Carbohydrates, Insulin and the Glycemic Index

Glycogen is the main component in the anaerobic glycolysis mechanism of ATP production and ATP is what fuels intense muscular contractions. Each set you do decreases the muscles’ momentary ability to produce ATP and the substrates (including glycogen) required to produce more ATP, and time is then needed for stores of these substrates to be replenished so that intense muscular work can be performed again (sufficient time is required both between sets and between workouts).

If muscle glycogen levels are low you won’t be able to produce sufficient ATP and you’ll be weaker and have less endurance for strength and size training. The relationship is quite simple: Low glycogen = low ATP availability = poor workouts = poor results. So, clearly, adequate muscle glycogen stores are essential for the hard training necessary to stimulate muscular size and strength increases. How do we most effectively “top up” our muscle glycogen stores? By the proper selection, quantities and timing of dietary carbohydrates.

Glucose: The Body’s Carbohydrate

There are a few paths by which the body manufactures glycogen. Muscle glycogen stores are replenished by blood-borne glucose (glycogen itself is essentially a chain of glucose molecules strung together), while the liver (which also stores glycogen) can make glycogen from fructose, lactates, alanine and other substrates. Because the liver has several pathways by which it can manufacture glycogen, most of the glucose absorbed from the gut actually travels through the liver without being absorbed and is, in a sense, “reserved” for muscle glycogen production. Only as muscle glycogen stores become replenished will the liver increase its production of glycogen from glucose.

The downside of the liver’s demand for glycogen is that when it runs out of glycogen it takes alanine, and the other substrates, from the muscles in order to make more (it can’t actually take the glycogen because once it is manufactured in the muscle it can’t be removed – it must be used for energy). The result is muscle atrophy (i.e. shrinkage).

The goal is, therefore, clear: Keep liver glycogen stores full so that muscle protein isn’t compromised and keep muscle glycogen levels full so that maximum ATP is available, via anaerobic glycolysis, for intense weight training. We know that glucose is vitally important for both of these processes (you don’t actually eat glycogen) so our attention should shift towards the best sources and timing of glucose intake. All carbohydrates are digested and converted into glucose by the body (except fructose – more on that later). So, now we need to focus on what kinds of carbohydrates (and which food sources) are best at delivering glucose to the muscles and liver.

The Four Categories Of Carbohydrates

Carbohydrates are divided into four main categories. They are:

1. Monosaccharides
   – glucose (also called dextrose), fructose (in fruits) and galactose (in milk)
2. Disaccharides
   – sucrose (table sugar), lactose (in milk) & maltose (in beer)
3. Polysaccharides
   – such as starches (in vegetables); these are chains of glucose molecules bonded together
4. Fiber
   – soluble (in oatmeal, some fruits, etc.) & insoluble (in whole grains, especially their bran)

The first two categories make up what are called the simple sugars (or simple carbohydrates), the last two are collectively called the complex carbohydrates.

The reaction of the body to the ingestion of the different categories of carbs can be quite different. Because monosaccharides and disaccharides are simple in structure, they tend to digest quickly; being rapidly converted into glucose by the body. This causes blood glucose (often referred to as “blood sugar”, a term which I’ll use in this article myself) to rise rapidly – the exception to this rule is fructose, which will be covered later. Polysaccharides however, because they are chains of glucose molecules bonded together, must be broken down considerably by enzymes in the gut before they can be absorbed. These digestive enzymes break the bonds between the glucose molecules making up the polysaccharides, allowing the body to utilize the carbs. But this process requires time, so polysaccharides digest more slowly than simple sugars, raising blood sugar more gradually and not as high. Because of the time spent in the gut they also provide a longer, more steady “flow” of glucose – they are the “slow-burners” of carbohydrates, so to speak (potatoes and yams are exceptions to this rule).

