A cardio bunny describes someone who does extensive cardio everyday as their main form of exercise.
I have three beliefs for fitness and exercise:
(1) All safely performed exercise is good exercise
(2) You should do exercises you enjoy
(3) For fitness to be a lifestyle, it must be sustainable
That said, the goal of this post is not to attack the preferences of a Cardio Bunny. If you're active, you feel great, and you're happy with your results then keep it up. However, the hard part about beginner fitness is having particular goals and not knowing what it takes to achieve them. Some fitness enthusiasts become Cardio Bunnies, simply because it has a low learning curve and offers quick weight loss results. Therefore, this discussion is for the Cardio Bunnies that are NOT happy with their results. I'm going to address the particular fitness goal of a body transformation and why Cardio Only Workouts aren't helping you achieve it.
Weight vs. Body Fat Percentage
To better understand how cardio transforms your body you must understand the relationship between Weight and Body Fat Percentage and how cardio impacts Body Fat Percentage. Body Fat Percentage is what determines your body's appearance. While Cardio Only Workouts can reduce your overall weight, they have almost no effect on your Body Fat Percentage. There are only two ways to permanently affect Body Fat Percentage and they are to change the amount of fat on your body and or to change the amount of muscle on your body.
Weight measures your entire body mass including water weight, muscle, skin, and bowel waste, etc. while Body Fat Percentage measures your fat mass in comparison to the rest of your body mass (muscle, skin, water weight, bowel waste). The lower the Body Fat Percentage, the more muscle definition and overall athletic appearance you will have. On the other hand, the higher the Body Fat Percentage, the less muscle definition, less athletic appearance, higher fat stores and increased health risks there are. As a quick gauge, women with bodies that are considered "fit" generally have a lower body fat percentage of 18-22%. Body Fat Percentages below this range may present health risks, and body fat percentages above this range, generally 30-35% may also present health risks.
Steady State Cardio Burns Muscle
Steady State Cardio like running is an effective method for weight loss but not for body fat loss due to its catabolic nature, or its nature to cause muscle breakdown. The cause of muscle breakdown can be separated into two primary reasons. The first reason is that cardio elevates cortisol hormone levels for prolonged periods of time which decreases insulin sensitivity. Reduced insulin sensitivity encourages your body to store body fat and shed muscle instead. The second reason is that muscle is heavy and extended cardio encourages the body to shed muscle mass so that the body can continue performing that workout efficiently. A lighter body is easier to carry through long bouts of cardio.
The image above is meant to depict how cardio effects a body transformation of a Cardio Bunny. Cardio Only Workouts help shed overall weight, however since cardio burns both fat and muscle it has little to no effect on Body Fat Percentage. Therefore, it does not transform your body's proportions into a leaner more athletic build.
Weight training AND cardio is the best combo for a body transformation...
Weight training challenges your muscles which signals to your body that you need them. As your body sheds weight, it will remember that you need those muscles and will shed your body fat instead. This will reduce your overall body fat percentage and produce that goal athletic appearance.
The above image represents the results of strength training for fat loss and only supplementing your workouts with cardio. It is meant to depict that through weightlifting, your overall weight loss may be less than doing only cardio because you're retaining more muscle. However, your result is a substantially decreased Body Fat Percentage that is responsible for a more curvy-athletic build.