Last week, John Berardi, Lonnie Lowery, Cy Willson, and that annoying Jared kid from the TV commercials discussed different aspects of carbohydrate metabolism. When last we left them, they were about to start explaining the differences between individual types of carbohydrates.

Listen, with carbohydrate classification schemes out of the way (and hopefully most of Lonnie's argumentative streak), let's address some of the individual types of carbohydrate floating around the nutrition and supplement worlds and discuss how they might or might not fit into a good nutritional regime. Let's talk:

Monosaccharides, fructose and glucose (dextrose)

Disaccharides, lactose and sucrose

Polysaccharide, maltodextrin

Food additive high fructose corn syrup

Polysaccharide starches, amylose and amylopectin

Polysaccharide fiber, cellulose

The sometimes carb, sometimes sugar alcohol (depending on whether bar manufacturers want to disguise carb content or not) glycerol

Fructose and glucose, as seen earlier, are single unit monosaccharides that, while structurally very similar, behave very differently in the body.

As Lonman mentioned, glucose is more rapidly absorbed in the GI and tends to elicit higher blood glucose and insulin responses. This makes glucose ingestion ideal for situations where rapid digestion or energy provision is required (during exercise, during a hypoglycemic episode, post exercise) but not so great for the remainder of the day when stable blood sugar and insulin are desirable. Glucose is found as a monosaccharide in fruits, vegetables, and honey as well as many manufactured sports drinks. Most blood glucose doesn't come from glucose ingestion but from other dietary carbohydrates that are eventually broken down into glucose.

Fructose, on the other hand, is metabolized differently due to its structure. In the liver, fructose is metabolized and can replenish glycogen (liver only) or can form triglycerides. Due to the fact that fructose doesn't cause a substantial rise in blood sugar (it's too busy filling liver glycogen stores and creating triglycerides), it doesn't stimulate insulin secretion to any large extent.

Now, the great debate among nutritionists has been whether the low insulin response is enough to outweigh the inevitable formation of some triglycerides. In my opinion, the best answer is that it all depends on how much fructose you ingest. With a very high daily consumption of fructose (from lots of fruit, but even from the more lipogenic high fructose corn syrup and from sucrose, which is in fact, a glucose and a fructose joined together), the lipogenic effects should probably be considered.

However, a moderate daily intake of fructose, especially from fruits, is encouraged. Just be sure not to consume fructose around exercise time. It has been shown time and time again to cause GI distress, increase ratings of exertion, and cause higher serum cortisol levels when consumed in conjunction with exercise. Fructose is found naturally in many fruits, berries and honey (foods that I highly encourage consumption of) as well as some dietary supplements, but in Western society most people get fructose from processed foods containing high fructose corn syrup.

Jared: Hey, I just heard someone talking about Fructopia. That's one of my fave drinks. You guys really know your stuff. And by the way, I'm feelin' better now! I'd better cut my walks back down to fifteen minutes.

JB: Good to know, Jared. Right now I was just about to talk about polysaccharides, like lactose, for example. Got anything to contribute?

Jared: Lactose? Um, I think that's the stuff in milk that gives me gas.

JB: That's right, o' baggy skinned one! You must be lactose intolerant and can't produce enough of the intestinal enzyme lactase. You see, lactose is a disaccharide consisting of glucose joined to galactose. Since lactose is too large to be absorbed in the intestines, it must be broken down into glucose and galactose for absorption. If not, the lactose just sits around in the GI tract fermenting, causing gas, diarrhea, and bloating.

Lactose is found in dairy products, with highest concentrations in milk. Fermented dairy products usually have very little lactose remaining as the lactose is converted to lactic acid. In addition, yogurt contains enzymes that help in the digestion of lactose. If you're lactose intolerant, like our friend Jared, you should avoid lactose. Otherwise, it's probably okay since it has a low GI and II.

LL: Johnman, I feel like I've got to interject a little anecdote. One of my acquaintances, Steve Hertzler, did some practical research on lactose intolerance back in 1996. He found that most maldigesters can handle up to six or seven grams (half cup of milk) before, uh, "distress" sets in. That's good information but its acquisition takes a dedicated hombre. The man has actually put rubber britches on his subjects, complete with gas collection valves! Ugh! I sometimes wonder: what career paths lead one to becoming a flatulence researcher? Okay, sorry, on with the show...

