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Carbohydrate loading, (carbo-loading or carbups for short) has been a staple of endurance training since Swedish scientists first developed the technique in 1967 (1).   Swedish athletes were pioneers of early endurance training, also developing the first type of interval training, known as "Fartlek Training", an archaic yet effective method of getting people back into cardiovascular shape and decreasing adipose stores (2).  During the late 1970's and early 80's, bodybuilders soon realized that the practice could be slightly tweaked to temporarily increase muscle-belly size as well as performance.  This article however, will focus on the two main approaches utilized by endurance athletes, which allow both the muscles and liver to store supra-physiological  (above normal) amounts of glycogen.  More glycogen = more fuel for aerobic exercise

The Science Behind Carbups:
Carbohydrates: various starches and sugars, are converted to glucose, and then either utilized immediately as energy, converted to glycogen via the enzyme glycogen synthase, stored in the muscles or liver, or, if muscle and liver glycogen stores are full, it is converted to and stored away as unsightly fat (2). Glycogen is broken down into glucose, the body's primary source of fuel.  When muscle and liver stores of glycogen become depleted, blood sugar drops, as does one's ability to perform at peak athletic levels.  At this point the athlete's body can do a couple of things: collapse from exhaustion, commonly known as "hitting the wall", break down nitrogen and amino acids and convert them to glucose through a rather complicated process known as glucogenesis, or oxidize fat stores through a process known as ketosis before "hitting the wall". However anyone who has partaken in or watched a marathon or extended endurance event knows that most of the time, athletes simply, "hit the wall" as soon as glycogen stores are diminished. 

Glycogen Depletion, Compensation, and Super-Compensation
The concept of "hitting the wall" may be what first peaked sport-scientists interest in glycogen stores and how they affect performance.  A muscle cell full of extra glycogen is also full of  extra water, providing super-hydration and warding off cramps, ATP-CP stores, providing increased anaerobic energy bursts (desirable to power/endurance athletes as well as weight lifters), and basically overloaded with all sorts of metabolic goodies such as electrolytes and others that would make up their own article.  As anyone who's partaken in a ketogenic diet for the purpose of future carbo-loading can attest to, once the body begins to run off of ketones.  Technically a ketone is any organic compound with a carbonyl group attached to two carbon atoms (3).  The lack of energy, cognition, and libido that comes with hypoglycemia is drastically decreased once the athlete enters ketosis.  The reason for this phenomenon is that the brain has found a new source of energy: ketones, and says to itself: glucose, we don't need no stinking glucose!”  (keep in mind your brain is still a bit loopy). 

In biological functions, “ketone bodies” are produced from states of fasting (even as little as the ten or so hours between dinner and breakfast), prolonged hypoglycemia, as is seen with decreasing carbohydrate consumption while increasing caloric expenditure, and metabolic disorders (3).  Ketogenic diets were first developed as a treatment for epilepsy, however prolonged accumulation of ketones in the body is not exactly a healthy diet considering that it causes a buildup of acetone, acetoacetone, and beta-hydroxybuturate (4).

However we're discussing the effects of short-duration ketogenic diets commonly known as Cyclical-Ketogenic Diets (CKD) or Timed-Ketogenic Diets (TKD), which do not cause massive buildups of ketones in the blood or brain, and have the positive effect of tricking the body into storing above normal amounts of glycogen once you add carbohydrates back into your diet, for up to 72 hours after “super-compensation” levels of glycogen are achieved.  So how does one know when this super-compensation is attained? What kinds of carbohydrates should be consumed, in what quantities, and how often?  The answers lie ahead.  However keep in mind that there are two, distinct and drastically different purposes for carbo-loading among bodybuilders or people who want to look extra “jacked” on the beach, and for endurance athletes, whose sole intent in conducting a carbups is muscular performance.

