As you progress through your Data-Driven Fasting journey, you will start to notice how different things other than food influence your blood sugars.
A deeper understanding of these various factors will help you to better interpret the number you see on your meter.
- 9.1 Many Things Affect Your Blood Sugars
- 9.2 How Does Sleep Affect Blood Glucose?
- 9.3 Should I Exercise?
- 9.4 How Does Exercise Impact Your Blood Glucose Levels?
- 9.4 Is It Better to Exercise Fasted?
- 9.5 How Can I Use My Glucose to Optimise My Workout Fuelling?
- 9.5 Why Does My Glucose Rise When I Don’t Eat?
- 9.6 Can I Fast When I’m Stressed?
As noted by the author of The Complete Guide to Fasting, Dr Jason Fung, back in 2016, when we were initially developing the ideas around DDF, many things can affect your blood glucose, such as stress, exercise, illness, and hormones.
Tracking your blood glucose is a powerful way to ensure you give your body what it needs when it needs it. However, it’s helpful to be aware of factors other than your food that can affect your blood sugars.
The intent of Data-Driven Fasting is to use your blood glucose as a guide. Don’t be hard on yourself if you feel compelled to eat before your blood glucose drops below your trigger. When this happens, try to be mindful of the other factors like exercise, stress, mood, and hormones (e.g., that time of the month for women) that can affect your blood sugars.
Remember, you only need to log a blood glucose below your trigger more often than not to make progress. Your blood glucose can also highlight other areas of your life that need improvement, like stress and sleep.
If your blood glucose is a little high, you know you still have plenty of fuel onboard and that all you need is nutrients. So, if you are hungry and choose to eat, try to prioritise nutrient-dense foods and meals with a higher protein percentage.
This will allow you to draw on your body’s stored fat and glucose while providing the nutrients you need to function optimally, prevent cravings, and preserve precious lean muscle mass.
9.2 How Does Sleep Affect Blood Glucose?
Getting good sleep is essential, especially when it comes to managing your cortisol and waking blood glucose. Don’t underestimate the power of a good night’s sleep to stabilise blood glucose and help manage your appetite!
We tend to sleep better if we’re not over-full or too hungry. So, try not to eat your largest meal, or only meal for the day, just before you go to bed. If you do, your metabolism will be elevated through the night as you burn off the energy from dinner or that late-night snack.
At the same time, you don’t want to be too hungry. Ketone levels rise once your glucose is depleted to ensure you are alert and energised to go out and hunt rather than lie down and sleep deeply.
If you start to notice your blood glucose is elevated after not sleeping well, do what you can to improve your sleep hygiene by limiting blue light from screens after dark, having a wind-down routine, and allowing enough time for sleep.
It’s also worth noting that high-protein meals will keep your metabolism more elevated than carbs and fat. Thus, it’s ideal to front-load your protein earlier in the day and top up on energy later (if required) when your blood sugars are lower.
9.3 Should I Exercise?
Resistance training is critical to send a signal to tell your body to maintain your muscle during weight loss. However, keep in mind that your blood glucose may go up or down when you exercise. You should, therefore, only use your blood glucose as an indicator of when to eat after your blood glucose has settled down.
If you typically feel hungry around your workout, you could choose to eat your Main Meal in the morning just before or after your workout, regardless of your blood glucose. Ideally, this should be protein-rich to support recovery. You can then treat other meals and snacks across the day as Discretionary Meals and only eat them if your blood glucose is below your trigger.
If you notice that your blood glucose is abnormally elevated after exercise, you can flag it in the DDF App to get a slightly higher trigger for the day.
While lower-intensity exercise can lower blood glucose levels, keep in mind that intense exercise can increase them. Although your post-exercise glucose levels may be higher than your trigger, it’s OK to eat quality food when you’re hungry after your workouts. In fact, it’s often ideal to eat after exercise when you first feel hungry, even if your blood sugar is still above your trigger, rather than waiting to the point you are ravenous because your blood glucose has plummeted.
9.4 How Does Exercise Impact Your Blood Glucose Levels?
Taking a moderate paced walk or doing some low-intensity exercise can deplete your blood glucose.
Exercising at a lower intensity when you can still breathe and talk easily will burn more fat and lower your glucose. Resistance training helps build metabolically active lean mass, which increases your overall metabolic rate.
Both resistance training and low-intensity exercise have their place, but they will affect your blood sugar differently. Meanwhile, higher intensity exercise will cause a release of stored energy from your liver and you may see blood glucose rise.
If you do intense exercise, your body will release glucose into your bloodstream to fuel the activity. This is not a problem, but it’s just something to keep in mind when testing your blood glucose.
