If you ask 100 people about their thoughts on fasted cardio, you’d probably get 100 different answers, coming down to two main themes:
- It is GREAT. You’ll get SUPER lean SUPER effectively.
- It is the worst thing in the world and will eat your hard-earned muscle. Avoid like the plague.
I’m here to give you an objective look and let you make the decision for yourself.
What IS Fasted Cardio?
Contrary to popular belief, fasted cardio does NOT necessarily have to be immediately upon waking. Fasted is simply a post-absorptive, low insulin state, where food is no longer being digested.
As T1Ds, we know that our short-acting insulin has completed its activity within 4 hours, with the majority completed within 2-3 hours (1). While digestion can last a while depending on what has been eaten and how much has been eaten, you could theoretically do fasted cardio at times other than first thing in the morning.
What are the benefits?
The theory behind fasted cardio, especially early morning fasted cardio, is related to insulin, glycogen, and fat oxidation. Let me lay out the fast facts:
- High levels of insulin shift metabolism to utilize carbohydrates as the fuel of choice. Thus, it is very difficult to oxidize fat.
- After an overnight fast, the body will have used up much of its liver and muscle glycogen, which should allow the body to turn to fat as the most available energy source.
- Carbohydrates consumed at rest or prior to exercise will dramatically blunt fat burning. Since they’re used more easily, they take priority over fat.
What does all this say in English?
When fasted, your body should have low insulin, lesser glycogen, and no carbohydrates to block fat from being burned, which should THEORETICALLY lead to greater fat utilization and greater fat loss. One study demonstrated an additional 20% calories burned during fasted cardio (2).
Anecdotally, from myself and from others, it has also shown effectiveness in reducing levels of bodyfat in the “stubborn areas” like lower back and lower abs when bodyfat levels are already low (i.e. <10% in males, <18% in females).
What are the drawbacks?
- The intensity of your workout may drop. Because your body is not loaded up on energy from a recent meal, the workout you normally dominate may become more difficult.
- You may be more susceptible to injury due to the low energy state.
- You may unintentionally decrease physical activity through the rest of the day, thus lowering total energy expenditure…which would pretty much negate any additional benefit of fasted cardio.
- Cortisol levels may skyrocket, leading to multiple effects…I’m going to hit this one hard:
I mentioned our old stress hormone friend cortisol in my post about Avoiding High Morning Blood Sugars and he’s back again with the same mechanism! Levels are highest in the morning, and cortisol is like Pac-man for body tissue – it loves to eat it. What is its primary target? The muscle tissue that you have been working so hard to obtain.
Cortisol is smart: it also increases with cardio intensity or cardio duration. So, trying to outwit the hormone by choosing a longer, lower-intensity cardio session or choosing a quicker, higher-intensity session, you’re out of luck.
Cortisol can increase blood glucose levels quickly. More on this in a minute.
Fasted Cardio with T1D: Things to Consider
Some individuals with T1D find it easier to manage their blood sugars while doing fasted cardio because they don’t have any insulin or food onboard to adjust for, which can make it more predictable.
Here are some more points to consider:
HIIT Cardio
This is very individual-specific, as are most themes in T1D. But for many people with T1D, working out first thing in the morning WILL lead to an increase in blood sugar levels, especially if you choose to do a HIIT workout.
Why? Simple. Like I mentioned in the “How To Avoid High Morning Blood Sugars” post, the liver is going to recognize that energy is needed for activity and start dumping glucose into the bloodstream.
Since you are also inducing stress on your body in the form of exercise, catecholamines (i.e. epinephrine) will be released, mobilizing fat for energy (YAY!), but also mobilizing protein stores as well for gluconeogenesis and therefore, glucose (boo).
Thus, not one, but TWO mechanisms for raising blood sugar are about to come into play. So, you may need to take a small bolus and/or increase your basal to combat this forthcoming spike..I’ve spiked to 400 before from a pre-workout blood sugar of 90.
It can take a little bit of trial and error to get the right pre-workout bolus or basal increase down, so don’t give up!
Steady-State Cardio
This may not be as pronounced for the steady-state cardio crowd (i.e. distance runners), but is still something to watch for, as blood sugars will likely increase a bit.
For those of y’all who choose to work out fasted but not immediately upon waking (i.e. 4-5 hours post-meal with nearly 0 active insulin at the start), the effect on your blood sugar will be as one normally expects, simply due to previous energy intake. Food for thought.
Solving the Muscle Loss Conundrum
Want to start, but concerned about muscle loss?
One way to combat this is to take a scoop of whey protein pre-workout.
Would this still be considered as fasting? Tomato, To-mah-to. It’s not a real meal.
When I work out in a fasted state, I swear by a half scoop of whey for this reason, and I like to think I’m doing pretty ok.
What Should I Do?
It’s All About the Calories, Baby
If the goal is fat loss, one needs to be in a caloric deficit, plain and simple. At the end of the day, the difference between fueled and fasted cardio is not massive. It’s much more important to actually DO the cardio at a reasonable intensity than doing it fasted and hating your life.
If you can fit fasted cardio into your schedule (and it doesn’t feel like torture), by all means, do what you do, but don’t worry if you can’t. Put in the work, and you will reap the rewards.
References
Dwayne
I have type 2 diabetes and my blood sugar goes way up if i exercise fasted. You used a couple terms i am not familiar with. Bolus and basal. Could you explain?
