There are benefits to doing cardiovascular exercise. It increases endurance, relieves stress and helps to ward off diseases like type 2 diabetes and heart disease, but more isn’t necessarily better.  You probably see people at the gym who are glued to the treadmill spending an hour or more working out at a steady intensity while periodically checking the numbers to see how many calories they’ve burned. Some people do this day after day, clocking as many as 15 hours a week of cardiovascular exercise. Then there are long distance runners who don’t think they’ve done a workout unless they run 6 miles or more. Cardiovascular exercise can be a good tool for weight loss, but doing too much of it can actually make it harder to lose body fat. Here’s why.

How Can Cardio Make It Harder to Shed Body Fat?

There are several ways long periods of steady-state cardiovascular exercise make it more challenging to shed body fat. Research shows that long cardio sessions reduce T3, the active form of thyroid hormone that keeps your metabolism operating at peak capacity. This is an adjustment the body makes to maintain energy stores. This phenomenon is so well described that it has a name, the low-T3 syndrome.

Another way prolonged cardiovascular exercise makes it harder to lose body fat is related to its effects on cortisol levels. Cortisol is a hormone produced during times of stress, and exercise is a type of stress, especially when it’s prolonged. Cortisol levels start to rise after about 60 minutes of exercise in response to low glycogen stores. Cortisol has the benefit of mobilizing fat to use as an energy source, but it also breaks down protein, which is not the desired outcome for most people. In addition, cortisol stimulates appetite, and the extra calories you take in goes to your tummy and waistline. Cortisol is a major contributor to visceral abdominal fat.

Long periods of cardiovascular exercise especially when combined with inadequate calories also lower leptin levels. Leptin is a major hormone involved in appetite control and metabolism. When leptin levels are low, it stimulates appetite and leads to a metabolic slow down as the body tries to conserve energy. On top of that, levels of another fat-burning hormone testosterone also decline with prolonged cardio.

A Double Whammy: Prolonged Cardio and a Low-Calorie Diet

To lose weight, some people combine long periods of cardio with a low-calorie diet. This further lowers T3, testosterone and leptin levels while boosting cortisol. Some of the metabolic slowdown that comes from too much cardio can be offset by eating adequate calories and carbs, but doing hours of cardio each week has other drawbacks. It increases the risk of overtraining and injuries.

What Does This All Mean?

When you do too much steady-state cardio, your body makes adaptations to hold onto fat for self-preservation. That’s why shorter periods of more intense cardio combined with a cross-training routine that emphasizes resistance training is ultimately better for fat loss and body composition. Combining long periods of cardiovascular exercise with a calorie and carb-deficient diet makes it difficult to get a lean, toned physique because you’re breaking down muscle and slowing down your metabolism. Choose a more balanced fitness program that emphasizes strength training. Don’t become a slave to cardio.

References:

Am J Physiol. 1993 May;264(5 Pt 2):R924-30.
European Journal of Applied Physiology.Volume 90, Numbers 5-6, 533-538.
European Journal of Applied Physiology.Volume 93, Number 4, 375-380.

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