The Difference In Fat Burning With Cardio Vs. Weight Lifting

As a personal trainer one of the things I get asked about most, or that I see the most confusion and misinformation about is weight loss and cardio.  You go to any gym and anywhere near peak hours and you’ll be lucky to get an open treadmill or bike if that’s your style.  Cardiovascular training has taken over as the primary exercise method for America when we enter the gym, and first and foremost I want to say that cardio is fantastic for your health.  Cardio training will make a stronger and healthier heart and vascular system, as well as offering other benefits.  That said, however, how many people do you see at the gym that use cardio machines every week that get smaller doing just that?  The answer is almost always none of them.  If someone simply does cardio, no matter how much, there is usually minimal positive impact on how much fat they are storing.  Before I get branded a fitness heretic, I again remind you that cardio is important for your health, and massively important for sports performance because of the specific way it improves endurance and conditioning, but that’s an entirely different article.


The Science

Before we go any farther we do have to take a step back into some of the science of metabolism.  Firstly, the body simply does not want to get rid of its fat stores.  Depending on how you define it, our species has been walking the plains and forests for somewhere between two hundred thousand and two million years, and the fact is starving was not a rare state of being, especially during the colder months depending on where you were.  Daily fasting for many hours while you searched for food was the norm, there was no such thing as a pantry, let alone a grocery store or restaurant.  We survived as a species because our bodies got very good at using as few calories as possible when we were not plentifully supplied with food.  Our adipose (fat) stores are what keep us alive between finding a large kill or a fruit tree in bloom, and so our body knows that if we dig too heavily into it, the first famine that comes along will wipe us out.  This may seem understandable to us, but there are tens of thousands of calories on even your average skinny person, but what if a famine lasts weeks or months?  Our body is not looking to actually be ‘fat’ in the modern sense the phrase, obesity is actually our sense of how many calories we have on hand gone haywire but the unfortunate result is the survivalist famine sensitive part of our brains is stronger than our chemical ability to realize we have too much in storage.

So what does our body use for energy then, anyway?  Our liver is our major on-the-go energy storage site, which can on average hold about 100 grams of stored sugar, or about 400 calories/4 hours of general use energy.  When we eat food, it gets broken down and the body actively uses the digesting food and the liver gets filled up until its around capacity and the rest goes to fat, if everything is operating under normal circumstances (non-normal circumstances are actually very common but we won’t talk about that within this text).

Our body then almost exclusively uses the sugar in our livers until it is depleted, and only then will it really tap into our fat stores in any significant way, and only then because it has no other choice.  Our body loves using the calories from fresh food and it’s liver because this means it’s energy reserve is safe from depletion and our survival isn’t threatened by the energy we are having to use.  Once the liver starts getting low or becomes depleted, surprise surprise, we get hungry and it wants to be filled again, all before it really touched our fat stores (and it will replace whatever it used when able).

I should also take a moment to point out, as I’m sure it will have occurred to some of you, if you hold off eating your body MUST dig into the fat, so you’ll lose it, right?  Yes and no.  It will cut into your fat for energy, but only to survive, as it starts sensing it isn’t getting the energy it needs for normal operation, it will start lower your metabolism by eating its own muscle (the part of our bodies that REALLY uses calories) and cutting down on how much everything else is functioning.  This will continue in greater degree until you eat or die.  The fat will eventually be lost, but only at threat to your life and your body, and the moment it has a chance to eat again it will protect itself from further such starvation situations by storing even MORE fat.  The lesson being, don’t try to avoid eating or under eating, caloric deficit diets are bad for fat loss.

So carrying on, the point we’re getting so far is that the body doesn’t like to use its fat unless it has to; when speaking on survival as an animal our attempts to get rid of our caloric warehouses is simply madness, though we know that food isn’t likely to dwindle any time soon and there are serious long term consequences to having that extra weight.  Our bodies, however, don’t know this.  We never spent time evolving to live in a world of plentiful food supplies, this only really happened in the last one to two hundred years (and only for the developed world, for the most part).  The result, it’s going to fight you tooth and nail, so you have to force it in most instances, for more reasons that are a bit too complex to list in this particular text.

Let’s look at one other bit of science and physiology before we move on to the exercise.  There are different kinds of muscle tissue, most simply described as fast twitch and slow twitch.  There are also 4 different specific chemical processes that our body can carry out to create the chemical ATP, which is what we actually break all food down to, to use as energy.  The first two processes are very quick, so can be used for powerful and fast motions, but they aren’t very efficient so they can only supply energy so long (which is why you can’t sprint or lift weights until your muscles actually break down).  The other two are slower, but are much more efficient so can be kept up for a lot longer, as long as you don’t expect them to actually let you sprint.  Our ‘fast twitch’ muscle fibers have mostly the two inefficient energy processes, and so that muscle type is a lot faster and more powerful, but fatigues easily.  Our ‘slow twitch’ muscle fibers mostly use the two more efficient energy systems, so they aren’t as fast or powerful, but they are a lot more efficient and can nearly last forever.


