Cardio and Metabolism Part Deux

In part one we discussed a little about thyroid and cardio. You can access it here if you missed it:

The skinny of it was that people who perform chronic cardio are at risk of causing problems with their thyroid function, especially if calorie intake drops too drastically. In one study we reviewed women who were eating 8kcal/kg/day (or 454 kcal for 125lbs women) and performing exercise had a decrease in T3, your primary thyroid hormone.

Lowering T3 is like taking your foot off the gas pedal and expecting your car to speed up, it just ain’t gonna happen no matter how frustrated you get or how much you scream.

Thyroid 102

Since we discussed about thyroid hormone itself today we’re going to discuss a little about thyroid testing.Welcome to thyroid 102.

Thyroid testing by many doctors usually consists of testing TSH or thyroid stimulating hormone. This is the hormone shot out by the pituitary in response to TRH or thyrotropin releasing hormone. When TSH is released it binds to the thyroid gland to produce T3 and T4. As T3 and T4 are released they tell the pituitary to slow production of TSH since the thyroid hormones are in place. When the thyroid hormones are used up the cycle repeats.

Testing TSH is important but doesn’t tell you everything. In fact, TSH can be “normal” and your thyroid status might still be messed up. Some people have a hard time converting T4 to T3. Too little carb intake may be one reason. But if all the doctor looks at is TSH and doesn’t pay attention to T4 and T3 and some others like reverse T3 the full picture of what might going on won’t be seen and thyroid according to TSH alone will appear normal.

In short talk to your doctor about doing a full thyroid panel which includes these other measures because many doctors will consider a full thyroid panel as TSH not all of them, but many of them.


Now that we’ve discussed thyroid a little more in detail, I think it behooves the conversation to discuss glycogen. Glycogen is a substance that is concentrated in the muscles and liver for use as energy. In fact muscle glycogen is the first thing that skeletal muscle will turn to for energy. And if you haven’t figured out from the name, glycogen is made from glucose.

Weight lifting and sprinting and running will use glycogen but at different levels. A bout of lifting can decrease glycogen by ~25-40% depending on duration and intensity where one 30 second sprint can deplete stores by 33% and 2 sprints by 47%. [1-3]

Glycogen needs to be repleted. Enter in carbs. People that are very active and using their glycogen without eating carbs to replenish them will have to use ketones for energy and/or breakdown protein to create glucose to then be stored as glycogen. For people who have lower activity levels this may be ok, key word being may. For individuals who are very active the level of glucose needed is going to increase to keep things even keel.

Gluconeogenesis in the liver can supply about 100g glucose/day at the most (it is likely to be less) in most people. [4] It is controlled hormonally by insulin and glucagon. Insulin stops the process and contrary to popular belief protein stimulates insulin production. There’s a great article on it here:

So when you aren’t getting enough glucose your body will turn on gluconeogenesis for blood glucose to be utilized throughout the body. This is all fine and dandy unless you are working really hard day in and day out and burning calories like a furnace. It’s going to be difficult to replace glycogen without glucose and glucose primarily comes from carbs.

People who are glucose deficient, or are expending lots of energy doing things like cardio and weights regularly will need a shift in metabolism to compensate. That’s when the body starts reducing T3. It needs to conserve somewhere so T3 goes down.

Homer's problem isn't's lack of burning glycogen.

Homer’s problem isn’t glycogen….it’s lack of burning glycogen. The opposite problem of having no or little glycogen and trying to run endlessly is also problematic.


Cortisol is the second part of this equation and is generally regarded by many anti-cardio folk as one major reason a person shouldn’t engage in regular cardio. When there is a deficiency of glucose cortisol will increase to help mobilize gluconeogenesis (glucose production from protein) and decrease glucose utilization.

As with everything else, certain amounts of cortisol are absolutely necessary for proper functioning. High cortisol is considered our stress hormone and too much can cause sleep problems, decrease in immune function, gaining weight while exercising and eating “right”, problems in the bedroom or lack of desire altogether, headaches and the list goes on.