Hitting the Wall Intentionally.
In order for all the key metabolic factors that allow glycogen super-compensation to occur, total glycogen depletion must happen first.  However we want this to occur during the carbo-depletion phase of the diet, not during your endurance event. Carbo-loading is generally recommended for events lasting 90 minutes or more (5).  Why 90 minutes you ask?  There's actually a quite precise reason: liver and muscle glycogen levels become drastically low after 60-80 minutes of “intense cardiorespiratory exercise” (6).  Rational thinking leads one to theorize that: If I start to run out of fuel between 60-80 minutes, but I need to run/cycle/climb/row/fight/ball for over 120 minutes. I need to put more than 90 minutes of fuel in my body!  Accomplishing this feat is easy.  It is in no way simple, but it's very easy.  Like driving a car: simple, not easy.

When it comes to carbo-loading for endurance events, there are two separate, but equal (depending on your metabolism) approaches.  They've both been around for over thirty years, so it's obvious they both work.  Finding which works best for you is what's important.  Method #2 is the oldest method, and is commonly accepted by many training organizations and coaching staffs. If you've never carbo-loaded before, the second method would be a good place to start.

Method #1: The New School Approach:
The second method for endurance carbo-loading takes a few ideas from the ways body-builders carbup.  Much credit is due in this respect to authors such as the late, great Dan Duchaine, and Lyle McDonald.  Without plagiarizing either author's work (too much), I'll try and convey the modern approach to endurance carbups.  The method I'm about to detail is not an exact duplicate of someone else's seven-day carbup plan, but the one used by myself during charity and sibling-rivalry swims off the Jersey Shore, as well as my sister, a tri-athlete/yoga-instructor/marathon runner.  We're both Endomorphs and have a family history of diabetes, as well as: Syndrome-X, or Mediterranean Metabolism, otherwise known as insulin resistance.  If your body tolerates certain carbohydrates better than others, you'll quickly learn which to use and which to avoid during your dry-runs.

The Program:
I like a nine-day program: five days of depletion, and four days of super compensation.  Why? Supra-physiological glycogen stores can be maintained for 72 hours (1).  Most people take 24 hours for their bodies to reach this state, most (8).  Some carbo-bunnies peak in 18 hours, some in 30.  Where's my reference? Most of them work out at Elements in TR. To err on the side of caution (as some athletes don't show signs of glycogen super-compensation until they pig out after an event), we'll give the body not only the first 24 hours to achieve super-compensation, but the next three days to fill any gaps. The event should either fall on day 8 or day 9.  Not so quirky anymore huh?  Naturally you can't change the day of the Boston Marathon, which is where the dry-runs come in handy.  After a few, you'll know whether you should compete on the eighth or ninth day. Or, go with the old school approach.  They both work: different stokes fer different folks

The Numbers:  Down & Dirty.
OK, so I stole the "down & dirty" line from Duchaine.  It's a great line though ; an omage to a pioneer in this crazy world of ours.  If you're not in shape when u start this, year a gonna be hurtin.  Here's how it breaks down. The diet is based on a 100kg (220 lb.) highly trained but out of practice, male, Endomorph athlete. 

Days one & two: the good news, you get 3500 calories per day: 90% protein-rich, low-glycemic carbs. .08% protein (get it wherever you can, man), .02% protein-enriched fats such as: DeeZ NutZ any nutz will do. Almonds and pumpkin seeds are among my favorites.  Bad news is, you'll be doing 60-80 minutes of your cardiovascular activities(s) of choice, at 50%-80% of your VO2-Max, or cardiorespiratory fitness. Days three & four lighten up in both directions.  If you Hit the Wall during these first two days, you're heading down the right path.

