While insulin is one of the most im-portant hormones in determining body composition, it is also one of the most misunderstood by bodybuilders. A clear concept of what insulin does, and how to control it, is key to achieving your physique goals. You can think of insulin essentially as a storage hormone, which is released by the pancreas after eating and stimulates cells to absorb and store nutrients from the bloodstream. Insulin is both anabolic and anti-catabolic. The major dilemma is that insulin promotes fat storage as well as muscle growth. You need to have some insulin around in order to grow (and to live for that matter) but too much will lead to excess bodyfat.
Control is the key. Diabetes is a disease in which insulin is either not pres-ent or else ineffective. Type I diabetes, which usually affects children, is an autoimmune dis-ease resulting in destruction of the cells in the pancreas that produce insulin . These patients lose weight even in the face of eating more, because their cells do not effectively absorb the nutrients . This underscores insulin’s important role as an anabolic hormone. Type II dia-betes usually affects adults, and in this case insulin is present but is not effective because the insulin receptor is not effective-ly activated by insulin. These patients are said to be “insulin resistant.” Let’s discuss insulin’s actions, espe-cially as related to bodybuilding, and then discuss how insulin secretion is con-trolled. This will lead to an understanding of dietary strategies to optimize insulin control and body composition. We will see that insulin occupies a central posi-tion in the control of carbohydrate, fat and protein metabolism. Insulin’s most profound effect is prob-ably on carbohydrate metabolism.
After a meal the food is broken down by stomach acid and by enzymes in the stomach and small intestine. After the nutrients are digested into molecular-sized fragments, they are absorbed across the lining of the small intestine into the bloodstream. Car-bohydrates are broken down into mono- or disaccharides, meaning one sugar molecule or two sugar molecules linked together. Proteins are broken down to the level of individual amino acids or short polypeptides, which are short chains of a few amino acids. These protein and carbohydrate breakdown products are absorbed into the bloodstream and transported directly to the liver by a special vein called the portal vein. So the liver gets “first dibs” at these nutrients and is the processing plant that controls their metabolism. Fats fol-low a different pathway, which we have discussed in other articles. Fats (except for medium chain fatty acids like Cap-Tri®) are packaged into special transport particles called chylomicrons, which do not enter the portal vein but instead are carried by the lymphatic system.
CapTri® acts more like a carbohydrate (and in fact has been referred to as a “carbolipid” in the literature) because it is absorbed by the portal vein and taken to the liver where it is immediately processed for energy production . After the carbohydrates reach the liver, they are converted to glucose, the form of sugar which is released into the blood-stream. One exception is fructose, a form of sugar present in fruit (and many sports bars, ironically). Due to the molecular structure of fructose, a significant frac-tion of it is converted into fat by the liver and is released into the bloodstream as a fat instead of a carbohydrate. (We have discussed the biochemistry of this in detail previously in #1 in the Sports Nutrition Guide). This is why we encourage our athletes to avoid fruit and fruit juice, and to carefully read the label before deciding on a sports bar. Parrillo Bars provide carbohydrate in a form, that is slowly converted to glucose, not fructose. So after a meal the amount of glucose in the bloodstream rises . This causes a rapid secre-tion of insulin. This insulin in turn acts as a signal for cells to absorb and store glucose . The tissues that are most affected are muscle, fat and liver.
Muscle tissue relies on carbohydrate (in the form of glucose) and fatty acids as its most common energy sources . At rest and during low intensity exercise muscle tissue relies most heavily on fat. Muscle cells normally are not very permeable to glucose, meaning that glucose cannot easily get inside the cell. During the fast-ing state blood glucose concentrations are low, and therefore insulin concentrations are low too, so the muscle cell relies more heavily on fat since the glucose can’t get in. Shortly after a meal blood glucose levels rise, causing an increase in insulin levels. The insulin binds to receptors on the cell membrane stimulating the uptake of glucose by muscle cells. Therefore for the first hour or so after a meal muscle cells use more glucose as fuel. Exercise also somehow causes muscle to rely more heavily on glucose as its energy source. This is regardless of the fact that insulin levels are low during exercise. Somehow the contractile process makes the muscle cell more permeable to glucose. So mus-cles use glucose as fuel during the fasting state (several hours after a meal) and during exercise.
