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Is Muscle Fatigue Inevitable When You Exercise While On A Low-Carb Diet?

Posted Sep 11 2008 6:18pm

Anthony Colpo flashes rock hard abs he got while on a low-carb diet

A man I have grown to respect and admire over the past year or so is Australian independent researcher and certified personal fitness trainer Anthony Colpo. He's one of those polarizing figures in the world of diet and health that you either absolutely love or completely eschew! And he wouldn't have it any other way!

When I interviewed him at my blog in June 2006, we got a giant gulp of Colpo mania in one swell foop! The man just has a way of putting things in very plain language to confound his critics while educating the common man with plain, unadulterated truth.

Colpo's much-awaited debut book last year The Great Cholesterol Con helped fill the void left by the sudden absence of his wildly popular He then began a new low-carb bodybuilding forum site called in August 2006 to share some of the secrets he has learned about strength, fitness, and fat-burning.

These principles he has been teaching in one-on-one sessions with people have been in such increasing demand that Colpo decided to share them in his newly-released e-book called The Fat Loss Bible. It's a comprehensive plan to help cut through the typical dietary advice by educating you with scientific truths. I will be reviewing this book soon here at my blog and Anthony has agreed to another interview.

In the meantime, I occasionally receive a fitness-related question from a reader that is well beyond my own personal experience and/or knowledge. Such is the case with the following e-mail concerning muscle fatigue while livin' la vida low-carb.

Here's what he wrote in the e-mail:

Hey Jimmy. I've lost 20 pounds on Atkins and only have 10 more to go! I feel pretty darn good overall. Anyways, in regards to my exercise routine I go to a spin class at my local gym.

For some reason, though, I have muscle fatigue especially in my legs. I started taking a fiber supplement three days ago. When I went on the national spinning web site and looked up nutrition, they said if you spin on low-carb diets you're going to get fatigued.

Personally, I think that's just BS! Maybe you can discuss on your blog the benefits of low-carb eating on an exercise program. Maybe I just need to continue the fiber supplement and see if the fatigue improves.

Please continue your awesome blog. I visit it at least TWICE A DAY. It's so exciting! God bless you.

While I have experienced some muscle fatigue at times while undergoing mostly low-intensity exercising on my low-carb lifestyle, the severity has not been as crippling to me as it has this reader. It really depends on how long and vigorous the individual's physical workout routine is.

Because of that, who better to ask this low-carb fitness-related question to than Mr. Low-Carb Muscle himself, Anthony Colpo? He was more than happy to respond.

Here's what Anthony had to say about low-carb muscle fatigue:


Your problem is a common one among low-carbers, and here's why: low-carb diets, as prescribed by most popular authors, are hopeless for meeting the metabolic demands of any meaningful volume of glycogen-depleting exercise.

You are probably asking, "What the heck is glycogen-depleting exercise?"

Low intensity activities, like walking, jogging, or light cycling rely heavily on fats for energy production. After adapting to a very low-carb diet, performance during these activities is not usually adversely affected.

Higher intensity activities (those performed at 75% VO2 max or greater) rely primarily on glucose to meet the energy needs of working muscles. Most of this glucose comes from the working muscles themselves, where it is stored as glycogen. These activities are known as 'glycolytic' and include such forms of exercise as body building, fast running or sprinting, mixed martial arts--and spin cycling.

Sometimes low-carbers can perform a low volume of glycolytic activity and not notice any untoward effects. That's because they're not depleting the glycogen stores of the working muscles to any great degree. If the only glycolytic exercise you do is a relatively brief weight training workout 2-3 times a week, you may feel just fine on a strict low-carb diet.

However, the picture changes dramatically after you introduce activities that make deeper inroads into your glycogen stores. These activities will deplete glycogen from your working muscles faster than your diet can replace it. The inevitable result is fatigue, premature exhaustion, and reduced performance. One of the telltale signs of glycogen depletion is a 'heavy' or fatigued feeling in the working limbs - which is exactly what you are experiencing.

I'll tell you what to do about this in a second, but first I need to warn you about something. Many low-carb devotees of the armchair variety insist that low-carb diets can indeed fuel glycolytic exercise, and that the symptoms you are experiencing are occurring only because you are not yet "fat adapted." According to these folks, you just need "to give it more time".

