days to lose fat to the point that they can’t keep performing. They then load up on carbs (with minimal fat) to replenish their glycogen stores, reset their hunger hormones (e.g., leptin, ghrelin, cortisol), and start the cycle again.
- Others use a cyclic ketogenic diet that involves staying low carb most of the time and then switching to high-carb on days that they train the bigger muscles that require more glycogen (e.g., squats and deadlifts).
- My friend Cain Foley (who wrote the excellent book Don’t Eat for Winter) alternated between high protein and fat meals in the morning and high carb low-fat meals later in the day around his workouts to get lean for a bodybuilding competition at 45. By staying out of the carb+fat danger zone, his meals were always satiating, and he was able to fuel his training with fast-acting carbs when required.
- Optimsiing Nutrition advisor Dr Ted Naimain also uses a similar approach where he uses low energy density carbs at in the evening to refill his glycogen stores after instense workouts.
- Closer to home, my wife Monica sticks to a fairly low carb way of eating to manage her Type 1 Diabetes. But when her blood sugar happens to go below the ideal range, she might eat some fruit (e.g., half an apple) to boost her blood glucose back into the normal range (note: the quickest and most effective way to raise glucose, and get out of a low caused by excess insulin, is glucose tablets)
- People using Data-Driven Fasting find they get great results when they prioritise protein at their first meal of the day to maximise satiety. They often find their blood sugars have dropped significantly below your trigger later in the day and can have some higher carb/low-fat foods, improving sleep and reducing cortisol.
If this all seems a little overwhelming, don’t worry.
- The majority of people who have plenty of glucose and fat stored in their body don’t need to worry about carb cycling. They need to prioritise enough nutrients without too much energy most of the time (i.e., nutrient density).
- As your blood glucose levels start to reach more optimal levels, you can use your blood glucose to guide the process to ensure you are giving your body what it needs, when it needs it.
Your natural cravings… with a little nudge
You may have noticed that you have different cravings at different times, depending on your activity levels and current diet.
If you’ve been avoiding carbs and/or more active than usual, you will have drawn down your available glucose stores, and your body will crave high carbohydrate foods.
When your blood sugars drop below what is normal for you, your ghrelin increases and your lizard brain (also known as your reptilian brain or the basal ganglia) wants you to reach for the doughnuts, pizza, chips and cookies (which are a combination of fat and carbs together) to quickly replenish your energy stores.
But this is not ideal if you want to lose weight. It just leads to uncontrollable overeating of these hyper-palpable foods. If we overfill both our fat and glucose stores simultaneously, we end up driving energy toxicity.
As you can see in the chart below (see this article for more details), if we stick to one side or another of the fat+carb danger zone, we will not tend to overeat. While we might prefer the doughnuts, cookies, pizza or creamy pasta, we only need to get a full signal from our carb OR fat fuel tank to be satisfied.
Rather than succumbing to our cravings for pizza and doughnuts when our blood sugars are low, we can prioritise high carb/low-fat foods and meals that will raise our blood sugars without overfilling our fat stores. Once you refill your glucose to normal levels, you will lose your cravings for carbs and achieve satiety with less energy than if we had gone all out with the pizza and chips.
According to Dr Clark Connery, occasionally spiking your glucose and insulin has the potential to send a signal to your thyroid that winter is over. We can reset our leptin levels and up-regulate our metabolism a little. You can check out our deep dive into the topic of metabolic flexibility here.
How to use your glucose to make better food choices
So when would high-carb foods be appropriate for you?
A blood glucose level that feels ‘low’ for you may feel high for someone else.
Hunger Training uses your blood sugar as a guide to deplete your excess glucose so your body can get on with using your stored body fat.
But if you wait too long or you are more active than usual, and your blood sugar drops significantly below Your Personalised Trigger, you can also use it as a guide to know when you actually need to refuel your carb tank.
If your blood sugar is low (for you) and you’re about to work out, then you would probably do well to have a few more carbs to increase the performance in your workout and build more strength.