During the fasting state insulin levels are too low to stimulate sig-nificant glucose uptake by muscle. If I may digress for a moment, a related question that often confuses people is what is the optimal aerobic exercise inten-sity to burn fat? At first you might think that low intensity exercise, like walking, would be best. It is true that during low intensity exercise, a higher percentage of the calories burned are derived from fat. However, it is also true that fewer total calories are burned. So higher intensity aerobics will burn more fat grams per hour, even though it works out to be a lower percentage of calories. And what we care about during our aerobics is the amount of fat we burn, not the percent-age. Furthermore, high intensity aerobics also induces changes in mitochondria, which make them more efficient at burn-ing fat. If you don’t believe this reasoning just look around and ask yourself who’s leaner, the people walking in the mall or the people running outside? Hands down, the runners are leaner than the walkers.
Although during high intensity aerobics muscles use relatively more glucose, they also use more fat. Insulin stimulates muscle cells to absorb glucose and to convert it into glycogen. Glycogen is a polymer (chain) of glu-cose molecules linked together, and is the storage form of carbohydrate in ani-mals (starch is the analogous molecule in plants). When the cell needs glucose for fuel, the glycogen is broken down into glucose units, which are then converted to energy. This is especially useful during high intensity anaerobic exercise such as weight lifting or sprinting. Glucose can be converted to lactic acid (a process which releases energy) even without oxygen. Aerobic metabolism requires the pres-ence of oxygen, so the rate of aerobic energy production is limited by the rate of oxygen delivery. Once this level of exercise intensity is exceeded, anaerobic metabolism can provide an additional energy boost for a few minutes. Only glucose (and not fat) can be converted to energy anaerobically. So while fat is an important fuel source for aerobic ex-ercise, carbohydrate is the primary fuel used to power weight lifting. Insulin exerts a profound effect on car-bohydrate metabolism in the liver.
One of the most important jobs of the liver is to maintain blood glucose concentrations. Under normal conditions the brain relies exclusively on glucose as its fuel source, so it is critical that blood glucose levels be maintained. If blood glucose levels drop too low (for example by the use of too much injectable insulin, or in the very rare case of insulin producing pancre-atic tumors) confusion results which can progress to coma and even death. So the liver soaks up a lot of glucose after a meal and stores it, and then slowly releases it as blood sugar levels drop several hours later. This acts to maintain a relatively constant blood glucose level even hours after a meal. This guarantees a constant fuel source for the brain and other body tissues. Muscle cells are unable to re-lease the glucose they store back into the bloodstream. Once glucose gets inside a muscle cell, it’s trapped there. This makes the liver’s job even more critical, and ex-plains why it gets first dibs at the nutrients as they leave the small intestine. Insulin exerts its control on liver carbohydrate metabolism at several key points.
Primarily it acts to stimulate the conversion of glucose into glycogen (by increasing the activities of phosphofruc-tokinase and glycogen synthetase) and by blocking glycogen breakdown (by inhib-iting phosphorylase). These actions work together to promote storage of glucose as glycogen. About 100 grams of glycogen can be stored in the liver. A hour or two after a meal, blood glucose levels start to fall. This reduces insulin secretion by the pancreas and reverses the steps above. Also, glucagon is secreted by the pancreas, which has the opposite actions of insulin. This results in the activation of the enzyme phosphorylase, which breaks down glycogen into glucose phosphate. The enzyme glucose phosphatase then removes the phosphate group from the glucose molecule, allowing it to leave the liver cell and enter the bloodstream. The phosphate group has a strong electric charge, which makes it thermodynami-cally unfavorable for glucose phosphate to cross the lipid membrane and leave the cell. Muscle cells lack glucose phospha-tase, and that’s why the glucose taken up by muscles is trapped there.
