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Carbs and GlycogenBy Gary Zeolla Carbohydrates come in three basic forms: monosaccharide, disaccharides, and polysaccharides. Monosaccharide's are simple sugars like fructose and glucose. Disaccharides are sugar molecules comprised of two simple sugars. Sucrose is one such example, which is comprised of one molecule of fructose and one molecule of glucose. Polysaccharides are also known as complex carbohydrates or more popularly starch. Complex carbs are long chains of glucose molecules. Carbs serve three main functions in the body. First, they can be immediately burned for fuel. Second, they can be stored as glycogen in the liver and muscles. Third, they can be stored as body fat. Looking at point two, all carbs are not equal when it comes to glycogen replenishment. "Supplements composed of glucose or glucose polymers are the most effective for replenishment of muscle glycogen, whereas fructose is most beneficial for the replenishment of liver glycogen" (Ivy). So glucose can be used to replenish muscle glycogen while fructose cannot. And glucose sugar or complex carbohydrates would both be possible sources of muscle, glycogen-replenishing carbs. However, of these two, complex carbs would be much preferred. … complex carbohydrates … are more effective in replenishing glycogen stores than simple sugars. This makes sense because complex carbs are released slowly whereas simple sugars are released very rapidly, potentially overwhelming the glycogen synthesis pathways and "spilling over" into fat stores. Furthermore, the increased insulin release resulting from simple sugars causes more of the sugar to be converted to fat (Parillo, p.20). I experienced this problem of simple sugars "spilling over into fat stores." When I was using dextrose (a.k.a. glucose) in my post-workout drink, I began gaining fat and feeling like my metabolism was slowing down. I was eating less and less but still gaining weight. But when I switched to maltodextrin (a complex carb), the problem reversed. I detail this experience in my article Post-workout Drink. I recently had someone email who had the same experience. He was gaining weight and eating less but couldn't figure out why. But then he came across my article and switched from dextrose to maltodextrin in his post-workout drink, and immediately he began losing the body fat he had gained. So maltodextrin would be much preferred to dextrose in a post-workout drink. An even better option would be brown rice syrup. Brown rice syrup is an extremely versatile and relatively healthy sweetener which is derived by culturing rice with enzymes to break down the starches, then straining off the liquid and cooking it until the desired consistency is reached. The final product is roughly 50% soluble complex carbohydrates, 45% maltose, and 3% glucose. The glucose is absorbed into the bloodstream immediately, the maltose takes up to one and a half hours to be digested, and the complex carbohydrates take from two to three hours, providing a steady supply of energy. Rice syrup has a shelf life of about a year, and once opened, should be stored in a cool, dry place (International Starch Institute). Maltose is a disaccharide composed of two glucose molecules, so all of the carbs in brown rice syrup can be used to replenish muscle glycogen. And with both the maltose and complex carbs being metabolized slowly, this ensures maximum muscle glycogen replenishment and minimal fat storage. As an added bonus, brown rice syrup retains the naturally occurring nutrients in brown rice. Brown rice syrup can be found at most health food stores. But the one problem with brown rice syrup is it requires a blender or Vitamix to blend the post-workout drink. Since I set up my home gym, I've been able to use brown rice syrup, and I've found it superior to maltodextrin. But if you can only use a shaker cup, then maltodextrin will have to do. For times other than immediately post-workout, grains and starchy vegetables like potatoes and corn would be ideal sources for replenishing muscle glycogen as they contain primarily complex carbs. Moreover, since carbs that are released slowly are more likely to be used for glycogen replenishment than fat formation, whole grains would be preferred over refined grains. Whole grains, especially stone-ground grains, tend to have lower glycemic ratings than refined grains. Similarly, sweet potatoes and corn have lower glycemic ratings than white potatoes. That said, an important point to note is, "… the amount of glycogen you can store is quite limited. The upper limit is generally believed to be 250-400 grams, depending on the amount of skeletal muscle you have" (Parillo, p.20). So it doesn't take a lot of whole grain or starchy vegetable consumption to fully replenish glycogen stores. What this means is once glycogen stores are fully replenished, any additional carbs, if they are not immediately burned for energy, will be stored as fat. So high-carb foods like whole grains should only be consumed in moderation. But exactly what "moderation" means will vary from person to person. The more active a person is, the more their glycogen stores will be depleted and thus the more complex carbs they can consume before glycogen stores will be filled and the body will begin to lay down fat. The ideal would be to consume just enough carbs for immediate energy purposes and to replenish glycogen stores, but no more. Soft Drinks, Sugar, and HFCS The primary carb source in carbonated and non-carbonated soft drinks is sugar (sucrose) or high fructose corn syrup (HFCS). Both of these are nothing but "empty calories." This in itself makes the consumption of such drinks unhealthy. But soft drinks are also problematic when it comes to the issue of glycogen replenishment. To review, the body can use glucose to replenish muscle glycogen, but it cannot use fructose. Fructose can only be used to replenish liver glycogen. Also, sucrose is a disaccharide consisting of one molecule of glucose and one molecule of fructose. "HFCS consists of 55 percent fructose blended with 45 percent glucose" (Challem). So only half of the carbs in sugar and a little less than half of the carbs in HFCS can be used to replenish muscle glycogen. Moreover, to summarize a rather complex situation, when a person exercises, the glycogen is depleted from the muscles that are being utilized. If the person then consumes complex carbs, the bonds between the glucose molecules will be broken down; the liver will then allow the glucose to pass through to the muscles until muscle glycogen stores are replenished. In this way, muscle glycogen will be available to provide energy for the next workout. The liver will then use any additional glucose from complex carbs to replenish its own glycogen stores. Once both muscle and liver glycogen stores are full, any additional