When blood sugar rises the pancreas secretes insulin in response. It is the function of insulin to increase the number of transporters at the surface of the cells that carry glucose inside (insulin itself doesn’t actually transport anything across). Once inside muscle and liver cells, glucose can be used either for energy or changed into glycogen and stored for future use. If glycogen stores are already full, more glucose is present than is immediately needed for energy, or there is simply more glucose present in the blood than the muscles and liver can absorb, then the excess will be converted to fat and stored in fat cells. Fat cells also absorb glucose – a process facilitated by insulin – but they don’t produce energy or glycogen, they only change the glucose to fat and store it. This is why some diets attempt to control both blood glucose and insulin levels to acheive fat loss.

The “problem” with simple sugars, generally, is that they raise blood sugar levels so quickly that the pancreas releases a large amount of insulin, causing the fat cells to become receptive to absorbing and storing glucose as fat. In addition, the high glucose levels may exceed the muscles’ and liver’s ability to absorb glucose, further causing glucose to be stored as fat in the fat cells. Then, because most of the blood-borne glucose is quickly absorbed into fat, muscle and liver cells (all facilitated by the large release of insulin), blood sugar levels plummit, leaving you tired and lazy. The brain’s preferred fuel is also glucose (and the brain gets priority), so the body’s solution to the now low blood sugar level is to free up glycogen from the liver, converting it to glucose, to get blood sugar levels back up again. This process amounts to, essentially, a fat-gaining roller-coaster ride.

The liver of an average man stores about 90 to 100 grams of glycogen and once this gets diminished proteins begin to be leached out of the skeletal muscles to be converted into glucose by the liver (obviously not a good situation for muscle strength and size). Under “normal” metabolic conditions, if you ate no carbs at all your body would convert up to 200 grams of protein a day into glucose just to feed the brain. Here’s a little section that I wrote about 10 years ago in one of my notebooks:

Insulin not only stimulates protein and glucose uptake into the muscle cells but also inhibits protein degradation and gluconeogenesis (the process of producing glycogen out of proteins). Between meals, as serum (blood) glucose levels fall, insulin levels decline also. Under these conditions protein synthesis halts and amino acids flow out of the muscle to be be delivered to the liver where they are oxidized directly for energy or are converted into glucose in order to raise blood sugar levels again.

Complex carbs, on the other hand, raise blood sugar slowly (generally) not leading to large secretions of insulin or the preferential storage of fat (unless, of course, energy requirements are already met and glycogen stores are already filled). This also has the effect of resulting in better glycogen storage in the muscles because glycogen storage enzymes aren’t “over-flooded” and excess glucose isn’t hastily converted into fat. In addition, having a moderate insulin level in the bloodstream (as promoted by complex carbs) has other very important muscle-building advantages: It keeps cortisol (a catabolic hormone) levels low and keeps proteins from being taken out of the muscles and brought to the liver for oxidation or conversion to glycogen; it facilitates the entry of certain amino acids into muscle cells; and it may inhibit androgen-binding globules which bind testosterone and keep it from breaking into its free (active) form.

The lesson to be learned here is that, for most of the time, complex carbohydrates should be your preferred source of carbohydrates.

The only category of carbs not mentioned up to this point is the fourth one: Fiber. Fiber cannot be digested by humans to yield energy. It does, however, have its uses. Soluble fiber (found in apples, oatmeal, citrus friuts, etc.), for example, has been shown to promote what is considered a “healthy” ratio of blood-borne cholesterol – although I am strictly against decreasing overall cholesterol levels when attempting to develop and maintain maximum muscle mass without the aid of anabolic drugs. Insoluble fiber (whole grains) absorbs water in the large intestine, thereby promoting “regularity” and “soft” stools (this wasn’t my intended focus when I started this article!). But, from a muscle building standpoint, the main benefits of fiber in the diet is that their presence in a meal lowers the rate at which carbs are digested (the benefits of which were discussed above) and soluble fiber has been shown to increase the insulin sensitivity of muscle (meaning better intramuscular glycogen storage and your body doesn’t have to secrete as much insulin in total). Soluble fiber has also been shown to lower estrogen levels (which is good when trying to lose fat). Before you go getting carried away with fiber though, be warned that more than roughly 35 grams in your daily diet can reduce the absortion of some key minerals from the gut. Some researchers have also noted that high fiber intakes lower testosterone levels, supposedly by binding testosterone just as it does cholesterol and estrogen. This is thought to be one of the reasons why vegetarians have lower testosterone levels than meat-eaters. The fact is, though, that most people in affluent nations don’t get anywhere near optimal amounts of daily dietary fiber.