Days three & four: you'll be getting 2500 calories a day, loosing 1K calories takes an extra bite at what little glycogen remains. 40% of your DCI comes from protein-rich carbohydrate sources: oatmeal, bulgur wheat, and certain breakfast cereals. Throw in a whole egg this time around, maybe a banana, a small scoop of protein powder if you're so inclined.  Drink a few mugs of Yerba Mate tea. You're gonna need some egg whites, diced ham, venison, and some peanut butter to up the protein & fat: both falling in at 30%.   Pre-workout stimulant mixes such as AE-Nutrition's Steel Edge,  and  SNS  Focus-XT are the two top products I've tried, which surprised the hell outta me, as their both in the lower price range.  It was like taking a Saturn out for a ride and smoking a Vette.  This definitely isn't the car industry.  In terms of a during workout boost, small doses of unflavored Vitargo mixed into one's water (I'm talking 10 g. to a 20 oz bottle) seems to help.  Most sports drinks have the electrolyte part right, it's just the sugars they're off on.  Another reason why pre/during workout drinks, even w/a bit of Vitargo or a Carb-Boom carb-gel packet helps keep that wall off in the distance.

Basically, as long as you get what you need, it doesn't matter what it comes from. If you were able to purchase this diet at McDonald's you'd increase your performance.. Today you'll be training over two sessions of 40-60 minutes.  Energy drinks are like Gatorade from planet Krypton.  Steel Edge, although on the inexpensive side of these luxury supplements, unfortunately is still a bit pricey. But it makes you rip like a bored-out 5.7 l running two bottles of octane boost; and that's after working for twelve hours.  Focus-XT is another formula that I think got overlooked because of its size, and therefore its price.  It's one of the cheapest products available, but w/a serving size of only 12g/ and 20 servings/container. 

Energy from external sources however, is only part of the battle.  You need your body running with as much glycogen as possible.  That's where Vitargo comes in. Vitagro is perhaps the most important scientific contribution to athletic since Testosterone. 

Vitargo, a patented carbohydrate made from Swedish waxy-maize starch  earns those damn Swedes a medal; they just don't quit when it comes to endurance innovations.  With 3000 times the molecular weight of dextrose, it is absorbed into an athlete's blood stream almost instantly, and causes absolutely no G.I. distress, unlike high-doses of dextrose or maltodextrin.  Plus there's your carb-gels, which are better for cycling and running, as there's nothing much to sit in your stomach.

Days five and six:  Similarly to the older method, we reduce exercise as well as calories.  You'll be taking in between 1.5K and 1.8K calories a day, like this: 15%-20% carbs: leafy vegetables, fibrous veggies, certain, fibrous grain substitute.  You'll be working out, probably by now by walking, jogging using an elliptical, or something in addition to our chosen regimen.  Getting in twenty minutes twice a day  is a bit much for most.  Push yourself, but don’t overdo it.  This program works just as well no matter how you deplete the glycogen; you might as well be safe about it.

Days Seven & Eight:  As I'm hoping you figured out by now, day eight is your first by.  You don't train, you eat (unless you have a weigh-in), you crank on the forums and video games.  Day seven will be difficult but it's important not to strain too much.  If the activity is too much, do something lighter, hell, even if it's just holding yourself up on second thought you may want to call the Doc if it gets that bad.  The important part is that you do something for 15-20 minutes twice per day. 800-1K calories:  Only 10%-15% (25-30g) .from carbs  such as fiberous veggies, certain high-fiber grains, trace amounts that go along w/ dairy and protein powder.  15%-20% (150-200g) protein from varied sources.  Protein consumption should be both spaced throughout the day, and served in one high-protein meal as many researchers believe this enhances amino-acid uptake.  Once your second workout on day seven is over, FEED  Just like sipping the water, don't eat too fast, but eat lightly and constantly.  During the 24 hours using products like Vitagro along with Taurine, Arginine-AAKG, and Citrulline Malate, the aerobic creatine will fill your muscles with these super-fuels.  Be sure to drink lots of water as well.  Unless you have high blood-pressure or are competing in a body building contest immediately after your marathon, no need to worry about sodium intake.

If the glycogen doesn't swell you up with both strength and confidence on day eight, you still have day nine to fill in and continue lounging.