JB: Thanks for sharing your "tale." Back to sucrose. Sucrose, like lactose, is a disaccharide that's formed when glucose and fructose join. Sucrose is, by far, the most abundant source of dietary carbohydrate in the Western world and while its glycemic index is lower than that of glucose, it's still substantially higher than fructose. Therefore, when sucrose is digested, it can have both a high glycemic index and be lipogenic. Not a great combination; therefore, perhaps sucrose intake should be moderated by anyone interested in their body composition. Just as I recommended saying no to fructose around exercise, I think this should extend to sucrose as well.

Dr. Lowery, why don't you tackle maltodextrin, high fructose corn syrup, and the starches amylose and amylopectin? Then Cy can wrap up with cellulose and glycerol.

Jared: Hold on a second. You guys are losing me. I just don't get it. I mean, I lost 3,000 pounds and don't even know what those words are! Just how much maltopectin or whatever is in one of those yummy Veggie Delight sandwiches? You know, they have less than six grams of fat! And by the way, why is that Cy dude growling at me?

JB: Just don't make any sudden movements, Jared. Lonnie, go ahead.

LL: Okay, maltodextrin, amylose, and amylopectin are polyglucose molecules. The primary differences are in their structure. Maltodextrin is shorter than the other two but still longer than mono and disaccharides. Amylose consists of long, straight chains of glucose units whereas amylopectin is a branched polymer. Amylopectin is rather like glycogen ("animal carbohydrate") but with fewer branches. Both forms of starch, amylose and amylopectin, occur in cereals, potatoes, legumes and other veggies, with amylose usually contributing 15 to 20% or so and amylopectin comprising the other 80 to 85% of total starch.

As far as application to athletes, a study by Costill's group in 1996 revealed twenty-four hour glycogen resynthesis rates in muscle to be in the order of glucose > amylopectin > maltodextrin > amylose ("resistant starch"). Actually, the first three groups were similar statistically and subsequent exercise performance wasn't different among groups. Only the amylose was inferior.

Other data suggest that glucose polymers may be slightly superior regarding performance, however. This conclusion stems from their decreased osmolality (in this case, faster gastric emptying during exercise). The difficulty in summarizing the legions of carbohydrate type-exercise studies lies in the different protocols. So many factors make for apples vs. oranges comparisons. Initial glycogen stores, exercise modality, drinks vs. solids, subjects' training status, and even temperature affect the research. Trends can be seen, despite the discrepancies, however. Perhaps most important is that para and post-exercise feedings are the key time to heed the GI and drink rapidly-digested/absorbed carbohydrates like glucose.

Next is high fructose corn syrup. What can I say? Many folks live on this stuff (fructose is a full 5% of the average American's diet) and they wonder why they're fat. John's already slammed the wicked glucose-fructose combination and this sweet syrup (fructose being 2.5 fold sweeter than glucose) offers it in a rapidly consumable, 64 oz., super-sized container. Not that it only appears in drinks – it's pervasive!

When one considers that there's about one teaspoon worth (4 grams) of sugar per ounce of sweetened beverage, it's no wonder that such drinks can quickly add up to hundreds of lipogenic kcal. (By the way, the GI of sucrose is only 59 and fructose is just 20 on a 100 scale, if memory serves; so much for the GI as a tool for judging the beneficence of a carbohydrate food.) That kind of man-made excess leaves your 40,000+ year-old genetic blueprint stumped. Sometimes I'm amazed that we can process it at all without ending up in some kind of glucosuric coma.

CW: Good job, Lonnie, I'll finish off the rest. Cellulose is a polysaccharide that, unlike glycogen or amylopectin, is an unbranched polymer of glucose (again, many glucose units jointed together). In cellulose, if anyone cares, the glucose units are joined by 1,4-beta-glycosidic bonds, whereas starch and glycogen are linked mainly by 1,4-a-glycosidic bonds and to a lesser extent 1,6-alpha-glycosidic bonds. I'll discuse the importance of this in a second.