Below are some examples of fiber-rich (fibrous) and polysaccharide-rich (starchy) carb foods.

Some Fibrous Carbs

• Asparagus
• Green Beans
• Broccoli
• Brussels Sprouts
• Cabbage
• Carrots
• Cauliflower
• Celery
• Cucumbers
• Egg Plant
• Lettuce
• Mushrooms
• Onions
• Peppers
• Spinach
• Squash
• Zucchini

Some Starchy Carbs

• Barley
• Lima Beans
• Red Beans
• Black-Eyed Peas
• Corn
• Whole Meal Flour
• Lentils
• Oatmeal
• Pasta
• Peas
• Popcorn
• Potatoes
• Rice
• Sweet Potatoes
• Tomatoes
• Shredded Wheat
• Yams

The Glycemic Index

Since the rate of carbohydrate digestion is of such importance, a comparative “list” has been made measuring how much a specific quantity of a given food raises blood sugar as compared to pure glucose solution (or, more recently, white bread). This list is called the Glycemic Index. To put the index into context, a higher number means that the food raises blood sugar more rapidly, a lower one means that the food raises blood sugar more slowly. The following list comes from Rick Mendosa – keep in mind that the entries in the index can vary; for example, cooking carbs generally raises their index value because it breaks down the fiber inside the food.

The key is, generally, to select foods that are low on the index. If going by the glucose-based index try to stick to food that are under around 50. If going by the white bread-based index stick to foods that are under around 70. The reason I say to stick to these rules “generally” is because the post-workout meal would vary substantially from these guidelines – but that’s for a different article.

Here are some other general rules about foods and the Glycemic Index (G.I.):

• Uncooked complex carbs are lower on the G.I. than cooked versions of the same food (pasta is an exception).
• Foods are lower on the G.I. when they are solid than when they are pureed.
• Solid versions of a food are lower on the G.I. than liquid versions.
• Cold carbs are lower on the G.I. than when those carbs are heated.
• Foods high in fiber are lower on the G.I. than foods low in fiber.
• Meals high in saturated fats may raise insulin levels higher than the G.I. of those meals would predict.
• Meals high in unsaturated fats lower the G.I. of foods by slowing gastric emptying, and don’t appear to raise insulin levels higher than expected.
• Adding protein to a meal increases the insulin release in response to that meal, but doesn’t appear to raise blood sugar levels (similar to saturated fats).

Predicting The Glycemic Index Of Mixed Meals

Remember earlier when I said that including fiber in a meal can lower the rate of digestion of that meal? Well, how would you figure out the Glycemic Index of a meal of say, canned baked beans and whole meal flour wheat bread? Simple, quoting Rick Mendosa: “you just multiply the percent of total carbohydrate of each of the foods by its glycemic index and add up the results to get the glycemic index of the meal”. In our example, let’s say you have a meal with a total carbohydrate content of 70 grams. 20 grams of these carbs come from the bread and the other 50 grams from the beans, so you figure out that the bread contributes 20/70 x 100 = 28.6% of the carbohydrate calories and the beans contribute the other 71.4% (50/70 x 100 = 71.4). Now multiply the Glycemic Index value (I’ll use the glucose-based version) of whole meal flour wheat bread (69) by 0.286: 69 x 0.286 = 19.7 and multiply the Glycemic Index value value of canned baked beans (48) by 0.714: 48 x 0.714 = 34.3. Now add the two numbers together 19.7 + 34.3 = 54 and that’s the Glycemic Index value of the combined meal. As you can see it’s a value somewhere between the index values of the two separate foods. This process illustrates how you can use low index value-carbs (like fibrous vegetables) to bring down the index value of a meal containing high Glycemic Index carbs.

