Cardio and Metabolism Part 1

Due to a heinous error in the World Wide Web half of this post was deleted somehow, someway by mysterious cosmic forces. Thus I’m obligated to give you the first half of this post and completely rewrite the other 1000 words or so of the second half. My apologies. It will be a couple of days before I’m able to get to the second half, …..again. Essentially this became a two part post so enjoy Part 1.

Every time I go to the gym I see something that sometimes drives me nuts. Not for my own personal sake mind you because this doesn’t affect me at all. But I worry about how it may adversely affect others who do it. I’m talking about people who go to the gym to run and run and run or use the eliptical for endless hours a week.

Please don’t misunderstand, I don’t believe that doing cardio is bad, quite the contrary. It can be beneficial. Yet if you’re sole purpose for a gym membership is to get on the treadmill then please consider a few things about how you’re metabolism is affected by endless running.

Please do not make this your goal!

Please do not make this your goal!

I’m not going to go into some of the benefits of a MODERATE amount of cardio. So if that’s what you’re looking for please don’t read on. I’m going to discuss the problems of doing more than a moderate amount of cardio.


Simply put too much cardio will wreak havoc on your thyroid. First lets do some thyroid 101 for anyone that needs to be up to speed. This won’t be super in depth seeing as this is a 101 course so if you want more information please use a good anatomy and physiology text.

Your thyroid gland is located in the neck and pumps out what is known as triiodothyronine (T3) and thyroxine (T4). T4 is a much less active form and T3 is the most active form. These are your thyroid hormones.

T4, while just mentioned is active, isn’t super active like T3 and in fact is transformed into T3 in the body. Some think that T4 may act as a pool for T3. it’s also interesting to note that carbs help this conversion. More on that in a bit.

Pretty much all your cells have thyroid receptors. They are important for growth and metabolism. Thyroid hormone is responsible for heat production and that’s why some people who are hypothyroid have cold intolerance, they just don’t generate enough heat to withstand the cooler ambient temperature.

If you haven’t guessed already, thyroid is also responsible for fat metabolism. This is also why some who are hypothyroid gain weight. If no T3 is getting to the fat cells then the fat cells aren’t receiving any signal to give up the fat for energy. This can make it difficult or impossible to lose weight if not enough thyroid hormone is around.

What does cardio have to do with thyroid?

The answer to this is that it can thrash thyroid hormone. [1-6]

When a person does a lot of steady state cardio the body requires a lot of energy to move that much. Energy requirements of the cells go up. Rightly so. You will burn more energy running like a maniac down the street than if you casually mosey on down. Cars are the same way. My truck burns way more fuel at 85mph than 35mph.

But that’s where the similarities between cars and bodies end.

Your body is an adaptive organism that wants to live. It’s programmed to live. When you start running endlessly it wants to conserve energy if nothing else has changed because it wants to live tomorrow. Your truck doesn’t know if it’s going to get fueled or not it just keeps burning gas at whatever rate you tell it. Your body doesn’t know if it’s going to get fueled or not or with how much so it shuts down energy production for you. How kind.

One quick way to shut down energy usage is to slow down the thyroid. This way your body can conserve energy for other things. Without as much T3 roaming your veins the cells aren’t getting the signal to burn and create heat.

Moderate to intense exercise actually can make the fat cells unwilling to give up their precious cargo for energy as well, compounding the problem. [7,8]

Another issue with thyroid is carbohydrate consumption. Many people, not all, but many are going low carb in the hopes of losing all kinds of fat. “Not so!” says the science.

In one trial of active men, three different diets were given at the same caloric values but differing carbohydrate intake. 88%, 44% and 2% percent of calories from carbs were the ratios and the group with 2% calories from carbs ended up with lower T3 and more nitrogen in their urine (ie more muscle wasting).[9]

In one of the trials already cited, women performed aerobic exercise and were given different amounts of calories in their diet, either 8kcal/kg/day or 30kcal/kg/day. The women who received 8kcal/kg/day had reduced T3 by 18%. Exercise quantity didn’t affect thyroid in the women, as long as they were receiving 30kcal/kg/day.[4] If you’re a 125lb woman that’s 1700 calories/day. If you were taking in 8kcal/kg that would be 454 calories/day. This is a fairly drastic cut and definitely not sustainable but makes sense. If you are working your body you have to feed it to keep energy production up or your metabolism will shut down to conserve energy.

In part two we’ll talk more about diet and how to avoid shutting down the metabolism while trying to lose a pound or two.



1.Baylor, L., and A. Hackney. “Resting thyroid and leptin hormone changes in women following intense, prolonged exercise training.” European journal of applied physiology 88.4-5 (2003): 480-484.

2.Boyden TW, Pamenter RW, Rotkis TC, Stanforth P, Wilmore JH. Thyroidal changes associated with endurance training in women. Med Sci Sports Exerc. 1984 Jun;16(3):243-6.

3.Wesche MF, Wiersinga WM. Relation between lean body mass and thyroid volume in competition rowers before and during intensive physical training. Horm Metab Res. 2001 Jul;33(7):423-7.

4. Loucks AB, Callister R. Induction and prevention of low-T3 syndrome in exercising women. Am J Physiol. 1993 May;264(5 Pt 2):R924-30.

5.Rosolowska-Huszcz D. The effect of exercise training intensity on thyroid activity at rest. J Physiol Pharmacol. 1998 Sep;49(3):457-66

6.Hohtari H, Pakarinen A, Kauppila A. Serum concentrations of thyrotropin, thyroxine, triiodothyronine and thyroxine binding globulin in female endurance runners and joggers. Acta Endocrinol (Copenh). 1987 Jan;114(1):41-6.

7.Jones, N. L., et al. “Fat metabolism in heavy exercise.” Clinical Science 59.Pt 6 (1980): 469-478.

8.Romijn JA, Coyle EF, Sidossis LS, Zhang XJ, Wolfe RR. Relationship between fatty acid delivery and fatty acid oxidation during strenuous exercise. J Appl Physiol. 1995 Dec;79(6):1939-45.

9.Bisschop, P. H., et al. “Isocaloric carbohydrate deprivation induces protein catabolism despite a low T3‐syndrome in healthy men.” Clinical endocrinology54.1 (2001): 75-80.



Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s