The main thing to realize is that we humans don't possess beta-glucosidases. These are enzymes that catalyze the hydrolysis of the bonds (break the bond) at the C-1 (beta configuration) position of each glucose unit. The function of these enzymes would be to break down cellulose into a bunch of free glucose molecules. Instead, we possess alpha glucosidases, which allow us to hydrolyze starches to d-glucose. So what does this mean? We can't digest cellulose and thus we consider it a "fiber." That's right, cellulose is the "fiber" contained in the various plants that you consume.

Oh, and on a side note, galactose, which is one of the two sugars in lactose, also has the anomeric carbon possessing beta configuration at the C-1 position. Luckily, animals like cows have certain symbiotic microorganisms in their rumen or their "first stomach" and thus are able to digest the cellulose in grass and things of that nature. I still look forward to the day where I can eat grass, don't you Jared?

Jared: Wait just a second! Are you trying to say I'm a cow or that I get intimate with cows? Because if that's what you're saying, it's not true. Why can't people just forget about that rumor? It's a lie, I tell you! A filthy lie!

CW: No, that's not what I meant at all, Mr. Sensitive.

Jared: Oh, huh, I was just testing you. Uh, change of topic... You know, if one day we can eat grass, just like the cows, I bet Tubway will be the first to come out with grass sandwiches. Baby, talk about low fat!

CW: We can't wait, Jared. Next topic – glycerol. Glycerol is classically described as a hygroscopic, trihydroxy alcohol, which simply means that it contains three hydroxyl groups and is able to take up moisture out of the air and retain it. It's also a clear, syrupy liquid that has a very distinct sweet taste. With those properties in mind, it's easy to see why those manufacturers were putting it into bars in order to keep them sweetened as well as provide some texture and moisture. The only problem was that they weren't counting the carb content since it's not a carbohydrate but instead an alcohol.

Well, that's fine provided that you inform customers that glycerol still has a caloric value and thus you should keep track of it just as you would any macronutrient. This was a very misleading and shady thing to do. Essentially, when you consider the calorie content of the glycerol in those bars, they'd be better off using sucrose. My suggestion would be to stay away from glycerol and whatever you do, don't combine it with sulfuric acid, Jared!

JB: I've written about glycerol in a previous "Appetite for Construction" column and the bottom line was that it can certainly be converted into a carb (for every 10 grams of glycerol you may get somewhere around 3 grams of glucose), therefore it's not necessarily calorie or carb-free, so to speak.

Okay, now that some of the biochemistry is out of the way, why don't we get down to the nitty gritty. I'd like to hear each of your rules on carb intake, you know, how many grams of carbs per day for different populations, percent carbs in the diet for different populations, proper timing of carb intake for different populations, etc.

LL: Interestingly, the necessity of any carbs at all has been questioned in the scientific literature. Can it be true that we don't need any? I'm continually amazed at how adaptable the human body is. Muscle glycogen stores can replenish quite well without post workout carbs, for example.

Of course, our brains prefer glucose as a fuel, which, along with other requirements, leads me to a rough preference for 50% of one's caloric intake. Based on available literature, I personally consume the majority of my carbs at breakfast (whole grain cereals with milk, oatmeal, flax pancakes, etc. equaling about 100 grams of carbs), then at midmorning (another 100 grams or so), at lunch (another 100 grams or so), and finally during and post-exercise (another 100 grams with protein, as a beverage). I try to minimize carbs in the evening but still probably eat about 50 grams worth.

If I were trying to lose fat, I'd probably confine my large carb meals (100g or so) to breakfast and post-exercise. These just can't be reduced without consequences. The other meals I'd cut back by half. If this sounds boring, so be it. I'm a big believer in moderation and allowing adequate time (i.e. a few weeks) to see changes.

CW: For those trying to gain muscle mass, I'm sticking to my "skinny bastard" recommendations. In other words, consuming foods that elicit a significant increase in plasma insulin levels and continually doing this throughout the day. It's that anabolic sledgehammer thing Lonnie has talked about.

As for the actual intake of carbs, for those trying to "gain weight" I've used a simple formula of three or four times the person's bodyweight in carbs. In other words, if the guy is 150 pounds then at least 450 to 600 grams of carbs need to be ingested every day.

And for those trying to reduce body fat stores, I suggest minimizing carb intake for the most part, except for a few key times. With these people, I'd suggest consuming foods that are high in fiber and don't cause much of an insulin release. Foods like old-fashioned oatmeal, yams, certain fruits, etc.