The Truth About Cardio

Taking this science to exercise, let’s take a look at cardio.  It’s fairly low to moderate intensity exercise that people usually do for 30-60 minutes.  Because you’re not sprinting and you’re not shifting heavy weights the body is not going to need to use your fast twitch muscle fibers, nor the two less efficient energy systems.  Assuming you’ve eaten not too long before going to the gym or hitting the road for cardio, in that hour you’re not really likely to burn through your livers 400 calories of readily available sugar.  Because the intensity was relatively low, there also will be minimal damage to your muscles that need to be repaired, though your heart and vascular system will have been trained to be healthier and more efficient, and your slow twitch muscle fibers will be incrementally better at being more efficient as well.  This also means the body will get better at burning making the calories it has last longer, decreasing the chance for substantial fat loss.  Better for sports performance and survival, but not as good for weight loss.

So where does the fat loss happen?  Well, with cardio it just doesn’t really occur in any substantial amount.  It’s not to say you didn’t use calories, but it wasn’t many and if you did get through your stored sugar to your fat stores it won’t have impacted it much at all.  The question then stands, is cardio just completely useless for fat loss?  The answer is a resounding yes and no. Doing low to moderate intensity cardio is not something I want to stop, even more people than are doing it now need to get onto a bicycle and improve their heart health, but it is without any doubt NOT the solution for fat loss.


High Intensity Cardio / Sprinting

Now high intensity cardio…that’s an entirely different story.  Once you jump into high intensity, where you are challenging your muscles enough that those two more efficient energy systems and their mostly slow twitch muscle fiber hosts can keep up with, you start getting less calorically efficient.  Instead of slowly burning calories with max efficiency systems, to put out the performance your body is now shoveling through its stored sugars to meet the demands you’re putting on it.  You’re much more likely to burn though those calories and get into your fat stores while you’re working out, even if it’s a shorter session.


If you do a cardio exercise as intense as sprints (which can be on a bike, even a recumbent bike, it doesn’t just have to be running), even if mixed in with more moderate intensity cardio, the damage to your muscle cells will be greater, and the repair involved will burn through even more calories.  Perhaps the greatest benefits, however, is that with sprinting of any kind your metabolism will continue to be rev’d for hours to come while your body essentially pays a ‘caloric debt’ from the high performance output.  You will also have trained your body to rely more on these less efficient muscles and energy systems that will end up burning more calories anytime they’re used.

We can now see that low to moderate intensity cardio will burn some calories and work our metabolism a bit, but it should mostly be done for its benefits to endurance, conditioning, and heart+vascular health.  High intensity cardio, and or cardio interspersed with sprints has a profoundly larger effect on your metabolism, and has a much better chance of cutting into your fat stores, no matter how much your body would like to avoid it.  However…if you are burning calories just from the actual work you are doing…and only the work you are doing in excess of those ~400 calories you have stored in your liver, you’re making progress but it’s going to take a lot more time than you’d likely prefer.


Resistance Training

The real fat burning award goes to resistance training, or weight training, depending on the term you’d prefer.  When you work against heavy resistance your body has to work very hard to do what you’re asking it to (relative to the weight used of course), and so less efficient energy systems and twitch fibers are primarily used, and the damage is pretty considerable so we get a lot of caloric expenditure.  The real metabolic effect, however, comes not from the act itself, or even from the rebuilding of the muscle, but the fact that doing resistance training will force your body to adapt by making more muscle fibers.


Other than your brain, pound for pound your muscles are the most calorically expensive thing in the body, and unless you really need your muscle tissue (aka use them with weight, not just cardio) your body likes to get rid of them when it can.  If you aren’t getting enough calories and the body thinks it may be headed for a famine, muscles will be the first thing to go (one of the many major problems with caloric deficit diets, as we talked about before).  When you build even more muscle, even when you’re not using them or recovering from a workout, just by existing they are burning more calories all the time.


If you haven’t done much resistance training, or done regular work through a job or hobby that has you moving heavy objects, choose a weight that is challenging at about 15-20 repetitions, and start there for several lifts that will together affect the majority of your muscles.  A good place to start would be squats, some kind of bench press, military press, pull downs, rows, and deadlifts (make sure a fitness professional, not just a friend, shows you how to do deadlifts correctly).  Raise your weights by about 10% a week and after about 3 weeks select a weight that you can do about 12-15 times and do the same thing.  Then go for a weight you can do 8-12 times, and then bout 4-6, before repeating the whole cycle with a whole new beginning weight.

Keeping your muscles challenged and your metabolism growing will require you to switch things up and slowly up your intensity.  Start choosing different exercises every couple of weeks, but ones that still affect your whole body when combined, and slowly spend less time in-between individual exercises so that rest time decreases.  This is where you start getting into metabolic circuit exercises, while still building strength and conditioning as well.  Many of the specific suggestions, however, can’t be gone into here in this article, but this would be a strong foundation for a fat burning plan: keep up your low to moderate cardio training a few days a week, but add in at least 1-2 high intensity/sprint cardio sessions, and 2-3 resistance training sessions, starting with less and building to more.  This will give you the greatest chances of success in putting your metabolism based hormones in a functioning balance, and cutting off as much fat even when you aren’t in the gym (note: most of it happens when you’re sleeping!  Losing 1-2 hours of sleep each night from what you need loses you up to %30 of your fat burning TOTAL).

As always, check with your physician or a relevant health professional before starting a higher intensity program.  I have all of my clients go through health assessments to make sure they will not have any risk of disease, injury, or death because of exercising, and even if you think you won’t be among them, sometimes people are surprised.  Get checked out and get working!

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