The presence of this enzyme in liver cells allows them to release their glucose into the blood-stream. Usually about 60 percent of the carbohydrate in a meal is temporarily stored by the liver, to be released later between meals. We reach a very important point here. The liver can only store so much gly-cogen, and then it gets full. If too much carbohydrate is consumed in a single meal, and liver glycogen stores are satu-rated, insulin will promote the liver to convert the excess glucose into fat. This fat then enters the bloodstream and is stored in fat cells. Regular readers on this column will know that it takes a huge amount of carbohydrate in a single meal to result in conversion of carbohydrate to fat. However, if you overconsume carbohydrates repeatedly, liver glycogen stores will eventually become saturated and the excess carbohydrate can be con-verted to fat. The keys are not to eat too many calories in any one meal or over the day. If you are trying to gain weight, we have found a daily calorie excess of 300-500 calories per day works well for most people. If you try to gain weight too quickly, by eating too large an excess of calories, then you will gain more fat along with the muscle.
To stay lean while gain-ing weight, gain slowly. Usually a pound a week is a good goal. If you put on fat easily, try to limit your caloric surplus to 250-300 calories a day. And even natu-rally lean people will begin to put on fat if their energy surplus exceeds 500 calories a day . The second point is meal pattern-ing. By this I mean small, frequent meals. By dividing your daily caloric allotment into many smaller meals, this will spread out and slow down the release of glucose into the bloodstream, thereby reducing insulin levels. If insulin levels become too high, this increases the proportion of calories which are stored as fat. Third, meal structure is important. By combin-ing starches with fibrous vegetables and protein this slows the release of glucose into the blood, also acting to help moder-ate the insulin response. These techniques are incorporated into the Parrillo Perfor-mance Nutrition Program. Probably a word about glycemic index (GI) is warranted here. Glycemic index is a way of monitoring how fast a par-ticular food is digested and appears as glucose in the bloodstream. This then is a rough index of the insulin response elicited by the food.
It is measured by feeding an individual food (ice cream or potatoes, for example) in a standard-ized amount (usually 100 grams) and then drawing blood samples at frequent intervals after the meal and measuring blood glucose. This sounds like a great idea at first, but the problem is it doesn’t work when applied to real life situations. People don’t eat meals consisting of just one single food (at least bodybuilders don’t). Bodybuilders know that they get the best results when they combine car-bohydrates and protein together at each meal . By consuming protein at each meal, this continually supplies the muscle cells with the amino acids they need to build protein. And by consuming carbohydrate at each meal, this provides a stimulus for insulin release, which promotes amino acid uptake and protein synthesis. So protein and carbohydrate should always be consumed together. This completely changes the rate of entry of glucose into the bloodstream for the mixed meal, thus the glycemic index of individual foods becomes essentially useless.
To site some examples, ice cream has a very low gly-cemic index, around 30-32. (Pure sugar or white bread are used as the standards, and are defined to have a GI of 100.) This is because the fat and protein in the ice cream slows down the entry of the sugar into the bloodstream. If you went by gly-cemic index, you might think ice cream would be a good bodybuilding food. It results in a low insulin response, so it should not make you fat. Unfortunately, it doesn’t work that way. Ice cream is a terrible bodybuilding food, because it can make you very fat. Potatoes and carrots on the other hand have a high glycemic index, around 100. So you might think they would make you fat. It turns out that potatoes and carrots are great bodybuild-ing foods. When they are combined with protein and fibrous vegetables their rate of digestion is greatly slowed, so that the GI of the mixed meal is much lower. Now we will consider the influence of insulin on protein metabolism. Insulin acts to promote the storage not only of carbohydrate, but also of protein (and fat, don’t forget).
