This can be a bit of a touchy subject because a lot of people genuinely enjoy cardio but when it comes to fat loss cardio is NOT the best approach
**DISCLAIMER - This is not to put down anyone who enjoys cardio and endurance activities. There are many great benefits for mental and physical health but when it comes to fat loss this is NOT an effective strategy. If fat loss is your ultimate goal, cardio exercise alone is not the way to go**
First let's discuss what results in weight loss. You need a caloric deficit - more calories burned than are taken in. Sure doing cardio can increase the number of calories burned in a day but ultimately you plateau very quickly and your metabolism will actually go down (the amount of calories you burn for any given task). The calorie deficit will get smaller unless you increase cardio intensity/frequency drastically (which is really tough to do and will still lower metabolism) or really limit the number of calories you take in (which can also decrease metabolism). Both of these are not necessarily the healthiest and are NOT the most effective approachs to weight loss.
How Does Cardio Decrease Your Metabolism
Your body wants to be as efficient as possible based on the stimulus it receives. So, what does that even mean? Your body adapts to the specific demand that we put on it and it wants to get better at those tasks/activities etc.
Let me explain.
Cardio tells the body that we need to conserve energy (seems shocking I know!) because we use it often for endurance activities. The body therefore becomes more efficient at conserving energy for said task. This means you burn less calories every time you do the same activity. The only way to increase calories expended during that activity is by increasing intensity (increasing time, frequency, speed etc.) Let me give you an example with hypothetical numbers.
Let's say you run 3 km at 7km an hour consistently 3 days a week. The first week you do this you burn 150 calories each time. Your body now adapts through various physiological processes and becomes more efficient because it knows that you need energy to do this run often. Now when you do this 3km run at 7km an hour you only burn 130 calories each time. Your body will become more and more efficient = less weight loss.
Another process in making the body more efficient and not expend calories with endurance type activities is to LOSE MUSCLE MASS!
Muscle is highly metabolically active at rest which means the more muscle you have the more calories you burn just at rest. With cardio our body will not only shed fat (for use of energy) it will also cause you to lose muscle in order to preserve energy! This results in a lower resting metabolism!
It is also important to note that cardio is not just the classic endurance activities we think of like running, biking, swimming, walking etc. It also includes fitness classes that have high repetitions that are repetitive in nature (20+ reps). Think of group fitness classes such as spin, step aerobics, zumba etc.
Summary / Key Take Aways:
Cardio will result in weight loss but this weight loss will include both muscle and fat. This will result in a less "toned" and "leaned" look as you will have less muscle.
Cardio will lower your metabolism over time because you are sending the signal to the body that you need to have energy in the tank for endurance activities. Therefor you burn less calories at rest (decreased resting basal metabolic rate).
In order to continue with weight loss (including both your muscle and fat) you need to continually increase your intensity/frequency/duration or your cardio activities (which unfortunately all of us have a limitation somewhere)
Cardio does have some great benefits (improving mental health, etc.) and IF you enjoy it you should continue to do it but you need to be aware that CARDIO is NOT the most effective strategy for FAT LOSS.
One other important thing to note with any type of exercise you do is that you need to be aware of proper biomechanics, muscle balances and listen to your body. Due to the repetitive nature of endurance activities and sports it is not uncommon to end up with overuse injuries. We always think that we need proper instruction to do things like weight lifting etc. but learning technique for endurance activities is equally important!
If you want to learn more or would like some guidance on your health and fitness journey you can always reach out!