Let's set a few things straight:

First of all, fat-adaptation almost always takes place within the first 10 days of switching to a low-carb diet. This is the cause of the notorious but short-lived energy 'crash' that many folks experience shortly after switching to very low-carb diets. In a study with competitive cyclists, Phinney et al found that 4 weeks of a ketogenic diet did not affect cycling performance at low intensities. The cyclists complained of the characteristic energy crash during the first 7-10 days, but after that their performance at 60-65% VO2max returned to normal. However, their ability to perform more intense activity (sprinting) deteriorated[1].

This was not due to a lack of "fat adaptation." Respiratory quotient (RQ) testing confirmed that subjects had indeed become fat adapted. Measuring RQ is one of the ways in which researchers check how a person's fat adaptation is progressing. This is typically expressed on a scale ranging from 1.0 (pure carbohydrate oxidation) to ~0.7 (pure fat oxidation). At the completion of the study, Phinney's elite cyclists displayed a mean RQ during testing at 60-65% VO2max of 0.72 - very darned close to 0.7![2] In other words, the cyclists were as fat adapted as they were ever going to be!

Another study by Helge and co-workers found that 7 weeks on a low-carb diet produced much less improvement in cycling performance than a high-carb diet[3].

To anyone intimately familiar with the metabolic demands of glycolytic activity, the findings of the above studies will come as no surprise. No matter how fat-adapted you become, glucose will always remain the primary energy source during sustained, high level exercise. The reason glucose is utilized as the primary energy substrate during high-intensity activities like sprint cycling is because fatty acids simply cannot be broken down fast enough to replenish ATP (the ultimate cellular energy source).

Furthermore, the findings of Phinney and Helge suggest that fat adaptation may actually impair the delivery of glucose to working muscles during glycolytic activities!

Does this mean you have to give up your low-carb diet if you wish to restore your performance during spin classes?

No way!

Despite the highly-polarized and often hostile sentiment that exists between low-carb and high-carb advocates, nutrition isn't an 'either-or' affair. There is absolutely nothing to stop you from utilizing a strategy known as 'carb-cycling'. While carb-cycling gets little mention in popular low-carb diet books, many athletes and bodybuilders have been utilizing it in one form or another for decades.

There are numerous ways to approach carb-cycling; I'll outline what I have found to be the safest, most convenient, and most effective form. It involves taking a large serving of carbohydrates in liquid form immediately after you finish your training. The period that immediately follows a glycogen-depleting workout is a unique one, especially the first 30-60 minutes. During this time, any carbohydrates consumed are preferentially shuttled straight to the muscles where they are used to restore glycogen. Contrary to popular mythology, post-workout carbs do not lurk around in the bloodstream causing damage, they are not converted to fat, and they do not suppress growth hormone release.

The amount of carbohydrate you consume after training depends on the type and duration of the activity. I thoroughly explain the science of post-workout nutrition in my new book The Fat Loss Bible, and I give full details on how to calculate your post-workout carb intake.

However, to get you started, I would suggest you begin drinking 75-150 grams of carbohydrate in liquid form, along with either 30-40 grams whey protein and/or 6 grams of a powdered amino acid formula that is rich in branched chain amino acids, immediately after your spin classes. It is important that you utilize both carbs and protein/aminos after training. Suitable forms of carbohydrate include maltodextrin (a.k.a glucose polymers) or rice syrup, diluted in around 500 ml water. You can add a little glucose or honey to sweeten the mixture. I recommend liquid carbohydrates over solid carbs because the former will be assimilated much quicker; remember, post-workout the goal is to get glucose to the working muscles as quickly as possible.

After finishing your post-workout drink, hit the shower, head on home, and then eat a solid meal when hunger returns or within 2 hours of finishing your class (whichever occurs first).

Try this, and I'm sure you'll be amazed at the difference this will make to your spin class performance!

All the best,

Anthony Colpo


1. Phinney SD. Ketogenic diets and physical performance. Nutrition & Metabolism, 2004; 1:

2. Phinney SD, et al. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism, Aug, 1983; 32 (8): 769-776.

3. Helge JW. Adaptation to a fat-rich diet: effects on endurance performance in humans. Sports Medicine, Nov, 2000; 30 (5): 347-57.

Special THANKS again to Anthony Colpo for his typical thorough and well-documented response to my reader's question. If you would like to learn more from him about his recommendations while engaging in intense fitness training, then be sure to check out The Fat Loss Bible for yourself. ENJOY!

Labels: Anthony Colpo, diet, exercise, Fat Loss Bible, fatigue, Great Cholesterol Con, low-carb, research, spinning, weight loss

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