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If you are doing prolonged cardio exercise several times a week, or perhaps even more often, chances are you’re not getting the physical results nor the health benefits you think you are. You want to look good and be healthy, right? Although all that jogging, swimming, running, and cycling may make you accomplished at all of these activities, it also can put you into a category known as skinny fat.

What is skinny fat?

Mark Hyman, MD, explains that the medical term for skinny fat is metabolically obese normal weight, or MONW. Basically, you can be “under lean but over fat—not enough muscle and too much fat,” especially the dreaded belly fat and love handles.

We’ve been led to believe that burning calories via cardio exercise will not only help us eliminate extra fat and create a great physique but also improve our overall health and better fight off disease. Unfortunately, that’s not true.

How cardio makes you skinny fat

In fact, cardio-exercise can be detrimental to the very factors you want to preserve; namely, muscle mass, muscle strength, a healthy metabolic rate and body fat percentage, and aerobic capacity. Although cardio exercise can improve the latter factor, it’s at the expense of the other four. Here’s why.

Cardio-loading raises your cortisol (a stress hormone) levels. The longer you run, swim, or bike, the higher your cortisol levels go and the longer it takes for them to return to normal. A 2015 study in the Journal of Sports Medicine and Physical Fitness evaluated master endurance runners before and after running a half marathon. Salivary cortisol concentrations increased by 59 percent immediately after the men had completed the race and stayed elevated until 60 minutes after the race.

A prior study measured cortisol levels in hair samples of 304 amateur endurance athletes–long-distance runners, triathletes, and cyclists–and 70 controls. The authors reported that the athletes had higher concentrations of cortisol in all three hair samples collected from each volunteer when compared with controls. This suggested that “repeated physical stress of intensive training and competitive races among endurance athletes is associated with elevated cortisol exposure over prolonged periods of time.”

Another effect of cardio-loading is on the immune system. A recent study in the Asian Journal of Sports Medicine looked at the cortisol, testosterone, and immunoglobulin levels in semi-endurance elite runners during their general prep and competition phases of training. The investigators found that “long and intensive exercises weaken the immune system, while moderate and short drills strengthened this system.”

The downsides of excess cortisol exposure are all the things you want to avoid by exercising; namely

  • Greater storage of fat
  • Muscle loss
  • Compromised immune function
  • Decline in memory, learning, and other cognitive functioning
  • Bone loss

Elevated cortisol also interferes with testosterone, which results in reduced T output and a low or poor testosterone-to-cortisol (T:C) ratio. What happens is this: the longer you engage in cardio, the more your cortisol levels rise. Your testosterone levels, however, peak after about 20 to 30 minutes, leaving you with a low T:C ratio, which results in muscle wasting. That’s because cortisol is catabolic; that is, it breaks down tissue.

In fact, cardio-loading leads to muscle loss by triggering a change in muscle fibers from those that favor testosterone (type 2 fibers) to those that favor cortisol (type 1 fibers) in as little as four months. Type 2 muscle fibers also promote fat loss. All of this adds up to mean that if you are engaged in prolonged cardio exercise, you may be supporting and nurturing fat storage and muscle loss.

Cardio-loading and weight loss

The human body is built for survival. When you cut calories and/or engage in exhausting workouts, you hope to drop pounds and get great abs. Instead, the body begins to store fat, lower your basal metabolic rate (to preserve energy), and burn tissue (muscle). Yes, much of the weight lost from reducing calories and/or endurance exercising is muscle.

A study presented at the European Congress on Obesity (2014) included 25 individuals who followed either a very-low-calorie diet (500 calories/day for 5 weeks) or a low-calorie diet (1,250 calories/day for 12 weeks). At the end of each of the diets, the individuals in both groups had lost a similar amount of weight (around 19 pounds). However, those who ate 500 calories per day lost about 3.5 pounds of muscle compared with those on the 1,250 calorie diet (1.3 pounds of muscle).

Do you want to be losing muscle as a result of your endurance exercise? In addition, cardio-loading can reduce your resting metabolic rate, which is the number of calories you burn while you are sedentary. This metabolic rate is responsible for about 60 to 75 percent of the energy (calories) that you burn, so you don’t want to cause it to decline, do you?

Skinny fat bottom line

When you lose weight associated with cardio-loading, you can begin to gain it back once your metabolic rate declines. In the meantime, you also have stored body fat and made it more difficult to lose it. The result is less muscle, weaker muscles, and a body that’s not quite what you had in mind. You are skinny fat.

Skinny fat alternative

To avoid being skinny fat, participate in exercise that focuses on building and maintaining muscle rather than burning calories. You don’t have to give up your cardio—I’m all for healthy aerobic capacity—but keep it moderate and well balanced with resistance training, which should be done several times a week.

Resistance training helps improve muscle strength, mass, tone, and/or endurance. You also will enjoy less stored fat, a higher metabolic rate (which means you will burn more calories when sedentary), and a lower risk of dying or developing degenerative diseases.

Need more convincing? In a 2014 study appearing in the Journal of the American College of Nutrition, investigators examined the impact of resistance training combined with a very-low-calorie-diet on lean body weight and resting metabolic rate. Twenty adults were assigned to one of two groups: an 800 calorie-per-day liquid diet for 12 weeks along with cardio exercise; or the diet with intensive resistance exercise.

Body weight decreased significantly more in the cardio than in the resistance group. Those in the cardio group lost a significant amount of lean body weight, but there was no decrease seen in the resistance group. Resting metabolic rate increased in the resistance group but decreased in the cardio group.

If you want to avoid being skinny fat, sport a more toned, muscular body, and burn more calories per day without getting out of your chair, then tone down the cardio-loading and rev up the resistance! You’ll look better, feel great, and age more gracefully.