If you want to train your body to burn more fat, you can choose to work out with a lower blood sugars. This is fine, so long as you don’t find yourself overindulging in the hyperpalatable carb+fat comfort foods later on.
Strategically timing your high carb/low-fat meals around your activity will enable you to enjoy a wider variety of foods that you enjoy and more opportunities to improve your nutrient density.
While you could get your respiratory quotient measured in a lab or even use a consumer device like Lumen to understand if you are burning primarily carbs or fat, the reality is that if your blood glucose is lower than normal, you will be burning more fat (and vice versa).
The cool thing about checking your blood glucose (other than it being super accessible and cost-effective) is that it also gives you an indication of whether you have excess energy onboard or need to refuel. You can use it to guide your fat loss journey AND meal timing.
What if I’m insulin resistant or have diabetes?
Many people from a low carb or keto background are unnecessarily concerned about avoiding ALL carbohydrates. However, some people do need to reduce their carbs to achieve stable, healthy blood sugars.
The simplest way to understand if you need to reduce your carbohydrate intake is to check your blood sugars before and after you eat. If they rise by more than 1.6 mmol/L or 30 mg/dL and take a long time to come back down, you should consider dialling back your carbs to get off the blood sugar – insulin roller coaster. A carb cycling approach probably isn’t going to be relevant for you.
If your blood sugars are in the normal healthy range, you can focus on maximising nutrient density and satiety most of the time and not worry about your blood sugars after you eat. There is no need to be concerned if your blood sugars rise and fall again quickly. In fact, low-fat foods may be more satiating than high-fat foods.
To explain how this would work in practice, let’s have a look at some scenarios.
Scenario 1. Significantly above Your Personalised Trigger
Let’s say you test your glucose, and you find you are significantly above Your Personalised Trigger. You realise that you probably overdid that last meal and have plenty of fuel onboard and can wait until you need to eat again.
2. Just above Your Personalised Trigger
There are times when people feel hungry, and their blood sugars are not coming down. Factors like stress, TOM for women, exercise, and sleep can artificially affect our blood sugars.
But if you are really hungry and just above your trigger, you know that you don’t really need fuel (from fat and carbs). If you’re going to eat, then you should choose the most nutrient-dense foods and meals that you reasonably can. It’s hard to go wrong if you give your body the nutrients it craves with less energy (from fat and carbs) if you know you already have fuel onboard.
3. Just below Your Personalised Trigger
If you wait and find your blood sugar is just below your trigger, then it’s time to eat and nourish your body. While getting nutrients is critical, you also need fuel.
This is where you can choose more energy-dense foods to refuel. Most of the time, you don’t want to wait until your blood glucose is too low because it will drive cravings for less than optimal foods.
4. Significantly below Your Personalised Trigger
If your blood sugar is significantly below Your Personalised Trigger, it’s time to eat.
Be aware that, because when your blood sugar is super low, we’re likely to reach for the peanut butter, pizza or other less than optimal food choices.
If you’re about to do an intense workout, you’re probably going to feel like trash, perform poorly and be even more likely to reach for the comfort foods later.
This is where we can choose higher carb low-fat foods to refill our glycogen stores (without also overfilling our fat stores at the same time). Not only will you feel better when you work out and recover faster, but you will also perform better, build more muscle and burn more fat.
Nutritious higher carb lower-fat food choices
Listed below are some high carb foods that you could use to boost your blood sugars if they are significantly below Your Personalised Trigger, especially if you’re about to do an intense workout. Incorporating low-fat, high-carb foods can provide more variety and nutrients that you may not be getting on a strict low carb or ‘keto’ approach.
If you’re trying to boost your glucose and insulin quickly, it’s worth noting that starchy foods (rice, potato, etc.) will be better than fruit. Fruit contains fructose which is converted to fat in the liver, and so doesn’t keep us away from the carb+fat zone we’re trying to avoid.
Fruits and vegetables
- bell peppers
- snow peas
- green beans
- Brussels sprouts
- sourdough bread
- brown rice
- sweet potato
- rye bread
- whole wheat bread
Don’t forget the protein
Don’t forget, regardless of whether you are eating low carb or low fat, obtaining adequate protein is still critical when you eat.