For those trying to reduce body fat stores, I suggest no more than their body weight in grams, so a 210 pound guy at 12 to 15% body fat shouldn't be consuming more than 210 grams of carbs per day. Now, this does leave some room for variation depending on the person's body fat levels. For instance, when it comes to a guy who's 350 at 25% body fat, I'm not going to suggest 350 grams per day, instead I'd suggest around 250 grams or less. Just keep in mind that this is my method of utilizing a "starting point" as it always works well. This way we have a level that's usually tolerable at the beginning and will still allow for fat loss. Now, after initiating the dieting plan, depending on the individual as well as their goals, we can adjust that particular intake accordingly.

As far as carb timing, I feel the most important times are in the morning upon waking, in the middle of the day, and post workout. I personally consume carbs in that fashion during the school year simply because I can't function as well without some form of carbohydrate in the morning. I also need some form of carb during the afternoon to ensure that my glycogen stores are at least partially full by the time I workout in the evening. Finally, after the evening workout, I consume a post-workout drink with carbs.

JB: Unfortunately, since I didn't invite any of the low-carb gurus to participate, this portion of the roundtable will be debate free.

Typically I tend to agree with Lonnie in that carb selections should be unprocessed, grainy, whole foods like beans, nuts, whole grains, oats, etc. My "Lean Eatin'" article here at T-mag gives a more thorough discussion of this. Lonman puts it best when he states that we simply weren't programmed to eat high sugar, high GI, processed foods. Therefore your carb choices should be low GI, low II, and unprocessed. As discussed in my article, these foods should make up about 80% of your daily carb fare with the additional 20% coming from high GI post-workout foods that help replenish glycogen (during periods of weight gain and during the early stages of weight loss).

Unlike Lonman, I don't make the morning vs. nighttime distinction since training throws a big monkey wrench into this. If you train in the morning, I'd recommend what Lonnie does – tapering off carbs as the day goes on. But if you train in the evening, I'd eat carbs after the workout to help with replenishment and recovery. Again, a lot of it depends on your goals. If you want to gain weight and have a fast metabolic rate, carb it all day. If you want to lose weight then the most important times to eat your carbs are probably the first meal of the day and then after training.

As far as how many carbs to eat when bulking up or losing weight, I tend to go with Cy's recommendations for a rough estimate. For a more definitive one, go check out my "Massive Eating" calorie calculations. These numbers are a great starting point. As per the definition, though, a starting point is where you start. You'll have to make adjustments, sometimes weekly, to maintain the type of progress you're looking for.

One strategy I often use for weight loss involves setting protein and fat intake constant (at about 1.5g/lb and about .5-.75g/lb) and then concomitantly reducing carb intake and increasing exercise volume in order to keep the weight loss coming. For weight gain I typically just reverse this scenario up to 500 to 600 grams of carbs, at which time I'll start to increase the protein and fat again.

As far as macronutrient ratios, I tend to think that concept is a bit too gimmicky and offers little value because if you're trying to gain mass; it doesn't matter if you're eating 40-30-30 when your total kcal intake is less than 2000kcal. Likewise, if you're eating a breakdown of 40-30-30 but 4000kcal, you're probably not going to lose fat. Therefore, as described above, I tend to recommend keeping protein relatively constant at about 1.5g/lb (300 grams for a 200 pound person) while fat, but mostly carbs, are adjusted slightly up or down based on your goals.

Jared: Know what I like? Those little squeezy bottles full of sauces at Tubway. I mean, how cool is that?

JB: Jared! Pay attention! Any closing remarks?

Jared: Uh, after all I've taught you today, I think my work is done here. You know, I've got an idea. Let's just all go to Tubway! Come on!

*Note: The names of certain characters and "weight loss" restaurants referred to in this article are almost fictional. Resemblance between them and any real life individual or establishment is purely satirical. In other words, don't sue us! We're just funnin' ya!

John Berardi, PhD, is the founder of Precision Nutrition, the world's largest nutrition coaching and education company. Berardi advises organizations like Apple, Equinox, and Nike. He's coached the San Antonio Spurs, the Carolina Panthers, US Open Champ Sloane Stephens, and 2-division UFC Champ Georges St-Pierre.