Over the next two weeks, I want to talk about cardio: what it is, how much you need, and, most importantly, how much is overkill and could be setting you back.
Most people agree that cardio is anything where you elevate your heart rate. When I was looking at the formal definitions, most will say that this requires rhythmic, repetitive movement of your limbs.
Because most people consider cardio as repetitive/rhythmic moments, when most people think of cardio, they think of activities such as running, cycling, and swimming. This is also why most people don’t include weight lifting when they think of cardio.
Therefore, it’s commonly thought that you need to do both: lift the weights and add on the running or biking.
But do you really? And if so, exactly how much should you do?
These are the questions I want to address over the next couple of weeks. We’re going to talk about the benefits and risks of different dosages of cardio and provide a framework for what I consider “optimal” based on my experience and research.
A big problem in the fitness world is that we equate competitions like marathons, to fitness and overall health. However, physical fitness and sports are completely different. They have different goals, you need to train differently, and they quite frankly, will have different health outcomes.
If you are just looking to be as healthy and fit as possible, and aren’t concerned with winning a competition or a race, these next two weeks will provide a framework for what will be the most optimal way to exercise to achieve your individual goals.
We want to implement the right dosage and type of exercise that will make our bodies optimal, without overdoing it and potentially reversing the benefits.
So let’s start by defining HIIT, and outlining the benefits, the risks, and if you should try HIIT, and if so, how much.
By definition, HIIT is a combination of short bursts of intense, all-out effort, followed by periods of rest. During the period of intensity, you’re usually reaching your near-maximum heart rate for a brief amount of time - anywhere from seconds to minutes.
HIIT has become increasingly popular because it’s time-efficient and beneficial for things like mitochondrial density and heart health.
Before we go any further, I’ve noticed mitochondria have received a lot of attention lately, so I want to discuss what these are, and why they are important for fitness. HIIT has been shown to improve the mitochondria size and function, and therefore, our metabolic health.
Mitochondria are the primary sources of cellular ATP synthesis. They are central in energy and redox metabolism. As many of us remember from our middle school science class, the mitochondria are the powerhouse of the cell.
It uses sugar and fat to fuel the cells in our body. When our mitochondria are larger and healthier, our metabolic health, metabolism, and muscular health all benefit.
Think of the mitochondria like a car engine. You always want the newest, biggest, and fastest engine possible. Even for an old car, that engine will help the old car move better, faster, and more powerfully.
Overall better metabolic health means you have healthy blood sugar, blood pressure, body fat percentage, cholesterol, and triglycerides levels.
Alternatively, when mitochondria are dysfunctional, that’s when we begin to develop health problems. In fact, there is a direct correlation between mitochondrial dysfunction and what’s called insulin resistance.
Insulin resistance is a precursor of diabetes but is getting lots of attention recently, as it seems to be much more common than we realized.
Insulin resistance is when the body becomes desensitized to insulin, and begins to produce more of it. This can mess with our body’s hunger signals, tricking you into thinking you’re hungry.
So it is important to control our insulin, and therefore control our hunger and other metabolic pathways.
Nutrition is important for controlling insulin, but exercise can be as well. Exercise, in the right dose and frequency, can also improve our insulin sensitivity.
Exercise improves insulin sensitivity because it stimulates mitochondrial biogenesis (or mitochondrial improvements). The healthier our mitochondria, the better our glucose uptake and insulin sensitivity, and overall the healthier our metabolic system.
However, like many other good things, exercise has a limit, where too much of it then becomes counterproductive.
Amidst all the clinical research we have in the fitness industry, the limit for exercise has not been defined. Unfortunately, inactivity is a pandemic, so for most people, getting them to do anything to avoid their sedentary lifestyle is a win.
However, as I’ve mentioned before, there needs to be a larger conversation held on how much exercise is the right amount, so that we are not inadvertently causing harm to our bodies. Unfortunately, because there isn’t much discussion surrounding it, people think that more is better, and they won’t face any negative side effects from overdoing it.
Exercise has what’s called a curvilinear relationship: meaning low amounts or no exercise have negative consequences to your body, modest to high amounts of exercise are the most beneficial and optimal for the health of your body, but high to extreme amounts of exercise display the kind of similar negative consequences that little or no exercise does.
This short-term study by Mikael Flockhart and colleagues shows that there is a negative side effect to overdoing it in HIIT workouts in the gym. They found that doing too much will reverse the positive effects on our mitochondria and insulin sensitivity, causing things like increased hunger cues.
This study took healthy subjects who exercised but were not competitive athletes and had them do HIIT workouts for 4 weeks.
Each week they added more HIIT sessions, working up from light training on week one (two sessions of HIIT repeating 4 min long intervals 5x on a bike with three min of rest in between) to moderate training on week two (added a third HIIT session and ramped up the length of some of the intervals to 8 minutes), and the 3rd week being daily interval training with all-out effort for 4-8 min intervals.
In week 4, they halved the time and intensity of exercise to act as a recovery week.
The researchers studied the participants’ physiological responses to the different doses of exercise by taking muscle biopsies and oral glucose tolerance (which measures how well their body was able to control their blood sugar).
The results: muscular power output increased from baseline through light and moderate training in weeks 1 and 2.
This is because of what’s called neuromuscular adaptation to exercise. By week 4, their muscles were essentially “getting used to” riding the bike, and their body adapted by improving within the first two weeks. Neuromuscular adaptation happens almost immediately and could be the reason for the increase in power so quickly.
The higher training regiment showed physical performance stagnation beginning in week 3.
Individuals’ muscular output, mitochondria efficiency, and insulin sensitivity all declined.
After the recovery phase where individuals’ training load was drastically reduced, the performance began to rise again, but mitochondria efficiency and blood sugar levels among the group were still less improved than their previous week 2 levels.
Overall the study proved more aggressive exercise routines can have negative effects on our metabolic health due to partial mitochondrial shutdown.
The study’s analysis demonstrated that if overall health is your goal, excessive HIIT workouts are not the best plan of action. This will only overwhelm the mitochondria as it is not given space to adapt to the intense and frequent stimulus.
The good news is that the negative effects of excessive exercise were reduced as soon as the exercise load decreased. It only took about one week of reduced training to improve the individual’s metabolic parameters.
This reminds us how quickly our bodies can adapt, and that you are not a lost cause if you’re realizing you may be harming yourself by hitting it too hard, too often.
Since this was a short-term study, the results are limited. I’d love to read how ramping up intensity over a longer period works for the body to adapt, but again, research studying the upper limit of exercise is very limited at this time.
I do think certain people can slowly increase their way towards a frequent, intense exercise routine and be totally fine. But from my expertise and experience what I’ve seen time and time again is this:
Someone begins to work out intensely and frequently, and they lean down because they are burning so many calories in their workouts.
In this time frame, they aren’t as focused on building muscle, they are more focused on cardio or burning calories
However, they eventually get to the point where their joints can’t tolerate the intensity and frequency of their workout routine, but they’ve also shot their metabolism because they have not worked on building muscle.
As a response, they switch to a more gentle program to take care of their joints but don’t change their eating habits. Because they are eating the same amount as when they would burn off what they ate, they shot their metabolism, and therefore will sometimes see weight gain.
On Thursday, I am going to dive into this a bit more, and how to find the right cardio workout for you. Until then, check out some of Evlo’s past blog posts, talking about everything from how to see better results in the gym to the biggest myths of the fitness industry.