When you’re eating higher carb foods, you will want to focus on leaner protein sources like the ones listed below.
- milk (low fat)
- egg white
- sirloin steak (fat not eaten)
- chicken breast (no skin)
How Data-Driven Fasting guides your food choices based on your blood glucose
This may all sound like a lot to take in, so we have programmed it into our Data-Driven Fasting App to guide your feasting and fasting journey, including links to our series of nutrient-dense recipe books that are tailored to different context and goals. The table below shows how we’ve programmed the DDF app to guide your meal choices based on your blood sugars.
|Significantly above trigger |
BG > 126 mg/dL (7.0 mmol/L) or 0.4 mmol/L (7 mg/dL) above trigger
|Your blood glucose is above Your Personalised Trigger. We recommend you delay eating or skip your next meal. If you’re REALLY hungry and your BG stays elevated (e.g., due to stress, poor sleep, intense exercise or TOM), try to have one less meal today than you usually do.|
|Just above trigger & and insulin sensitive BG |
< 7 mg/dL (0.4 mmol/L) or above trigger AND post-meal BGs are < 30 mg/dL (1.6 mmol/L) above trigger
|Your blood glucose is just above Your Personalised Trigger. You should eat only if you are REALLY hungry. If you choose to eat, prioritise nutrients and a higher protein % (i.e., fewer carbs and/or fat). For nutrient focused, high-satiety meal and food ideas check out Nutrient Optimiser or the following recipe books: Fat Loss, Maximum Nutrient Density, or High Protein:Energy.|
|Just above trigger & insulin resistant |
Equal to or less than 7 mg/L (or 0.4 mmol/L) above trigger AND post-meal BGs are greater than 30 mg/dL (1.6 mmol/L) above trigger
|Your blood glucose is just above your trigger. You should only eat if you are REALLY hungry. If you choose to eat, prioritise nutrients and a higher protein % (i.e., fewer carbs and/or fat). For nutrient focused, high-satiety, lower carb meal and food ideas check out Nutrient Optimiser or the following recipe books: Fat Loss, Maximum Nutrient Density, or Blood Glucose and Fat Loss.|
|Below trigger and insulin sensitive |
Up to 3 mg/dL (0.2 mmol/L) below trigger AND post-meal BGs < 30 mg/dL (1.6 mmol/L) above trigger
|Congratulations. Your blood glucose is below your current trigger. It’s time to eat! For nutritious meal and food ideas, check out Nutrient Optimiser or the following recipe books: Maintenance, or Maximum Nutrient Density.|
|Below trigger and insulin-resistant |
Up to 3 mg/dL (0.2 mmol/L) below trigger and post meal BGs > 30 mg/dL (1.6 mmol/L) above trigger
|Congratulations. Your blood glucose is below your current trigger. It’s time to eat! For nutritious lower carb meal and food ideas, check out Nutrient Optimiser or the following recipe books: Maintenance, or Low Carb & Blood Sugar.|
|Significantly below trigger |
> 3 mg/dL (0.2 mmol/L) below trigger
|Your blood glucose is significantly below your trigger. If you’re feeling particularly hungry, feel free to enjoy some more energy-dense foods or higher carb foods and meals to raise your blood sugar a little (especially if you’re about to work out). For nutritious meal and food ideas, check out Nutrient Optimiser or the following recipe books: Bodybuilder, or Athletes & Bulking.|
Start optimising your metabolic flexibility now
To learn more about Data-Driven Fasting, you can:
- check out the DDF Facebook Group,
- download the free Data Driven Fasting Manual, or
- join the next DDF 30 Day Challenge.
- Metabolic Flexibility Deep Dive | Dr Clark Connery
- How quickly can you make progress with Data-Driven Fasting?
- How much difference does chasing your blood glucose trigger make to the weight loss process?
- Data-Driven Fasting: How to Lose Weight and Reverse Type 2 Diabetes Without Tracking Food
- How to reverse type 2 diabetes and optimise your blood sugar