People often don’t feel hungry after exercise because of this elevated blood glucose. However, if you feel hungry after your workout and your blood glucose is still artificially elevated, don’t be afraid to eat! You may find that eating your Main Meal before or after your workout helps you avoid more intense hunger later in the day, which often leads to poorer food choices.
If you do a heavy workout like high-intensity interval training or intense weightlifting that depletes glycogen stores, don’t be surprised if you find yourself hungrier later in the day as your body tries to refill your glucose and fat fuel tanks. We suggest you use your blood sugar levels to experiment with your meal timings to ensure you refuel before your blood glucose gets too low and you end up in an all-out binge.
It’s virtually impossible to accurately calculate calories burned during exercise. It’s also easy to overeat due to increased hunger after your workout. However, once the glucose surge has settled down, your blood glucose will give you a good idea of whether you are over or under-fuelling over the long term.
To cater to the effect that exercise has on blood sugars, you may find it helpful to have your nutrient-dense, high protein-focused Main Meal around your workout. You can then use your blood glucose trigger to determine other Discretionary Meals during the day once your blood glucose normalises. When you log your blood sugar after a workout in the DDF app, your trigger will temporarily increase, so you can go ahead and eat even if your blood sugar is elevated.
9.4 Is It Better to Exercise Fasted?
If you want to burn body fat, have plenty of glucose on board, and are not hungry, exercising in the morning before you eat a great idea. However, if you want to perform explosive exercise, run faster or lift heavier, you don’t want to have low blood glucose before your workout.
9.5 How Can I Use My Glucose to Optimise My Workout Fuelling?
Your blood glucose can give powerful insights to help you decide when and what to eat around exercise. CGMs have even been banned in cycling competitions due to the potential performance advantages.
Even if you’re not an elite cyclist, you still want to keep your blood glucose in the normal healthy range to maximise performance. Overdoing carbs can cause sharp rises that lead to glucose crashes later. Similarly, you also want to avoid running out of glucose (known as ‘bonking’).
Outlined below are some scenarios to show how you can optimise your workout selection and fueling to align with your goals to enable you to leverage the insights from your glucose monitor.
Lower Intensity Short Duration Exercise
Low-intensity exercise is a great way to lower glucose at any time.
One of the primary observations from many people who have tried CGMs is the importance of being active throughout the day to lower glucose.
If your blood glucose is elevated, a gentle walk is a great way to bring it down at any time of the day, e.g.:
- in the morning before you eat when your blood glucose is elevated due to the dawn phenomenon, or
- after a meal to blunt the rise in glucose after you eat.
Lower intensity exercise mainly uses fat, but it will also slowly deplete your blood glucose. The good news is that if you are using your glucose to guide when you eat, you will be able to eat again sooner because you have used the excess glucose in your system.
If you want to get technical, your goal is to keep your intensity low enough to stay below your “lactate threshold”. This is the intensity at which your body can use the lactate produced for energy without it building up to higher levels in the blood.
But unless you’re an elite cyclist, you’re probably not testing your lactate or VO2max. You can also use a heart rate monitor and keep your heart rate under 180 minus your age. This is also known as “Zone 2” training.
But the most practical way for most people to monitor intensity is to monitor their breathing. For example, if you can still carry out a conversation with complete sentences and breathe through your nose, you’re in Zone 2 and will deplete rather than raise your glucose. Likewise, if you were talking on the phone, the person on the other end would know you’re exercising, but it wouldn’t hamper the conversation.
You could potentially go all day and not get tired at this lower intensity. And possibly, more importantly, you can get up the next day and do it all over again without being overly tired. This pace may initially feel slow, but in time, as your body learns to oxidise more fat for fuel rather than glucose, during exercise, the speed and power you can produce at a lower intensity will increase.
You don’t have to go all out if you’re new to exercise. Monitoring your step count on your phone can be a great way to start. If you look at your phone and find your current average step count is 2000 per day. Next week, aim for 3000. Once you can consistently do that for a week, aim for 4000 each day.
If you’re curious, you could check your glucose before and activity the activity. You should see it drop slowly but not so low that you get ravenously hungry afterwards.
Short Duration High-Intensity Exercise
Once your body has mastered using fat for fuel at a lower intensity, more intense exercise such as resistance training or high-intensity interval training (HIIT) is a great way to build strength and aerobic capacity. But your body will need some glucose to fuel your more intense activity.
Unless your glucose is lower than normal for you before your workout, you don’t need to eat before an intense workout. Your body has plenty of stored glycogen that it can release to fuel your activity. If you test your glucose, you may see a rise in glucose during your workout.
However, you need to be ready for the crash in glucose afterwards. If you use a ton of glucose in your high-intensity workout, your muscles will be depleted afterwards and suck the glucose from your blood into your muscles to refuel.