Again cortisol, in the right amounts is super important. Too much for too long can eventually lead to adrenal fatigue and then more powerful hormones like epinephrine take over because your adrenals just can’t take it anymore. Then you’re really up the creek without a paddle.

So cortisol becomes chronically elevated and T3 becomes low….essentially a recipe for fat gain, lethargy, anxiety and sliding down a slope that if not corrected will cause chronic health problems.

What to Do?

Some people may be thinking to themselves, “I do heavy cardio routines and have been losing weight and feeling great”. Good for you. No really, I’m serious. I don’t deny there is bioindividuality that can be a boon for a select few. But for people that are going to engage in something that is stressful on the body they have to be willing to treat the body in the right way and not the “right” way.

If you are losing weight doing what you’re doing and feeling good, then keep it up. At some point though you will hit a plateau. It’s as that point that you have to be careful about doing endless hours of cardio and not feeding your body because the effects may be very detrimental including all those things just mentioned.

As noted in the study from part 1 [5] women who ate sufficient amounts of calories were able to maintain T3. But if they ate too little T3 dropped. Bad mojo that is. I don’t know that 30kcal/kg/day is required to keep that T3 up, maybe 25 instead. For 125lbs (56.8kg) woman that is about 1400 kcal/day of food intake.

Now this isn’t a set number if you are a female and weigh 125lbs. It might need to be a bit more or a bit less depending on your energy expenditure and what you’re trying to accomplish but you can use it as a guide. While this study wasn’t done in men, using similar numbers again gives us an estimate. For a 175lbs (79.5kg) the equation gives us just under 2000kcal/day.

Remember too that carbohydrates are not evil, just like cholesterol isn’t evil, just like insulin isn’t evil, just like cherry pie isn’t evil. In fact I think I could go for a piece right now. But too many carbs, above and beyond what you need isn’t so healthy if done regularly. Eating an entire cherry pie 1 time probably isn’t going to do a whole lot other than make you feel like a glutton and bloated. Doing it regularly is problematic.

Moral of the story, get enough to provide for what your body needs. There is no exact number. If you are feeling lethargic and are having problems with energy and weight gain and are working out like there is no tomorrow then check to see if your at a large calorie deficit. If you are already fairly lean then shoot for an isocaloric diet (a calorie amount to maintain bodyweight) and give your body a break.Look at getting around 3-4g of carbohydrate/kg/day for a couple of days and let your metabolism get what it needs to reset.

Summary and Conclusion

Cardio workouts are a good way to keep your heart and body healthy provided you’re smart about it. Going for long distances every single day and not supplying your body with the macronutrients it needs to sustain such activity is not the way to do it. Get Liftin!

Too little fuel will shut down thyroid and increase cortisol and make any attempts to maintain or achieve a desired look or feel nearly impossible.

People that perform moderate amounts of exercise and eat few carbohydrates and have no problems with thyroid or weight gain are probably right where they should be activity and calorie wise. But be careful. Remember that carbs aren’t evil.

People that are having problems with it need to give their bodies a break and recouperate. Remember that you really can’t fool your body. Treat it right and it will treat you right.


1.Haff, G. Gregory, et al. “Carbohydrate supplementation and resistance training.” The Journal of Strength & Conditioning Research 17.1 (2003): 187-196.

2.Haff, G. Gregory, et al. “Carbohydrate supplementation attenuates muscle glycogen loss during acute bouts of resistance exercise.” International journal of sport nutrition and exercise metabolism 10.3 (2000): 326-339.

3.Esbjörnsson-Liljedahl, Mona, et al. “Metabolic response in type I and type II muscle fibers during a 30-s cycle sprint in men and women.” Journal of Applied Physiology 87.4 (1999): 1326-1332.


5.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.


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