During the Event: Once you've chosen your favorite energy sources and have a healthy supply of water (at least inside you), the rest is in your hands.  If you trained smart and followed either set of guidelines in this article, you have nothing to worry about; save for the inherent dangers of your chosen, endurance-testing activities.

Post event: Sip water; any endurance athlete reading this knows why.  No matter how thirsty you are: S-I-P.  Ice appropriate body parts, apply stinky balms and salves, and eat whatever you want for a few days.  Then start another dry run once the DOMS starts to fade, usually two days.

Method Two: Tried & True
The oldest method for carbo-loading doesn't involve a depletion phase at all. In fact, depleting before taking on this regimen could even be counter-productive.  Plan your carb-up for seven days before your "dry run", Types of carbs during certain times of this carbup are not nearly as complicated as in the first technique. Grain, complex starches, and some fruit juice while at rest, sports drinks, water, and a fast-acting carb source during training. 

Day one: 90 minutes training, 60% dietary carbohydrate intake (DCI) in the form of complex carbohydrates: rye/wheat bread, oatmeal, starches.  Day two: 40 minutes training 60% DCI, same types of food. Day three: repeat day two.  Day four: 20 mins. training, 70% DCI, from lower glycemic starches, some fructose. Day five: repeat day four.  Day six: rest/no training.70% DCI low glycemic starches, some fructose, low-glycemic sugars during the day, pasta/starches at night mixed w/ protein & some fat to slow digestion. Day seven, competition day: 70% DCI (7).  Morning: juice/Vitagro.  Most pre-event carbs from fruit or easily digestible starches, water, sports drink of choice, water w/ small amount Vitagro & a good dose of Steel-Edge or Focus-XT, along with some Carb Boom shots, or even a Clif bar- one of my all time favorites. I once Met the man who invented them in Vermont.  He made ‘em in his pop's garage and set out to sell 'em to ski resorts and dead-headz before becoming a millionaire.


Conclusion:
A healthy athlete, depending on previous levels of training, can expect to improve anywhere between 15%-20% over 10-20 weeks of consecutive training (7).  Quite a variable, huh?  Those ACE folks must really know their stuff to get an accurate reading like that.  Completing frequent depletions and carbups can maximize results in the least time (1,5,8). Besides, despite what a “personal trainer's manual says, any athlete knows what muscle memory is, and how quickly it rebounds you from a layoff.  Old-School precision, if you will, is a good start for beginners;  However why would an expert rely on  archaic carbup programs that merely laid the groundwork for what we know now? If you're not a beginner, most training techniques in books (apparently even in books that train trainers) rely on a false idea: that people will react the same to certain training principles, when it is the subtle variations that make understanding exercise-performance as much an art form as a science.  Science has taught us much about how the human body performs, what makes certain things perform better or differently. I'm not talking better living through chemistry, I'm just referring to the leaps and bounds kinesiology and sports-performance training has made since, say 1967?

 References:
1. Carbohydrate Loading Wikipedia. www.en.wikipedia.com
2. Skeletal Muscle Structure and Function. National Federation of Professional Trainers. Archives.  Online: November 17, 2004. Available: http://www.nfpt.com/Library/Articles/muscl1.html
3. Ketosis. Wikipedia. www.en.wikipedia.com
4. Ketogenic Diets Wikipedia. www.en.wikipedia.com
5. Mac, Brian. Endurance Training. Internet. Online: November 18, 2004. Available: www.brianmac.demon.co.uk/enduranc.htm
6. Bryant, C. Et al. ACE Personal Trainer Manual. 3rd edition. 2003 American Council on Exercise pp 239.
7. Bryant, C. Et al. ACE Personal Trainer Manual. 3rd edition. 2003 American Council on Exercise pp 238.
8. McDonald, Lyle. The Ultimate Diet 2.0.  2003. 1st ed.  Self-Published. Pp. 63-64.

Carbohydrate Loading for Endurance Athletes