Christel Oerum
Those are insulin terms. Basal, or long-acting insulin, is the background insulin. Bolus is when you inject rapid-acting or short-acting insulin to cover food, or stressful situations such as fasted exercise. If you don’t use insulin to manage your diabetes and want to reduce the exercise spikes you can experiment with eating a small meal before your workout
C
Very interesting article. A trainer was recommending fasted walking to help lean out legs. I’ve been faithfully doing it and normally I drop weight easily. Out of curiosity, I just took my blood sugar–my blood sugar went UP during this fasted walk! Scary that fasted cardio can also increase cortisol. So, if you want to get the benefit of cardio in the morning, could you have a small snack with a bit of protein and then still burn some calories and a bit of fat, and keep the cortisol levels more in check? Thank you for the great article
Christel Oerum
A small snack won’t hurt you. If you inject insulin you could also just try a small bolus or an increase in your basal
May
Really useful article, thank you! I am T1D and often have what my physician described as a ‘hormonal’ glucose spike in the mornings. Early workouts are best (i.e. without fast-acting insulin in my body) and weight workouts consistently bring my glucose levels down. Cardio tends to leave me higher, though. How do I get the benefits of the fasted cardio without ending up too high blood sugar or taking fast-acting and then not burning any fat?
Christel Oerum
You can try and minimize the hormonal impact by doing steady state cardio (no interval training, super hardcore cardio or weights) or simply inject a small correction dose. You’ll still be burning calories
Aurelio Santos
Hi!
After reading the comments, being a Type 2 Diabetic wanting to do fasting workouts, it is recommended to have some nuts, for example, to avoid the glucose spike and the cortisol raise?
I’m sorry but english is not my mother tongue and some things get lost in translation.
Thanks for any tips on this! I tend to double my glucose level after an hour of fasting intense workout and it is soooo frustrating.
Christel Oerum
You could try with a few nuts or protein (such as a few slices of meat or cheese). If your blood sugar comes down again by itself rather quickly after the workout you could also just let your body work it out on its own. Could be a good idea to run it by your doctor and get his/her medical opinion
shalini sinha
Hi, I would like to understand what impact can fasted cardio have on T2D. Is it going to be any different than what this article explains?
Ben
Hey Shalini,
If you are insulin dependent, it should be a similar situation. If you are NOT, the physiology will be the same for the most part, but the blood sugar spike may not be as pronounced since your body still produces insulin to a degree.
Jeanne
I am insulin resistant and have terrible trouble losing weight. My husband is a bodybuilder, so I eat clean and often, 11-1300 calories a day and have tried all exercise known to man. Right now I’m doing 80 Day Obsession and an derling stronger (8 pounds lost in eight weeks with 230 to go). Ivecheard fasted Cardio is not good for someone like me. Is this true?
Alyssa
I just want to make sure I understand:
1. Cortisol levels are at their highest in the morning; doing fasted cardio could increase those levels even more?
2. If I were planning to do 1iu of insulin pre fasted cardio and then 25-30min LISS; would I reap the benefits of fat loss from my blood glucose being “undisturbed” because my body will turn to stored fat for fuel?
Ben Tzeel
Hi Alyssa,
To respond to your points above:
1. Cortisol spikes at ~7am and trends downward through the rest of the day. Fasted cardio can increase it, yes, but it is already elevated – for example, if you did fasted cardio at 10am, prior to starting, your levels will be lower than if you did the same cardio at 7am. The elevation of the exercise is less than that of the natural cortisol rhythm.
2. This will depend on the individual – that 1u of insulin is to cover any sort of hepatic glucose output from glycogen breakdown that would result in a BG increase. I can’t say how much fat you will burn – that depends on your respiratory quotient, which is dependent on a variety of factors about you – but it is likely to be a higher amount of fat than carbohydrates/glucose, yes.
Mick
So let me see if I have got this right. Lets say I wake up and decide to go for a medium to fast-paced walk for 20 to 25 minutes before I go to work and my blood glucose levels are 13.5 mmol/L (243 mg/dL) prior to the walk. If I were to take insulin to lower my levels it would therefore not allow my body to utilise fat for energy and therefore defeat the object. However, if I were to not take insulin before the walk my blood sugars would actually rise due to the breakdown of glycogen to glucose? Could you please tell me if I understand the situation correctly? I am a bodybuilder who is T1D and want to know how to reduce fat levels down to around 5% safely and productively without losing too much hard earned muscle mass.
Thanks for your help in advance
Ben Tzeel
Great question! Here’s how I’d think about this situation:
1) what type of cardio am I doing? In your case, it’s medium to fast paced walking, so I would classify that as lower to maybe medium intensity.
2) because it’s a lower intensity type cardio, now consider its effect on blood glucose- less likely to induce an enormous cortisol release or hepatic glucose output, and typically DECREASES blood glucose.
3) since you started a little on the higher end of the blood glucose spectrum, and any fasted cardio COULD increase blood glucose, a small amount of insulin may not only prevent hepatic glucose output, but also assist in jump starting a blood glucose drop. A small amount will NOT blunt fat burning effects
Katherine Dilworth
You are a true motivation dear. Thanks for sharing this awesome piece of information with all of us. Will share this article with one of my friend who is suffering from the same. I hope he’ll also get inspired by you. Thank you once again. Best wishes and Regards.
Caroline
Interesting article! I’m T1D and fasted cardio definitely does not work for me. My blood sugar drops too low – opposite of what you’ve mentioned. But we’re all different so this is not surprising! I usually have to eat around 30g carbohydrate at breakfast in order to then do cardio and not go low.
Leana
Awesome article!