Cardio training: low or high intensity?
Cardio splits people who train into two camps: some swear by long, steady treadmill sessions, while others prefer short sprints at a high heart rate. Which option is better? The honest answer is that it depends on your goal — and here you’ll find a clear comparison, with no marketing spiel.
Why does cardio work at all?
Aerobic training is not merely a fat-burning tool — it shapes how your entire body functions. When you do cardio regularly, several things happen at once:
- your resting heart rate drops,
- your resting blood pressure falls,
- muscular endurance increases,
- your metabolism improves,
- your body fat % goes down,
- stress decreases,
- you have more energy left for everyday tasks.
The good news: cardio doesn’t have to mean hours on a stationary bike or dull plodding on a treadmill. Basketball, swimming, a bike ride, football, boxing — all of these count as cardio too. Choose an activity you genuinely enjoy. That way you’ll still be going in the eighth week, not just through the first two weeks of January.
What actually happens in your body
To understand why low and high intensity have different effects, we need to take a quick look under the hood. Promise: it won’t get too complicated.
The breakdown of fat in the body is called lipolysis. Behind it is an enzyme called lipase, which splits triglycerides into three free fatty acids (FFA) and one glycerol molecule. Cardio training is one of the things that gets lipase working.
Next comes mobilization — getting triglycerides and FFAs into a form the muscle can use as energy. The final stage is oxidation, where the mobilized fat is actually burned. If fat was mobilized but the muscle didn’t use it for energy, it simply goes back to being triglycerides. Annoying, but that’s how the body works.
High-intensity cardio — more calories in total
Studies show something interesting: as intensity rises, the amount of free fatty acids (FFA) in the bloodstream falls. The first impression is that high intensity burns less fat. But that isn’t quite the case.
Although fat mobilization decreases as intensity increases, the total amount of fat oxidized goes up — simply because you burn more calories. Fat makes up a smaller share of the calories burned, but the total you start from is larger. In the end, that means more fat burned on the scale.
With low-intensity cardio, the body uses more FFA for oxidation than it does at high intensity. Where does the rest of the fat come from at high intensity? From intramuscular triglycerides — tiny droplets of fat located right between the muscle fibers. They are physically closest to where energy is produced, so they get used faster than the stored fat deep under the skin.
A simple worked example
Let’s say that at low intensity you burn 200 calories in 30 minutes and 90% of that comes from fat. That makes 180 calories from fat and 20 calories from other sources.
Over the same time at high intensity, you might burn 300 calories, of which 75% comes from fat — that gives 225 calories from fat and 75 calories from other sources. The numbers are illustrative, not an exact formula, but the direction is clear: over the same period, higher intensity gives you more fat burned and more calories burned in total.
So which cardio option should you bet on?
Short answer: it depends on your goal. There is no single answer — both have their advantages. Let’s look at each profile.
High intensity suits you if you’re not preparing for stage condition and don’t need to worry about every gram of muscle mass. It works well when time is short and you want to burn as many calories as possible in a short workout. It’s also an effective route for lowering body fat % quickly.
Low intensity suits you if you’re preparing for a competition and are afraid of muscle catabolism. It also suits you if you prefer longer, calmer movement — walking, a bike ride or swimming at an easy pace — and want as much as possible of the energy you spend to come from fat. 200 calories burned at low intensity can be more effective at lowering body fat % than 200 calories burned at high intensity, especially if you’re already in a slight calorie deficit.
Cardio FAQ
How often should you do cardio?
For cardiovascular health, 3 times a week for 30 minutes is usually enough. To lose fat, you can add 1–2 more workouts, but make sure your strength training volume doesn’t suffer.
Does cardio ruin your strength training results?
Not if you get enough protein and calories. The problem only arises when the volume and intensity of cardio keep climbing but your diet stays exactly the same.
Does fasted cardio burn more fat?
In the short term yes, but over the whole day the difference is small. It matters far more that the workout happens regularly at all — not whether your morning coffee comes before or after.
Author: Shane Giese/Bodybuilding.com
Source: Turg.Fitness.ee
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