carbs consumed will be stored as body fat. However, an entirely different situation exists when fructose is consumed. Since fructose cannot be used to replenish muscle glycogen, there is no reason for the liver to let it pass through to the muscles. Instead, the liver will use the fructose to replenish its own glycogen stores. Once those stores are full, any additional fructose consumed will be converted into body fat. But the muscle glycogen will remain depleted, so there will be no energy available, and the exerciser will most likely drag through the next workout (summarized from Parillo, pp. 19-21). Be sure to note the most important point here, "… once liver glycogen stores are full, the liver says, ‘We've got all the glycogen we can hold, so any more carbs coming in here we'll just convert to fat'" (Parillo, p.22). In other words, if after a hard workout the exerciser consumes a large amount of fructose, say in the form of a large soda, the liver stores of glycogen will be filled rather than muscle glycogen. Even the glucose in soda will most likely not be used for replenishing muscle glycogen since soda is metabolized too quickly. Then later, with the liver glycogen stores filled, even if complex carbs are consumed, those carbs will not be used to replenish muscle glycogen. They will instead be converted into body fat. Meanwhile, the muscle glycogen stores will remain depleted. And once again, come the next workout, the exerciser will drag though the workout. If this pattern continues, rather than becoming more lean and muscular from exercise and gradually increasing in endurance capacity, the person will most likely gain body fat and continue to drag through the workouts. And with no results forthcoming, the person very possibly give up on exercise altogether. But the problem will not be with the workout program but with the person's eating pattern. The bottom line of all of this is that sucrose and HFCS are much more likely to be stored as body fat than complex carbs. Of course, this discussion does not relate to just soft drinks. It would apply to any food that is high in sucrose or HFCS content, such as cakes, cookies, pie, ice cream, candy, and the like. If a person's diet consists of high amounts of sucrose and HFCS from any of these sources, that is a perfect prescription for the laying down of body fat and reduced energy levels. Fruit Consumption The first argument is based on fruit's high sugar content. It is said that sugar of any sort is not good for you, even when it occurs naturally in fruit. Moreover, it is said that fruit today has higher sugar contents than yesteryear due to hybridization practices. The latter is true. All you have to do is compare the size and sweetness of an apple growing in the wild with an apple found at a grocery store. The grocery store apple most likely will be larger, have a brighter and more consistent red color, and most importantly, be sweeter. So basically, you'd have to eat two or three wild apples to get the same sugar content as one grocery store apple. However, this argument would not mean any fruit consumption is unhealthy. It would simply mean to consume fruit only in moderation. In other words, if one grocery store apple equals three wild apples, only eat one grocery store apple, not three. The second argument against fruit is more directly related to the above discussion on glycogen replenishment. To review, glucose is especially good for replenishing muscle glycogen but not liver glycogen. Fructose is the exact opposite; it can be used to replenish liver glycogen but not muscle glycogen. Fructose is also known as "fruit sugar" since fruit is the main food source of it. So the argument of the detractors is that by consuming fruit, the athlete is "wasting" carb intake on a form of carbs that cannot be used to replenish muscle glycogen. This will in turn lead to lower glycogen storage and lowered exercise performance. Moreover, "Instead of being stored as glycogen, fructose gets directly converted to fat by the liver" (Parrillo). For this reason, fruit should be avoided. However, there are some problems with this reasoning. First, the sugar in fruit is not pure fructose. All fruits in fact contain a mixture of fructose, glucose, and sucrose. The exact amounts of each vary for different fruits, but the popular conception that fruit is pure fructose is not correct. And the glucose and the glucose in the sucrose could be used for muscle, glycogen-replenishment. Add to this the fact that the rate at which fruit is digested is slower than soda or desert foods. This is due to the high fiber content of fruit, This slower digestion would increase the likelihood the glucose in fruit will be used to replenish muscle glycogen and lessen the chance the sugar in fruit will be stored as body fat. Moreover, the amount of sugar in fruit is not really that great. It is far less than that found in soft drinks or desert foods. For instance, one average apple contains about 20 grams of carbs while one slice of double-layer chocolate cake with chocolate icing contains over 100 grams of carbs (DietPower). In addition, out of the total amount of carbs that an athlete would consume in a day, the fructose from a couple servings of fruit would be a very small percentage and thus would have minimal effect on lowering muscle glycogen storage. Or to put it another way: … don't be deterred from eating fruit because it contains fructose. Although fruit contains a high percentage of fructose, the total amount is relatively low because the total amount of carbohydrate is relatively low … This is in contrast to cake or cookies, which contain tremendous amount of sucrose (remember, sucrose is made up of half glucose and half fructose). Therefore, although desert foods may contain a lower percentage of fructose than fruit does, they yield a greater amount of fructose per ounce (Faigin, p. 145; italics in original). Moreover, while foods like soft drinks and desert foods are nutritionally worthless, fruit contains a wealth of nutrients, so fruit is a much better way to satisfy your sweet tooth than the consumption of those foods. So eat fruit. It is a healthy food. Just don't overdo it. As the saying goes, all things in moderation. Bibliography: Challem Jack, The Nutrition Reporter™ – http://www.thenutritionreporter.com/fructose_dangers.html DietPower software. Copyright 1992-2004 by DietPower inc. Faigin, Rob. Natural Hormonal Enhancement. Extique Publishing: Cedar Mountain, NC, 2000. Food Resource, Oregon State. Flavor and Flavor Components: Sweetness - http://food.oregonstate.edu/flavor/sweet.html Ivy JL. Glycogen resynthesis after exercise: effect of carbohydrate intake. Int J Sports Med. 1998 Jun;19 Suppl 2:S142-5. PMID: 9694422. Parrillo, John. "Fructose: The Ideal Carbohydrate Source for Gaining Fat." John Parrillo's Performance Press, pp.18-23. |