Rather than insulin ‘pushing’ glucose into your cells, this process uses your GLUT1 glucose transporters (i.e., non-insulin-mediated glucose uptake). Waiting too long to eat after a workout can lead to lower blood sugars than your body is comfortable with and intense cravings for energy-dense foods to quickly replenish the energy deficit created by the workout.
So, you should be ready with a robust, protein-focused meal after your workout that will blunt your hunger and give your muscles what they need to recover and grow. If your blood sugars are lower than normal for you, you can use some faster acting carbs to bring your blood glucose up into the normal range and keep Lizzy at bay.
Longer Duration Low-Intensity Exercise
If you’re doing very long duration activity (e.g., triathlons, marathons etc.), it’s essential to train your fat-burning metabolism with lower intensity exercise and get used to having lower glucose levels. This training style means that, when it comes to performing, you have plenty of glucose in reserve for the intense efforts. While you may be able to use mainly fat for the majority of an event, competitions are won and lost in the sprints at the end. Having some glucose in reserve is critical.
When it comes to fuelling your activity, fast-acting carbs that raise your blood glucose levels excessively can lead to crashes later. So, it can be useful to monitor your blood sugars before, during and after exercise and ensure that your blood sugars are not rising excessively (e.g., by more than 30 mg/dL or 1.6 mmol/L) after you eat or crash below your normal level. Evening out your fuelling by finding the balance of fat and carbs that suit your activity and metabolism can lead to more stable and sustainable energy levels. If your blood sugars don’t rise excessively, you can afford to fuel with more fast-acting carbs. But if you’re not yet as metabolically flexible as a Tour De France cyclist, you may need to lean towards more fat to ensure a steadier flow of fuel.
Of course, if you find yourself with lower blood sugars towards the end of the activity, some fast-acting carbs can be helpful. This is why endurance athletes are constantly pounding gels and bars during the event to refill their glycogen stores quickly to prevent bonking.
You can find the fuelling regime that works for your metabolism and training with some trial and error. While most high-performance athletes require tons of fast-acting carbohydrates, many people who compete in more extended duration events have found it is more useful to lean towards a higher fat diet to maintain more stable energy and less dependence on in-race fuelling.
Once you’ve trained your fat-burning metabolism, you can throw in carbs and fats to fuel up on race day for the best performance.
9.5 Why Does My Glucose Rise When I Don’t Eat?
It may seem counter-intuitive, but your blood glucose after meals or upon waking can increase as you start to lose weight, particularly if you don’t have great metabolic health and (or) have a lot of weight to lose.
If you are overweight, your body has a lot of stored energy to unload when you don’t eat for a while. So, given the opportunity, your body is eager to dump your stored energy in the form of glucose and fat into your bloodstream to make up the difference.
You should see your waking blood glucose trend down over the long term, especially once your weight is stable. However, many people see their waking blood glucose drift up a little in the short term. They also find their blood glucose can increase during the day, even if they haven’t eaten.
This is not cause for concern. Your body is just releasing stored energy from its fuel reserves into your bloodstream. You can see rising glucose when you don’t eat as a sign that your body is releasing stored energy, so you don’t need to eat now.
If other health markers like body fat, weight, and waist are moving in the right direction, then there is a good chance you will be in a much healthier position long term once things stabilise.
Rather than trying to fast for longer, which can cause your blood glucose and ketones to rise even more, it’s ideal to find a sustainable daily eating pattern. Eventually, you should see your blood glucose levels fall, and your body fat levels decrease as you deplete your liver glycogen.
9.6 Can I Fast When I’m Stressed?
The extra adrenaline and cortisol from stress can cause your blood glucose to rise, regardless of what you eat (or don’t!). Stress can also cause your lizard brain to awaken, and you may eat for comfort to prepare for an emergency like during a famine or to run away from a predator.
Aside from waiting to eat, you can lower your blood glucose by doing what you need to relax and de-stress. Breathing exercises, meditation, a walk in the sun, sleeping in, a hot bath, and journaling are powerful and free ways to calm your mind. Don’t be surprised if actively managing your stress helps you lose weight!
Many people find they get the best results with Data-Driven Fasting when they relax, don’t overthink the process and stop trying quite so hard to manage the outcomes. Trying to understand every nuance can lead to increased stress, which is counterproductive.
In the Data-Driven Fasting 30-Day Challenge, we encourage people to relax and follow the process. It’s only four weeks of your life! If it doesn’t work out, you can always go back to what you were doing before.
- Data-Driven Fasting Challenge
- Data-Driven Fasting Program
- Data-Driven Fasting: How to Lose Weight and Reverse Type 2 Diabetes Without Tracking Your Food
- Get the DDF Manual
- QuickStart Guide
- The DDF app
- Join Our Community
- Frequently Asked Questions