In the absence of in-sulin, protein synthesis drops to zero . There are several ways insulin promotes protein storage . For one, it stimulates the transport of some amino acids inside muscle cells. Most notably the branched chain amino acids leucine, isoleucine, and valine. These also happen to be the most abundant amino acids incor-porated into muscle proteins. So if you use a BCAA supplement, such as Par-rillo Muscle Amino, you should take it with meals. This will promote maximum absorption into muscle cells. Last month I discussed that growth hormone also stimulates amino acid uptake by cells and protein synthesis. So both hormones have an anabolic effect on protein synthesis. Insulin stimulates the transcription of cer-tain genes, to increase the amount of RNA inside cells. It also stimulates the activity of ribosomes, the machines that connect amino acids together to make proteins. These three actions have a synergistic ef-fect to increase protein synthesis. Insulin is also anti-catabolic acting to inhibit protein breakdown. It probably does this by reducing the activity of lysosomes, digestive factories inside cells that de-grade old proteins. Insulin also decreases the rate of gluconeogenesis. This is the production of glucose (think “glucose genesis”) from amino acids by the liver. When blood glucose levels get too low, and the liver runs out of glycogen, muscle cells break down their own proteins and release amino acids into the bloodstream. The liver has the ability to convert these into glucose. This process is kind of a last resort that the body is forced to rely on in order to maintain glucose availability for the brain. Unfortunately, fat cannot be converted into glucose, so you have to break down muscle protein in this situ-ation.
This is obviously a bodybuilder’s worst nightmare. This will happen in cases when either overall caloric intake or else carbohydrate intake is too low. Another instance is in prolonged inten-sive aerobic exercise, such as marathon running. This is why marathon runners, while being quite lean, also have small legs. To avoid this from happening, don’t restrict calories too severely and don’t go too low on carbs. We find that fat loss of one to two pounds a week is optimal. Usually if you try to lose faster than that most of the additional weight lost will be muscle. Also, the very low carb diets set you up for this problem. Generally I would not recommend going lower than one gram of carbohydrate per pound of body weight per day, and certainly never less than 100 grams per day . When insu-lin levels get too low, protein synthesis virtually stops, muscle breakdown is ac-celerated and muscle cells release large amounts of amino acids in the blood. The liver picks these up and converts them into glucose. This is why people with (untreated) diabetes experience muscle wasting and weakness. There seems to be a synergistic effect between insulin and growth hormone. Ex-periments have been performed in young rats in which their pancreas and pituitary glands were removed. This renders them deficient in both insulin and growth hor-mone. In this situation growth stops. When either hormone is injected into the animals alone, a little growth results but not much. When both hormones are injected together dramatic growth results. So while both hormones are anabolic and anti-catabolic, neither one can do the job by itself. The reasons for this are not entirely clear, but may have to do with the fact the each hormone stimulates the uptake of a different set of amino acids. What about the use of injectable insulin as a drug by some bodybuilders? First I will say that this is dangerous. While all drugs have potential side effects, insulin has the ability to kill you right now, if you inject too much of it.
The rationale for bodybuilders using insulin has two parts. First, we have discussed the synergistic effect of insulin and growth hormone above. So bodybuilders who use growth hormone think they will get better results from their GH if they combine it with insulin. Second, growth hormone and anabolic steroids cause some degree of insulin resistance, so it takes more insulin to get the job done. What about using insulin as a stand alone drug, without anabolic steroids and GH? This won’t work, it will just make you fat. Studies in rats have demonstrated that injecting su-pra-physiologic (excess) insulin promotes excess fat storage. The only reason to use injectable insulin is if you’re a diabetic who needs it, and even then using more than you need will just make you fat. One of the goals of the bodybuilding diet is to reduce insulin levels, so what sense does it make to inject it? If you want to ex-periment in a natural way with increased insulin levels, just eat a diet high in sugar. See what I mean? Next month we will continue our dis-cussion of insulin and look at how insulin affects fat metabolism. Also, we’ll go into more detail about dietary strategies to op-timize insulin response .
1. For more information about the physi-ologic effects of insulin, refer to Guyton AC and Hall JE. Textbook of Medical Physiology. W.B. Saunders Company, Philadelphia, 1996.
2. For detailed information about how to construct your diet for optimal insulin control, refer to the Parrillo Performance Nutrition Manual.