Liver Glycogen, Muscle Glycogen, Fat And Fructose

Earlier I stated that fructose behaves a little differently than the other simple sugars. And if you look at the glycemic index you’ll notice that fructose scores very low indeed. Some people have used this low G.I. rating of fructose, and the fact that it is much sweeter than sucrose or glucose, to promote its use as a replacement sweetener when on a fat-loss diet. Seems sensible, right? Lower G.I. = less insulin secreted = less fat stored. You may also recall that I said that the liver can manufacture glycogen by several different pathways, one of which involves fructose. In fact, fructose can be used very efficiently to replenish liver glycogen. So given these facts you may decide to use fructose selectively to help promote fat loss and to replenish liver glycogen after workouts.

But, it isn’t quite that straightforward. Fructose cannot be used by muscle cells for glycogen replenishment. Therefore, when fructose is absorbed in the gut it is dealt with by the liver where one of two things happens: It is either converted into liver glycogen (which can then be released as glucose) or it is converted into fatty acids. If liver glycogen stores are not full then it will be converted mostly into liver glycogen but if liver glycogen stores are full, or there is an overabundance of fructose, it will be converted into fatty acids. On the other hand, glucose is the preferred substrate for muscle glycogen synthesis, so much of the glucose that is ingested may be absorbed by the muscle cells for muscle glycogen replenishment. If muscle glycogen stores are full, the glucose is not needed immediately by the muscles for fuel, or there is simply too much glucose in the blood stream, then the excess glucose will either be converted to fat or returned to the liver where it will either be converted to liver glycogen (if liver glycogen is low) or converted to fatty acids (if liver glycogen is high). Incidently, this is how sugars – particularly fructose – so effectively raise blood triglyceride levels. If both the muscles and the liver are slow to utilize this glucose, or cannot absorb an overabundance of it, then the fat cells will absorb it, convert it into fat and store it.

So, what we have is fructose is useless as far as replenishing muscle glycogen stores is concerned, but is capable of replenishing liver glycogen. Glucose is superior for muscle glycogen replenishment and any excess will go towards liver glycogen replenishment – if needed. If all glycogen stores are replenished (muscles and liver) then excess glucose will be converted to fatty acids. And flooding your system at anytime with large amounts of any carbohydrates will result in fat production and storage. Now here’s the practical part:

The muscles of a 154 pound male athlete can store about 400 grams of glycogen and the liver about 90 grams. If you depleted some of this by exercising then your body would be in need of enough glucose to replenish both of these stores of glycogen. If you ate only fructose as the carbohydrate portion of your post-workout meal you’d only replenish liver glycogen and leave your muscles starving for glucose. In addition, the liver can only make glycogen out of about 50 grams of fructose at a time, so any more than this and you’re just producing fat and probably giving yourself diarrhea. On the other hand, if you used glucose as your post-workout carb source you’d replenish both muscle and liver glycogen (with muscle glycogen being replenished first) and, because both the muscles and liver were utilizing it, you could eat much more of it without worrying about storing fat. The moral of the story is not to go crazy on products sweetened with fructose (after training …or anytime for that matter) and not to rely excessively on fruit as your post-workout carb source (most ripe fruits are high in fructose – hence the name fructose).

Am I saying that fruits are bad? NO!! They’re full of vitamins, enzymes, soluble fiber and probably hosts of things we haven’t even discovered yet. But for fueling workouts and while on a fat loss diet, they’re probably not something you want to load up on.

Incidently, sucrose (table sugar) is made from one molecule of glucose connected to one molecule of fructose. It has been suspected that this is why table sugar makes people fat – it is half fructose, and the other half (glucose) rapidly raises insulin levels – the perfect fat promoting molecule.

So Many Questions, So Little Time

What about low-carb diets? Carb loading? Pre- and post-workout carb intake, etc.? Well, those things will have to wait for another time. There are many more intricacies to the whole carbohydrate story than I’ve covered here, and I’ve simplified and omitted some things to make the understanding more applicable to weight training, but the information above should provide you with a good nutritional background on carbohydrates and how they fit into a weight trainers nutrition program. With proper understanding you can start making your own informed decisions about your diet.

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